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A simulation model for a shrub ecosystem in the semiarid Karoo, South Africa

Wiegand, T., S. J. Milton, C. Wissel. 1995. Ecology 6:2205-2211


Plant community dynamics in semi-arid regions appear to be "event-driven'". The aim of our model is to attain an understanding of the main processes determining the spatial and temporal dynamics of a shrub community in the semi-arid Karoo on a large temporal scale and to identify the significant events which drive this shrub community. Vegetation at the study site covers 15-20% of the soil surface and is dominated by five shrubs: Brownanthus ciliatus (Mesembryanthemoideae), Ruschia spinosa (Ruschioideae), Galenia fruticosa (Aizoaceae), Pteronia pallens (Asteraceae) and Osteospermum sinuatum (Asteraceae). Grasses and annuals play little part in the dynamics of this vegetation. The model is based on detailed life-history data for the five dominant species and on monthly long-term rainfall data for this region. The method of "dynamic automata" is employed to model individual plants. Growth, death, seed production, germination and seedling establishment are modelled over long time scales in annual time-steps under the influence of the stochastic and unpredictable rainfall in ungrazed rangeland. In the absence of grazing, survival of seedlings depends on their competitive ability during the seedling stage and their ability to compete with established plants in neighboring cells.


The model shows that the dynamics of this shrub community are typified by episodic and discontinuous changes in species composition with intervening quasistable phases lasting some decades. The reason for this episodic behaviour is that both recruitment and mortality of plants depend on particular conditions: Using 93 complete data sets with monthly rainfall data taken at the weather station in Prince Albert we show that rainfall is only sufficient for seedling recruitment in 44% of all years for B. ciliatus and in less than 30% of all years for the four other species. We identify two types of abrupt and discontinuous changes in species composition: (1) big recruitment events which can only occur if plant density is low and if rainfall conditions are extraordinarilly favourable and (2) big mortality events which can only occur if cohorts, originating from big recruitment events, die within a short period of time and if little further recruitment has taken place. However, this behavior is not a property of the biota but is generated by rainfall input to the model. By using a different rainfall input, the model can also display regular cyclic succession. Therefore there appeared to be no contradiction between sudden, discontinuous changes and gradual, continuous and reversible changes in vegetation composition.


Key-words: individual-based; simulation model; automata; non-equilibrium; semi-arid ecosystem; spatial and temporal dynamics; event-driven system.

Vegetation change in semiarid communities: simulating probabilities and time scales
Wiegand, T., and S. J. Milton. 1996. Vegetatio 125:169-183.


In arid regions, the effects of grazing or sparing management on natural communities of long-lived plants generally take decades to become evident. Event-driven dynamic behavior, unpredictable and low rainfall and complicated interactions between species make it difficult to assess probabilities and time scales of vegetation change. To gain a better understanding of the main processes and mechanisms involved in vegetation change, we have developed a spatially explicit individual based model that simulates changes in plant communities over long time spans. The model, based on life-history attributes of the five dominant component plant species of a typical Karoo shrub community, follows the fate of each individual plant within the community, the sum of which is community dynamics. The model explores the differential effects of a realistic range of rainfall pattern on the abilities of these species to compete, survive, grow and reproduce. The specific aim of the model is to identify key processes of vegetation change and to calculate probabilities and timespans for transitions between different vegetation states. Such knowledge is needed for species conservation and sustained animal production.


We show that the time-scale for changes of the dynamic state of the system are long compared with human lifespans. Employing the full range of possible rainfall scenarios showed that short-term community dynamics (years to decades) and species composition depend strongly on the short-term (years) sequence of rainfall events. In all simulation experiments the final vegetation state varied by more than 37% after a 60 year simulation period. Simulating resting of an overgrazed part of the shrub community indicated that little improvement in rangland condition was likely during a period of 60 years. Even such active management, as (simulated) clearing of unpalatable shrubs, resulted in only a 66% probability that degraded shrubland would be in good condition after 60 years resting. Simulated overgrazing of a rangeland in good initial condition only became obvious 40 or 50 years after the initiation of heavy grazing, and after 70 years the mean vegetation state eventually reached that of an overgrazed rangeland.


Keywords: event-driven dynamics, grid based model, grazing, individual-based simulation model, Karoo.

Simulated plant population responses to small scale disturbance in semi-arid shrublands
Wiegand, T., M. R. J. Dean, and S. J. Milton. 1997. Journal of Vegetation Science 8:163-176.


We used a spatially explicit simulation model to examine the impact of small scale disturbance on the temporal and spatial dynamics of a typical Karoo shrub plant community, and to gather insight into the interplay between disturbance architecture and population dynamics. Establishment, growth, mortality, seed dispersal and competitive interactions were modelled over long time scales in annual time-steps under the influence of stochastic and unpredictable rainfall. Disturbances were created by the digging of aardvarks (Orycteropus afer) and bat-eared foxes (Otocyon megalotis) or by humans as management action. 


We examined the impact of three different disturbance regimes on long-term population dynamics by varying the type, rate and size of the small scale disturbances. The impact that a disturbance regime has on long-term community dynamics depends on complex interactions between disturbance characteristics and life-history attributes of component species. Plant density decreased with overall disturbance rates; this effect was independent of the type of disturbance. A given type and rate of disturbance did not influence all species within a guild (e.g. coloniser species) in the same way. The reason for these differences was that species were responding not only to the disturbance but to changes in competition intensity from other species and changes in their reproductive potential relative to other species. Such interactions resulted in a sequential change in dominant species within guilds as disturbance rates increased. An increase in the overall disturbance rate did not always produce the trend in evenness expected from the intermediate disturbance hypothesis, but was influenced by the relative abundance of different types of disturbances.


Keywords: individual-based simulation model; Karoo; South Africa; spatial and temporal dynamics; spatial modelling.

Population dynamics, disturbance, and pattern evolution: identifying the fundamental scales of organization in a model ecosystem. 
Wiegand, T., K. Moloney, S. J. Milton. 1998.  American Naturalist 152:321-337. 


We used auto- and crosscorrelation analysis and L function analysis to analyze spatio-temporal pattern evolution in a spatially explicit simulation model of a semiarid shrubland (Karoo, South Africa), and to determine the impact of small-scale disturbances on system dynamics. Without disturbance, local dynamics were driven by a pattern of cyclic succession, where “colonizer” and “successor” species alternately replaced each other. This results in a strong pattern of negative correlation in the temporal distribution of colonizer and successor species. As disturbance rates were increased, the relationship shifted from being negatively correlated in time to being positively correlated — the dynamics became decoupled from the ecologically-driven cyclic succession and were increasingly influenced by abiotic factors (e.g., rainfall events). Further analysis of the spatial relationships among colonizer and successor species showed that without disturbance periods of attraction and repulsion between colonizer and successor species alternate cyclically at intermediate spatial scales. This was due to the spatial “memory” embedded in the system through the process of cyclic succession. With the addition of disturbance, this pattern breaks down, although there is some indication of increasing ecological organization at broader spatial scales. We suggest that many of the insights that can be gained through spatially explicit models will only be obtained through a direct analysis of the spatial patterns produced.


Key words: individual-based simulation model, disturbance, Karoo shrubland, Ripley’s L-Analysis, spatio-temporal dynamics, time-series correlation analysis.

Assessing the risk of extinction for the brown bear (Ursus arctos) in the Cordillera Cantabrica, Spain. 
Wiegand, T, J. Naves, T. Stephan, A. Fernandez. 1998. Ecological Monographs 68:539-571.


The status of the brown bear (Ursus arctos) in Spain has suffered a dramatic decline during the last centuries, both in area and numbers. Current relict populations are suspected to be under immediate risk of extinction. The aim of our model is to attain an understanding of the main processes and mechanisms determining population dynamics in the Cordillera Cantabrica. We compile the knowledge available about brown bears in the Cordillera Cantabrica, Northern Spain, and perform a population viability analysis (PVA) to diagnose the current state of the population and to support current management. The specially constructed simulation model, based on long-term field investigations on the western brown bear population in the Cordillera Cantabrica, includes detailed life history data and information on environmental variations in food abundance. The method of individual-based modeling is employed to simulate the fate of individual bears. Reproduction, family breakup, and mortalities are modeled in annual time steps under the influence of environmental variations in food abundance, mortality rates and reproductive parameters. In parallel, we develop an analytical model that describes the mean behavior of the population and that enables us to perform a detailed sensitivity analysis.


We determine current population parameters by iterating the model with plausible values and compare simulation results with the 1982-1995 time pattern of observed number of females with cubs of the years. Our results indicate that the population suffered a mean annual decrease of ~4-5% during the study period 1982-1995. We found that this decrease could be explained by a coincidence of high poaching pressure with a series of climatically unfavorable years during the period 1982-1988. Thereafter, population size probably stabilized. We estimated that the population currently consist of 25 or 26 independent females and a total of 50-60 individuals. However, our viability analysis showed that the population does not satisfy the criterion of a minimum viable population if mortalities remain at the level of the last few years of 1988-1995. The "salvation" of at least one independent female every three years is required. The population retains relatively high reproductive parameters, indicating good nutritive conditions of the habitat, but mortality rates are higher than those known in other brown bear populations. The most sensitive parameters, adult and subadult mortality of females, form the principal management target. Our model shows that the series of females with cubs contains valuable information on the state of the population. We recommend monitoring of females with cubs as the most important management action, both for collecting data and for safeguarding the most sensitive part of the population.


Key words: brown bear; endangered species; extinction; individual-based stochastic simulation model; population dynamics; Ursus arctos; viability analysis.

Simulation models for semi-arid rangelands of Southern Africa.
Wiegand, T., F. Jeltsch, S. Bauer, K. Kellner. 1999.  African Journal of Range and Forage Science 15:48-60. 


In semi-arid regions, the effects of grazing or sparing management on natural communities of long-lived plants generally take decades to become evident. Event driven dynamic behaviour, unpredictable and low rainfall and complicated interactions between species make it difficult to gather sufficient understanding of vegetation dynamics to be able to develop guidelines for sustainable management. Simulation models that consider the essential processes that determine vegetation dynamics offer scope for quantitatively exploring long-term vegetation dynamics of arid and semi-arid rangelands. In this paper we review three models that were aimed to provide an understanding of the vegetation dynamics and management of different typical vegetation types in South Africa; including the Karoo shrubland, the shrub-grassland of the southern Kalahari, and pure semi-arid grasslands. 


Key words: Kalahari, Karoo, semi-arid grasslands, shrubland, vegetation dynamics.

Finding the missing link between landscape structure and population dynamics: a spatially explicit perspective.
Wiegand, T., K. Moloney, J. Naves, F. Knauer. 1999.  American Naturalist 154:605-627. 


We compile and exemplify a general modeling framework that allows for a systematic investigation of the impact of changes in landscape structure on population dynamics. The essential parts of the framework are a landscape generator with independent control over landscape composition and physiognomy, an individual-based spatially explicit population model that simulates population dynamics within heterogeneous landscapes, and scale-dependent landscape indices that depict the essential aspects of landscape which interact with dispersal and demographic processes. Landscape maps are represented by a grid of 50 x 50 cells and consist of good-quality, poor-quality or (inhabitable) matrix habitat cells. The population model was shaped in accordance to the biology of European brown bears (Ursus arctos), and demographic parameters were adjusted to yield a source-sink configuration. Results obtained with the spatially explicit model do not confirm results of earlier non-spatial source-sink models where addition of sink habitat resulted in a decrease of total population size because of dilution of high-quality habitat. Our landscape indices, that describe scale-dependent correlation between and within habitat types, were able to explain variations in variables of population dynamics (mean no. of females with sink home-ranges, the mean no. of females with source home-ranges, and mean dispersal distance) due to different landscape structure. When landscape structure changed, changes in these variables generally followed the corresponding change of an appropriate landscape index in a linear way. Our general approach incorporates source-sink dynamics as well as metapopulation dynamics, and the population model can easily be modified for other species groups. 


Key words: habitat connectivity, heterogeneous landscapes, scale-dependent landscape indices, population dynamics, spatially explicit population models, source-sink dynamics.

Live fast, die young: estimating size-age relations and mortality pattern of shrubs species in the semi-arid Karoo, South Africa.
Wiegand, T., S. J. Milton, K. J. Esler, and G. Midgley. 2000. Plant Ecology, 150: 115-131. 


We present a technique for estimating size-age relations and size-dependent mortality patterns of long-lived plants. The technique requires two sets of size data of individual (non-marked) plants that should be collected with a time-lag of several years in the same area of a study site. The basic idea of our technique is to assume general (three parameter) families of size-dependent functions which describe growth and mortality that occurred between the two data gathering events. We apply these growth and mortality functions to the size data of the early data set and construct predicted size-class distributions to compare it, in a systematic way, to the size-class distribution of the later data set. In a next step we calculate the size-age relations from the resulting growth functions, which yield the smallest difference between observed and predicted size-class distribution. Applying this technique to size data of five dominant shrub species at the Tierberg study site in the semiarid Karoo, South Africa produced new insight into the biology of these species which otherwise cannot be obtained without frequent measurements of marked plants. We could relate characteristics of growth behavior and mortality, for certain subgroups of the five species, to the life-history attributes evergreen vs. deciduous, succulent vs. woody, and early reproductive vs. late reproductive. The results of our pilot-study suggest a broad applicability of our technique to other shrublands of the world. This requires at least one older record of (individual) shrub-size data and performance of resampling. 


Key words: growth models, Karoo, mortality, size-age relation, shrub community.

Rule-based assessment of suitable habitat and patch connectivity for the Eurasian lynx in Germany
Schadt, S., F. Knauer,  P. Kaczensky, E. Revilla E., T. Wiegand, and L. Trepl. 2002. Ecological Applications 12:1469-1483


Conservation biologists often have to take decisions for which little information derived from detailed field studies is available. The urgency of most conservation issues and scarcity of financial resources make it impossible to obtain empirical data, but rather require that conservationists make the best out of the existing information to improve current management. In Germany lynx recovery and reintroduction initiatives are controversially discussed, one concern being that there is not enough suitable habitat left to support a viable population. The extent and the spatial arrangement of potentially suitable habitat is unknown and almost no data are available to address this question. The aim of this study was to predict the location of potentially suitable habitat for lynx in Germany, to obtain rough estimates of the potential maximum number of resident lynx, and to estimate connectivity between patches of suitable habitat. In a rule-based model habitat preferences of lynx were described as availability of forest cover, defined by patch size, degree of fragmentation and spatial structure. The model rules were implemented on a Geographic Information System (GIS) to produce a map showing the resulting patches of suitable habitat. Optimal corridors between patches were modeled with a cost-path analysis, based upon habitat type specific probabilities for lynx to cross. Our results show that patches of suitable habitat vary greatly in size, with 10 areas large enough to sustain more than 20 resident lynx. Overall, the 10 areas could sustain about 380 resident lynx. Uncertainty analyses of various critical model assumptions and parameters show that the predictions of our model are more or less stable, mainly due to the strong constraining impact of forest distribution. Our analyses suggest that reintroduction programs should focus less on small and isolated areas, but should include large scale connectivity into the decision making process. Our model is an example of a cross-boundary analysis of landscape structure based upon the needs of a species with large spatial requirements, an approach suitable as a basis for co-ordinating reintroduction initiatives and focus conservation efforts on the most promising areas.


Key words: Eurasian lynx, Lynx lynx, rule-based model, predictive habitat model, patch connectivity, GIS, costpath-analysis, limited resources, large-scale approach, decision making process, conservation, species reintroduction.

Assessing the suitability of central European landscapes for the reintroduction of Eurasian lynx.
Schadt, S., E. Revilla, T. Wiegand, F. Knauer, P. Kaczensky, U. Breitenmoser, L. Bufka, J. Cerveny, P. Koubek, T. Huber, C. Stanisa, and L. Trepl. 2002. Journal of Applied Ecology 39:189-203


1. After an absence of almost 100 years the Eurasian lynx Lynx lynx is slowly recovering in Germany along the German-Czech border. Additionally, many reintroduction schemes have been discussed controversially in various locations. We present a habitat suitability model for lynx in this region as a basis for further management and conservation efforts aimed at recolonisation and population development.


2. We developed a statistical habitat model using logistic regression to quantify the factors that describe lynx home ranges in a fragmented landscape. As no data were available for lynx distribution in Germany, we used data from the Swiss Jura Mountains for model development and validated the habitat model with telemetry data from the Czech Republic and Slovenia. We derived several variables describing land use and fragmentation, introducing also variables that described the connectivity of forested and non-forested semi-natural areas on a larger scale than the map resolution.


3. We obtained a model with only one significant variable that described the connectivity of forested and non-forested semi-natural areas on a scale of about 80 km². This result is biologically meaningful, reflecting the absence of intensive human land use on the scale of an average female home range. Model testing at a cut level P > 0.5 correctly classified more than 80% of the Czech and Slovenian telemetry location data of resident lynx. Application of the model to Germany showed that the most suitable habitats for lynx were large forested low mountain ranges and the large forests in east Germany.


4. Our approach illustrates how information on habitat fragmentation on a large-scale can be linked with local data to the potential benefit of lynx conservation in central Europe. Spatially-explicit models like ours can form a basis for further assessing population viability of species of conservation concern in suitable patches.


Key-words: GIS, large-scale approach, logistic regression, Lynx lynx, spatially explicit connectivity index, species reintroduction, statistical habitat model.Key-words: GIS, large-scale approach, logistic regression, Lynx lynx, spatially explicit connectivity index, species reintroduction, statistical habitat model.

Using pattern-oriented modeling for revealing hidden information: a key for reconciling ecological theory and application
Wiegand, T., F. Jeltsch, I. Hanski, and V. Grimm. 2003. Oikos 100: 209-222.


We suggest that the conscious use of information that is “hidden” in distinct structures in nature itself and in data extracted from nature (= pattern) during the process of modeling (= pattern-oriented modeling) can substantially improve models in ecological application and conservation. Observed patterns, such as time-series patterns and spatial patterns of presence/absence in habitat patches, contain a great deal of data on scales, site-history, parameters and processes. Use of these data provides criteria for aggregating the biological information in the model, relates the model explicitly to the relevant scales of the system, facilitates the use of helpful techniques of indirect parameter estimation with independent data, and helps detect underlying ecological processes. Additionally, pattern-oriented models produce comparative predictions that can be tested in the field.


We developed a step-by-step protocol for pattern-oriented modeling and illustrate the potential of this protocol by discussing three pattern-oriented population models: (1) a population viability analysis for brown bears (Ursus arctos) in northern Spain using time-series data on females with cubs of the year to adjust unknown model parameters; (2) a savanna model for detecting underlying ecological processes from spatial patterns of tree distribution; and (3) the incidence function model of metapopulation dynamics as an example of process integration and model generalization.

We conclude that using the pattern-oriented approach to its full potential will require a major paradigm shift in the strategies of modeling and data collection, and we argue that more emphasis must be placed on observing and documenting relevant patterns in addition to attempts to obtain direct estimates of model parameters.


Keywords: Brown bear; conservation biology; ecological application; error propagation; incidence function model; indirect parameter adjustment; Kalahari; pattern-oriented modeling; population models; savanna, uncertainty, virtual ecologist

Endangered species balancing between natural and human constrains: the case of brown bears (Ursus arctos) in northern Spain
Naves, J., T. Wiegand, E. Revilla, and M. Delibes. 2003, Conservation Biology 17:1276-1289.


We developed a conceptual framework for classifying habitat quality that requires the construction of separate habitat models for each demographic key-feature, to be applied when the factors that determine different demographic processes differ substantially. For example, survival of large carnivores is mainly determined by human-induced mortality, while nutritional condition determines the reproductive rate. Hence, a two-dimensional habitat model built for reproduction and survival yields five hypothetical habitat categories: matrix (no reproduction and/or very high mortality), sink (low reproduction, high mortality), refuge (low reproduction, low mortality), attractive sink (high reproduction, high mortality) and source (high reproduction, low mortality). We applied this framework to two endangered brown bear sub-populations in the Cantabrian Mountains, Spain. Our aim was to generate working hypotheses on the quality and the spatial arrangement of bear habitat for analyzing the present conditions of the different population nuclei, for identifying core areas of high conservation value, conflictive areas, or areas with unoccupied potential habitat. We used a geographic information system (GIS) and two spatial long-term data sets on presence and reproduction, and performed logistic regressions for building a two-dimensional habitat model. The analysis reveals that both populations seem to exist under sub-optimal habitat conditions: the eastern population occupies mainly areas of sub-optimal natural habitat and relatively low human impact, while the western population is mainly located in areas with high human impact, but otherwise good natural quality. To test hypotheses on demographic features of the obtained habitat categories we classified data on historic extinction in the north of Spain (14th to 19th centuries) within the two-dimensional model. Extinction probabilities within each habitat category confirmed the hypotheses: most extinctions occurred in matrix habitat, and the least in source habitat.


Keywords: attractive sink; endangered species; extinction; habitat model; human impact; logistic regression; matrix; refuge; source-sink theory; Ursus arctos

Expansion of brown bears (Ursus arctos) into the eastern Alps: a spatially explicit population model.
Wiegand, T., F. Knauer, P. Kaczensky, and J. Naves. 2004. Biodiversity and Conservation 13:79-114. 2004.


We present a spatially-explicit population model for analysing the expansion of brown bears (Ursus arctos) after the reintroduction program in central Austria. The model is based on field investigations into brown bears in Austria and Slovenia and on current knowledge of brown bears. The landscape of the eastern Alps is represented by a GIS-derived raster map defining local habitat suitability and five major spatial barriers to dispersal. The population model follows the fate of individual bears and simulates reproduction, dispersal, home range establishment, and mortality in annual time steps. We indirectly adjust unknown or uncertain model parameters with 10-year data on the number of females with cubs in central Austria and determine key variables of population dynamics, such as population sizes and growth rates within different population nuclei, dispersal distances, or mortality rates, for model parameterisations that reproduces the data on females with cubs.


We estimated a current (1996-2000) growth rate of the population in Austria and adjacent parts of Italy of some 14%; a high proportion of this growth was due to immigration from Slovenia. Consequently, the growth rate of the subpopulation in central Austria, which probably is isolated functionally (i.e., no exchange of females) from the nuclei along the Austrian-Slovenian border, yielded some 7%. This subpopulation may comprise seven residents, and we estimated for females a 33% risk of extinction during the 1992-2000 period. Validation and confirmation of our model results with data on bear densities that were not used for model construction and parameterization supported our findings. The high female mortality rates together with the vulnerability of the small population to chance events (i.e., demographic stochasticity) are the most pressing threat for the population in the eastern Alps. Our approach could be widely applied for analysing dynamics of rare and endangered species in which the paucity of data precludes an appraisal of the state of the population using standard methods


Keywords: extinction, individual-based model, landscape, management, population dynamics, spatially explicit population model

Reducing uncertainty in spatially explicit population models
Wiegand, T., E. Revilla, and F. Knauer. 2004. Biodiversity and Conservation 13:53-78.


It has been argued that spatially explicit population models (SEPMs) cannot provide reliable guidance for conservation biology because of the difficulty of obtaining direct estimates for their demographic and dispersal parameters and because of error propagation. We argue that appropriate model calibration procedures can access additional sources of information, compensating the lack of direct parameter estimates. Our objective is to show how model calibration using population-level data can facilitate the construction of SEPMs that produce reliable predictions for conservation even when direct parameter estimates are inadequate. We constructed a spatially explicit and individual based population model for the dynamics of brown bears (Ursus arctos) after a reintroduction program in Austria. To calibrate the model we developed a procedure that compared the simulated population dynamics with distinct features of the known population dynamics (= patterns). This procedure detected model parameterizations that did not reproduce the known dynamics. Global sensitivity analysis of the uncalibrated model revealed high uncertainty in most model predictions due to large parameter uncertainties (coefficients of variation CV » 0.8). However, the calibrated model yielded predictions with considerably reduced uncertainty (CV » 0.2). A pattern or a combination of various patterns that embed information on the entire model dynamics can reduce the uncertainty in model predictions, and the application of different patterns with high information content yields the same model predictions. In contrast, a pattern that does not embed information on the entire population dynamics (e.g., bear observations taken from sub-areas of the study area), does not reduce uncertainty in model predictions. Because population-level data for defining (multiple) patterns are often available our approach could be applied widely.


Keywords: individual-based model; model calibration; pattern-oriented modeling; population dynamics; spatially explicit population model; Ursus arctos; uncertainty

Do Grasslands have a Memory: Modeling Phytomass Production of a semiarid African Grassland
Wiegand,T., H. A. Snyman, K. Kellner, and J. M. Paruelo. 2004. Ecosystems 7:243-258.  


We analyzed data sets on phytomass production, basal cover, and monthly precipitation of a semiarid grassland in South Africa for good, medium, and poor rangeland condition (1) to investigate whether phytomass production per unit of basal cover differed among rangeland conditions, (2) to quantify the time-scales of a carry-over effect from production in previous months, and (3) to construct predictive models for monthly phytomass. Finally, we applied the best models to a 73-year data set of monthly precipitation data to study the long-term variability of grassland production.

Mean phytomass production per unit of basal cover did not vary significantly between rangeland conditions, i.e., vegetated patches in degraded grassland have approximately the same production as vegetated patches in grassland in good condition. Consequently, the stark decline in production with increasing degradation is a first-order effect of reduced basal area. Current year precipitation accounted for 64%, 62%, and 36% of the inter-annual variation in phytomass production for good, medium, and poor condition, respectively, and we found that 61%, 68%, and 33%, respectively, of the unexplained variation is related to a memory index that combines mean monthly temperature and a memory of past precipitations. We found a carry-over effect in production from the previous 4 years for grassland in good condition, and from the previous 1 or 3 month for medium and poor. The memory-effect amplified the response of production to changes in precipitation due to alternation of prolonged periods of dry or wet years/months at the time scale of the memory. The interannual variability in phytomass production per unit basal cover (CV = 0.42 0.50 for our 73-year prediction, CV = 0.57 0.71 for the 19-year data) was greater than the corresponding temporal variability in seasonal rainfall (CV = 0.29).

Keywords: basal cover, climate, carry-over effect, desertification, drought, grassland condition, long-term data set.

Rings, Circles and Null-Models for Point Pattern Analysis in Ecology
Wiegand, T., and K. Moloney
2004 Oikos 104: 209-229

A large number of methods for the analysis of point pattern data have been developed in a wide range of scientific fields. First-order statistics describe large-scale variation in the intensity of points in a study region, whereas second-order characteristics are summary statistics of all point-to-point distances in a mapped area and offer the potential for detecting both different types and scales of patterns. Second-order analysis based on Ripley's K-function is increasingly used in ecology to characterize spatial patterns and to develop hypothesis on underlying processes; however, the full range of available methods has seldomly been applied by ecologists. The aim of this paper is to provide guidance to ecologists with limited experience in second-order analysis to help in the choice of appropriate methods and to point to practical difficulties and pitfalls. We review (1) methods for analytical and numerical implementation of two complementary second-order statistics, Ripley's K and the O-ring statistic, (2) methods for edge correction, (3) methods to account for first-order effects (i.e., heterogeneity) of univariate patterns, and (4) a variety of useful standard and non-standard null models for univariate and bivariate patterns. For illustrative purpose, we analyze examples that deal with non-homogeneous univariate point patterns. We demonstrate that large-scale heterogeneity of a point-pattern biases Ripley's K-function at smaller scales. This bias is difficult to detect without explicitly testing for homogeneity, but we show that it can be removed when applying methods that account for first-order effects. We synthesize our review in a number of step-by-step recommendations that guide the reader through the selection of appropriate methods and we provide a software program that implements most of the methods reviewed and developed here.

Key words: Bivariate second-order analysis; edge correction; irregularly-shaped plot; heterogeneous point patterns; Monte Carlo simulation; null models; O-ring statistic; pair-correlation function; Ripley's K-function; spatial association.


Effects of Habitat Loss and Fragmentation on Population Dynamics
Wiegand, T., E. Revilla, and K. A. Moloney. 2005, Conservation Biology 19:108-121.


We used a spatially explicit population model that was generalized to produce nine ecological profiles of long-lived species with stable home ranges and natal dispersal to investigate the effects of habitat loss and fragmentation on population dynamics. We simulated population dynamics in landscapes composed of three habitat types (good-quality habitat ranging from 10-25%, poor-quality habitat ranging from 10-70%, and matrix). Landscape structures varied from highly fragmented to completely contiguous. The specific aims of our model were (1) to investigate under which biological circumstances the traditional approach of using two types only (habitat and matrix) failed and assess the potential impact of restoring matrix to poorquality habitat, (2) to investigate how much of the variation in population size was explained by landscape composition alone and which key attributes of landscape structure can serve as predictors of population response, and (3) to estimate the maximum fragmentation effects expressed in equivalent pure loss of goodquality habitat. Poor-quality habitat mattered most in situations when it was generally not considered (i.e., for metapopulations or spatially structured populations when it provides dispersal habitat). Population size increased up to 3 times after restoring matrix to poor-quality habitat. Overall, habitat amount accounted for 68% of the variation in population size, whereas ecological profile and fragmentation accounted for approximately 13% each. The maximal effect of (good-quality) habitat fragmentation was equivalent to a pure loss of up to 15% of good-quality habitat, and the maximal loss of individuals resulting from maximal fragmentation reached 80%. Abundant dispersal habitat and sufficiently large dispersal potential, however, resulted in functionally connected landscapes, and maximal fragmentation had no effect at all. Our findings suggest that predicting fragmentation effects requires a good understanding of the biology and habitat use of the species in question and that the uniqueness of species and the landscapes in which they live confound simple analysis.

Key Words: ecological profiles, individual-based spatially explicit population model, landscape metrics, landscape
structure, matrix heterogeneity, metapopulation, source-sink

Fragmented landscapes, road mortality and patch connectivity: modeling dispersal for the Eurasian lynx in Germany *
Kramer-Schadt, S., E. Revilla, T. Wiegand, U. Breitenmoser. 2004. Journal of Applied Ecology 41:711-723


1. Although many reintroduction schemes for the Eurasian lynx Lynx lynx in Germany have been discussed, the implications of connectivity between suitable patches have not been assessed.


2. We introduce an individual-based, spatially explicit dispersal model to assess the probability of a dispersing animal reaching another suitable patch in the complex heterogeneous German landscape, with its dense transport system. The dispersal model was calibrated using telemetric data from the Swiss Jura and based on a map of potential lynx dispersal habitat.


3. Most suitable patches could be interconnected by movements of dispersing lynx within 10 years of reintroduction. However, when realistic levels of mortality risks on roads were applied, most patches become isolated except along the German–Czech border. Consequently, patch connectivity is limited not so much by the distribution of dispersal habitat but by the high mortality of dispersing lynx. Accordingly, rather than solely investing in habitat restoration, management efforts should try to reduce road mortality.


4. Synthesis and applications. Our approach illustrates how spatially explicit dispersal models can guide conservation efforts and reintroduction programmes even where data are scarce. Clear limits imposed by substantial road mortality will affect dispersing lynx as well as other large carnivores, unless offset by careful road-crossing management or by the careful selection of release points in reintroduction programmes.


Key-words: conservation, large carnivores, large-scale approach, Lynx lynx, movement, spatially explicit individual-based model, species reintroduction



*'This is an electronic version of an article published in Journal of Applied Ecology: complete citation information for the final version of the paper, as published in the print edition of Journal of Applied Ecology, is available on the Blackwell Synergy online delivery service, accessible via the journal's website at http://www.blackwellpublishing.com/journals/jpe or http://www.blackwell-synergy.com.

Effects of matrix heterogeneity on animal dispersal: from individual behavior to metapopulation-level parameters.
Revilla, E., T. Wiegand, F. Palomares, P. Ferreras, and M. Delibes. 2004. The American Naturalist 164: E-130-E153 .

Mounting theoretical and empirical evidence shows that matrix heterogeneity may have contrasting effects on metapopulation dynamics by contributing to patch isolation in non-trivial ways. We analyze the movement properties during interpatch dispersal in a metapopulation of Iberian lynx (Lynx pardinus). On a daily temporal scale lynx habitat selection define two types of matrix habitats where individuals may move: open and dispersal habitats (avoided and used as available, respectively). There was a strong and complex impact of matrix heterogeneity on movement properties at several temporal scales (hourly and daily radiolocations, and the entire dispersal event). We use the movement properties on the hourly temporal scale to build a simulation model to reconstruct individual dispersal events. The two most important parameters affecting model predictions at both the individual (daily) and metapopulation scales were related to the movement capacity (number of movement steps per day and autocorrelation in dispersal habitat), followed by the parameters representing the habitat selection in the matrix. The model adequately reproduced field estimates of population-level parameters (e.g. interpatch connectivity, maximum and final dispersal distances), and its performance was clearly improved when including the effect of matrix heterogeneity on movement properties. Assuming a homogeneous matrix results in large errors in the estimate of interpatch connectivity, especially for close patches separated by open habitat or corridors of dispersal habitat, showing how important it is to consider matrix heterogeneity when it is present. Movement properties on the interaction of dispersing individuals with the landscape can be used as a mechanistic representation of dispersal at the metapopulation level. This is so when the effect of matrix heterogeneity on movement properties is evaluated under biologically meaningful spatial and temporal scales.


Key-words: autocorrelated random walk, Lynx pardinus, matrix fragmentation, individual-based spatially explicit simulations, interpatch connectivity, standard of plausibility.

Modeling Species Distributions to Improve Conservation in Semiurban Landscapes: Koala Case Study 

Rhodes, J. R., T. Wiegand, C. A. McAlpine, J. Callaghan, D. Lunney, M. Bowen, and H. P. Possingham. 2006. Conservation Biology 20:449-459


Species distribution models are commonly used to inform landscape and conservation planning. In urban and semiurban landscapes, the distributions of species are often determined by a combination of natural habitat and anthropogenic impacts. Understanding the spatial influence of these two processes is crucial for making spatially explicit decisions about conservation actions. We present a logistic regression model for the distribution of koalas (Phascolarctos cinereus), in a semiurban landscape in eastern Australia, that explicitly separates the effect of natural habitat quality and anthropogenic impacts on koala distributions. We achieved this by comparing the predicted distributions with the predicted distributions assuming anthropogenic variables are fixed at their mean values. Similar approaches have relied on making predictions assuming anthropogenic variables are zero, which will be unreliable if the training data set does not include anthropogenic variables close to zero. Our approach is novel because it can be applied to landscapes where anthropogenic variables are never close to zero. Our model showed that, averaged across the study area, natural habitat was the main determinant of koala presence. However, at a local scale, anthropogenic impacts could be more important, with consequent implications for conservation planning. We demonstrate that, by using this modeling approach and presenting predictions visually as a map, provides important information for making decisions on how different conservation actions should be spatially allocated. This approach is particularly useful for areas where wildlife and human populations exist in close proximity.


Key words: anthropogenic impacts, conservation planning, logistic regression, natural habitat quality, mixed effects model, spatial distribution model


Lynx reintroductions in fragmented landscapes of Germany: projects with a future or misunderstood wildlife conservation?

Kramer-Schadt, S.,.E. Revilla, and T. Wiegand. 2005.  Biological Conservation 125: 169-182

Eurasian lynx are slowly recovering in Germany after an absence of about 100 years, and additional reintroduction programs have been launched. However, suitable habitat is patchily distributed in Germany, and whether patches could host a viable population or contribute to the potential spread of lynx is uncertain. We combined demographic scenarios with a spatially explicit population simulation model to evaluate the viability and colonization success of lynx in the different patches, the aim being to conclude guidelines for reintroductions. The spatial basis of our model is a validated habitat model for the lynx in Germany. The dispersal module stems from a calibrated dispersal model, while the demographic module uses plausible published information on the lynx_ life history. The results indicate that (1) a viable population is possible, but that (2) source patches are not interconnected except along the German–Czech border, and that (3) from a demographic viewpoint at least 10 females and 5 males are required for a start that will develop into a viable population with an extinction probability of less than 5% in 50 years. The survival rate of resident adults was the most sensitive parameter, and the best management strategy for the success of reintroduction would be to reduce the mortality of residents in the source patches. Nevertheless, the extremely low probability of connectivity between suitable patches makes most of the reintroduction plans isolated efforts, and they are therefore questionable in the long run. With such a model, the suitability of the single habitat patches can be assessed and the most appropriate management scheme applied. This study shows that simulation models are useful tools for establishing the comparative effectiveness of reintroduction plans aimed at increasing the viability of the species.

Keywords: large carnivores, Lynx lynx L., mortality scenarios, population viability analysis, spatially explicit individual-based model, species reintroduction


Detailed assessment of microhabitat suitability for Aedes aegypti (Diptera: Culicidae) in Buenos Aires, Argentina.

D. Vezzani, D., A. Rubio, S. M. Velazquez, N. Schweigmann and T. Wiegand. 2005. Acta Tropica 95: 123-131


Little information is available on the ecology of Aedes aegypti Linnaeus at the southern extreme of its distribution (Buenos Aires, Argentina), particularly on microhabitat suitability. The aim of our study was to identify at a detailed scale, microhabitat factors that correlate with the presence of preimaginal stages of the mosquito. In March 2001 we performed a spatial census of all containers located in a 1-ha patch within a cemetery in Buenos Aires City. On a reference map (1:700) we plotted the position of graves and surrounding corridors, the location of containers, the shade projected by each plant between 10:00 and 16:00 hours and vegetation cover. We classified vegetation by height, substrate by composition and shadow by level of exposure to sunlight. We performed univariate and multivariate logistic regression analyses with 9 constructed independent variables, some of them at scales of 0.5, 1, 2, 3, and 10 m. Of 850 receptacles examined, 101 contained preimaginal stages of Ae. aegypti. Level of exposure to sunlight, type of substratum, vegetation height and distance of containers to vegetation were significantly associated with the presence of breeding sites at the studied scales. Final multivariate models were significant at scales of 2 (c2(3)=25.693, p<0.001) and 3 m (c2(3)=26.440, p<0.001), and 65.9% and 66.8% of our data were correctly classified respectively for each scale. Our results suggest that sites less exposed to sunlight, with taller and closer vegetation, and in shaded and vegetated neighbourhoods were the most favourable microhabitats for Ae. aegypti breeding.


Key words: mosquitoes - Aedes aegypti - microhabitat suitability - vegetation - shade - statistical habitat model - Argentina


Context-dependent negative and positive interactions between adult shrubs and seedlings in a semi-arid shrubland

Riginos, C., S. J. Milton, and T. Wiegand. 2005. Journal of Vegetation Science 16:331-340


Question: In semi-arid systems, rainfall gradients can cause plant-plant interactions to shift from negative to positive or vice versa. However, the importance of a second major abiotic factor, soil nutrients, has rarely been considered. We consider different combinations of both factors and ask: do net adult-seedling interactions become less competitive and more facilitative with increasing overall abiotic harshness?


Location: Succulent Karoo, Western Cape, South Africa.


Methods: We examined the interactions between seedlings and adult shrubs at two sites. Sites differ in rainfall, and each contain two habitats: nutrient-rich mounds associated with underground termitaria and a relatively nutrient-poor matrix. We carried out a spatial pattern analysis of community-wide seedling-adult associations. We then conducted field and greenhouse experiments to test the effects of soil and the presence of neighboring shrubs on the growth and survival of six seedling species.


Results: At the higher rainfall site, both competitive and facilitative effects of adults on seedlings were found but did not differ by habitat, despite the more benign conditions in the mound habitat. At the lower rainfall site, adult shrubs generally had neutral effects on seedlings in the matrix habitat. In the nutrient-rich mound habitat, however, adult shrubs had strong and consistently competitive effects on seedlings.


Conclusion: Seedling-adult interactions could not be predicted by a simple overall gradient of abiotic harshness, demonstrating the need for more complex, mechanistic models to predict plant-plant interactions. We suggest that rainfall and soil nutrients affect seedling-adult relations through their interactive effects on the life-history attributes of the species involved.


Key words: Competition; facilitation; nurse-plant effect; point pattern analysis; Succulent Karoo


Pattern-oriented Modeling of Agent-based Complex Systems: Lessons from Ecology

Volker Grimm, Eloy Revilla, Uta Berger, Florian Jeltsch, Wolf M. Mooij, Steven F. Railsback, Hans-Hermann Thulke, Jacob Weiner, Thorsten Wiegand, Donald L. DeAngelis,  Science 310:987-991


Agent-based complex systems are dynamic networks of many interacting agents; examples include ecosystems, the immune system, financial markets, and cities. In the search for general principles underlying the internal organization of such systems, bottom-up simulation models such as cellular automata and agent-based models are widely used. So far, however, no general framework for designing, testing, and analyzing bottom-up models has been established. However, recent advances in bottom-up ecological modeling have come together in a general strategy we call Pattern-oriented Modeling. This strategy provides a unifying framework for decoding the internal organization of agent-based complex systems and may lead toward unifying algorithmic theories of the relationship between adaptive behavior and system complexity.


Extending Point Pattern Analysis for Objects of Finite Size and Irregular Shape* Appendix (100K)
Thorsten Wiegand, W. Daniel Kissling, Pablo A. Cipriotti, and Martin R. Aguiar, 
Journal of Ecology 94: 825-837
1 We use a grid- and simulation-based approach to extend point pattern analysis to deal with plants of finite size and irregular shape, and compare the results of our approach with that of the conventional point approximation. The plants are approximated by using an underlying grid and may occupy several adjacent grid cells depending on their size and shape. Null models correspond to that of point pattern analysis but need to be modified to account for the finite size and irregular shape of plants.

2 We use a mapped area of a grass-shrub steppe in semiarid Patagonia, Argentina, to show that the shrub community is essentially randomly structured, but that shrubs facilitate grasses in their immediate neighbourhood.

3 The occurrence of this random spatial structure provides important new information on the biology of shrub populations. In general, previous data from semiarid and arid ecosystems have showed that adult shrubs tend to show over-dispersed patterns, whereas juveniles are clumped.

4 We find that the point approximation may produce misleading results (1) if plant size varies greatly, (2) if the scale of interest is of the same order of magnitude as the size of the plants and (3) if the plants of a given pattern are constrained through competition for space by the presence of other plants. The point approximation worked well in all other cases, but usually depicted weaker significant effects than when the size and shape of plants were taken into account.

5 Our approach to quantifying small-scale spatial patterns in plant communities has broad applications including the study of facilitation and competition. Ecologists will be able to use the software available to take advantage of these methods.
Key-words: Community structure, facilitation, grid-based implementation, neighbourhood effects, null models, pair-correlation function, point pattern-analysis, Ripley’s K-function, spatial pattern, second-order spatial statistics.


*This is an electronic version of an article published in Journal of Ecology: complete citation information for the final version of the paper, as published in the print edition of Journal of Ecology, is available on the Blackwell Synergy online delivery service, accessible via the journal's website at http://www.blackwellpublishing.com/journals/jec or http://www.blackwell-synergy.com.


Abrupt population changes in treeline ecotones along smooth gradients* Appendices (246K)
Thorsten Wiegand,  J. Julio Camarero, Nadja Rüger, and Emilia Gutiérrez,
Journal of Ecology 94: 880-892

1 We developed a spatially explicit and individual-based simulation model describing the dynamics of tree populations across treeline ecotones. Our aims were to identify minimal factors and processes able to generate treeline types with abrupt vs. smooth transitions in different variables (tree height, age, density), to investigate the role of positive feedback in pattern generation, and to find out why krummholz appears at some treelines, but not at all. We hypothesised that a different balance between smooth growth and mortality gradients across the treeline ecotone could account for differences between commonly observed treeline types.

2 The model contained only processes and factors we regarded as essential for producing a treeline ecotone and was parameterized with an extensive individual-based data set from Pinus uncinata treelines in the Spanish Pyrenees. However, parameters expected to influence treeline type were systematically varied.

3 The simple model was able to generate major treeline types differing in abruptness and krummholz abundance. The most important factors determining treeline types were the relative strength of the growth and mortality gradients, followed by facilitation strength, whereas demographic parameters accounted for more subtle differences.

4 We developed a tentative classification scheme to locate treelines types within a two-dimensional state space spanned by two parameters defining the growth and mortality gradients. Only certain combinations of growth inhibition, mortality, and facilitation allowed emergence of certain treeline types characterised by contrasting abruptness values and the presence or absence of krummholz.

5 High krummholz densities emerged only under positive feedback and strongly increasing growth inhibition across the ecotone. Abruptness in adult tree density was positively correlated with facilitation strength and growth inhibition. In contrast, treelines with abrupt height transitions occurred only, where both low growth inhibition and a strongly increasing mortality occurred across the ecotone.

6 Our analysis suggests that treeline features are not arbitrary but there is a clear signal in the pattern which allows for inference of the underlying processes. Our approach of a systematic comparison of model predictions and various observed patterns can be widely applied for testing hypotheses on the functioning of ecological systems and for deriving specific explanations to be verified in the field.

Key-words: demographic model, individual-based model, pattern-oriented modelling, Pinus uncinata, Pyrenees, spatially explicit population model, positive feedback.


*This is an electronic version of an article published in Journal of Ecology: complete citation information for the final version of the paper, as published in the print edition of Journal of Ecology, is available on the Blackwell Synergy online delivery service, accessible via the journal's website at http://www.blackwellpublishing.com/journals/jec or http://www.blackwell-synergy.com.

Spatial patterns and competition of tree species in a Douglas-fir chronosequence on Vancouver Island.

Getzin, S., C. Dean, F. He, T. Trofymow, K. Wiegand, and T. Wiegand. Ecography 29: 671-682.


Douglas-fir forest is a major forest in the Pacific Northwest region. While the successional dynamics and large scale structure of the forest is well studied, the fine-scale spatial characteristics at the stand level are still poorly understood. Here we investigated four hypotheses on the fine-scale spatial structure of the forest on Vancouver Island. Our primary purpose is to understand how the three dominant species, Douglas-fir, western hemlock and western redcedar, coexist and partition space along a chronosequence comprising of three stands of immature, mature and old-growth. We used the O-ring statistic and Ripley’s L-function to quantify the change in spatial distribution and association of the species along the chronosequence. Evidence on intra- and inter-specific competition was also inferred from correlations between nearest-neighbor distances and tree size. Our results show that (1) variation in local site characteristics was highly important for the unexpected aggregated pattern of Douglas-fir in old-growth, (2) surviving trees of the species were less aggregated than their pre-mortality patterns, thus rejecting the random mortality hypothesis and being strong evidence for intra-specific competition, (3) tree death was a random process among larger overstorey trees in the plots, and (4) inter-specific competition declined with increasing plot age as a result of spatial resource partitioning. Here we highlight the importance of spatial heterogeneity for the long-term coexistence of shade-intolerant pioneer Douglas-fir and shade-tolerant colonizer species during forest succession.



Spatial ecology of a root-parasite—from pattern to process

Watson, D.M., Roshier, D.A., and T. Wiegand. Austral Ecology 32: 359-369.


Occurrence patterns of parasitic plants are constrained by the distribution of suitable hosts and movement patterns of seed vectors and, accordingly, represent a simplified system to study many aspects of spatial ecology and determinants of distribution. Previous work has focused on the aerially hemiparasitic mistletoes, and it is unclear whether root parasites are affected by similar factors. Here, we evaluate spatial patterns in the root-parasitic Santalum lanceolatum in an arid shrubland in northwestern New South Wales, central Australia. In this region, the principal host is a long lived nitrogen fixing shrub Acacia tetragonophylla closely associated with ephemeral creek-lines. The location of 765 individuals of both species was mapped along a 250 m section of creek-line using a total survey station, and occurrence patterns of the root parasite related to host distribution and landscape context. We used Ripley’s K-function and the O-ring statistic to determine whether the distribution of S. lanceolatum was random, aggregated or regular; the spatial scales at which these patterns occurred; and to quantify any spatial associations between the parasite and its host, A. tetragonophylla.  While acacias were closely associated with the creek-line, S. lanceolatum plants were more tightly clustered, displaying significant clustering at two spatial scales (1.2 m and 8.8 m). We suggest that host quality may act as an important constraint, with only those acacias growing in or near the creek-line being physiologically capable of supporting a parasite to maturity. Insights gained from spatial analysis are used to guide ongoing research in this system, and highlight the utility of the O-ring statistic for understanding patterns of distribution affected by multiple processes operating at critical scales.


Keywords:  Dispersal, Distribution, Host quality, O-ring statistic, Santalum.

Revealing driving forces of mid-cities urban growth patterns using spatial modeling: a case study of Los Angeles (Chile). 

Aguayo, M., T. Wiegand, G. Azócar, K. Wiegand, and C. Vega. Ecology and Society: 12 (1): 13.

[online] URL: http://www.ecologyandsociety.org/vol12/iss1/art13/  


City growth and changes in land use patterns cause several important social and environmental impacts. In order to understand the spatial and temporal dynamics of these processes, we must identify and analyze the factors that drive urban development, especially those that can be used to predict future changes and their potential environmental effects. The objectives of this research were to quantify the relationship between the urban growth and its driving forces, exemplified for Los Angeles, a city in central Chile, and to predict the spatial growth pattern based on historical land use changes. This involved (i) analysis of a set of multi-temporal images (1978, 1992, 1998) and characterization of the land use change spatial pattern; (ii) the construction of digital coverage in GIS; (iii) the selection of predictive variables through univariate analysis; (iv) construction of logistic regression models using 1978 - 1992 growth vs. non growth as dependent variable; and (v) the prediction of the probability of land use change through application of the regression model for the 1992-1998 period. To investigate the influence of spatial scale we constructed several blocks of models i) comprising only distance variables (e.g., distance to highway), ii) comprising only scale-dependent density variables (e.g., density of urban area within a 600m radius), iii) combining distance and density variables and iv) combining distance and density variables at several spatial scales. All blocks included the environmental variables. We found that combining distance and density variables at several scales are required to appropriately capture the multi-scale urban growth process. The best models correctly predict about 90% of the observed 1992-1998 land use changes. Distance to access roads, the densities of the urban road system and urbanized areas at different scales, and soil type were the strongest driving forces of the growth pattern. In turn, other environmental variables were less important or not significant for explaining the urban growth process. Our approach which combines spatial modeling tools and GIS significantly advances our understanding of urban growth patterns, it provides an important contribution to urban planning and management, and can be widely applied.


Patterns for parameters in simulation models

Kramer-Schadt, S., E. Revilla, T. Wiegand, and V. Grimm. 2007. Ecological Modelling 204:553-556

Population simulation models are often used in conservation biology to assess human impact on species s urvival, but have been under heavy critique due to parameterization problems. The general notion is that only models for which parameters were directly assessed from field data can showthat a certain process isworking, in contrast to using field data to fit models. We would like to provide an update regarding the relationship between simulation model parameterization and the use of field data as ‘pattern’ for revealing ‘structurally realistic’ parameters and processes. ‘Pattern-oriented modelling’ is an inverse modelling technique in ecology that considers the use of multiple field data pattern simultaneously to filter the parameterizations whichwere successfully tested against all available data on system dynamics.We highlight this technique with an example of our own research and conclude that this approach is especially suitable for models in conservation of rare and elusive species, where data are gener ally scarce.


Analysis of spatial pattern in early stages of primary succession on former lignite mining sites.

Felinks, B. and T. Wiegand 2008.  Journal of Vegetation Science 19:267-276

Questions: 1. Does random colonization predominate in early stages of primary succession? 2. Do pioneer species facilitate the establishment of later arriving species? 3. Does an initially random distribution change to an aggregated pattern with ongoing succession? Location: Lignite mining region of Lower Lusatia, Eastern Germany.


Methods: Individual plants were mapped along a 2 x 28m transect during three successive years and classified into two groups (1) the pioneer Corynephorus canescens and (2) 'all other species'. Using the pair-correlation function, univariate point pattern analysis was carried out by applying a heterogeneous Poisson process as null model. Bivariate analysis and a toroidal shift null model were applied to test for independence between the spatial patterns of the two groups separately for each year, as well by exploring spatiotemporal patterns from different years.


Results: Corynephorus canescens and 'all other species' showed in the first year an aggregated pattern on a spatial scale > 40cm and in the second and third year a significant attraction for distances between 4 – 12cm, with an increasing radius in the third year. The analyses of interspecific spatiotemporal dynamics revealed a change from independence to attraction between distances of 4 and 16cm when using Corynephorus canescens as focal species. However, applying 'all other species' as focal points result in a significant attraction at distances up to 60cm in the first year and a diminishing attraction in the second and third year with distances £ 6cm. Conclusions: Facilitative species-species interactions are present in early stages of primary succession, resulting mainly from pioneer species acting as physical barriers and their ability to capture diaspores being drifted by secondary dispersal along the substrate surface. However, due to gradual establishment of perennial species and their ability of lateral extension by vegetative dispersal, facilitation may influence spatial pattern formation predominantly on rather short temporal and fine spatial scales.


Keywords: Facilitation, Pair-correlation function, Point pattern analysis, Species interactions, Succession


Spatial patterns in the sociable weaver (Philetairus socius).

Giesselmann, U.C., T. Wiegand, J. Meyer, R. Brandl, and M. Vogel. 2008.  Austral Ecology  32:359 - 369

The spatial organisation of individuals or groups of individuals within a population can provide valuable information about social organisation and population dynamics. We analysed the spatial distribution of communal nets of the colonial living sociable weaver (Philetairus socius) on two farms in the Kalahari. Sociable weavers build large communal nests on big savannah trees, forming a persisting point pattern of occupied and unoccupied trees. We used two spatial statistics, Ripley’s K and the pair-correlation function, to describe characteristics of the patterns over a range of distance scales. We found consistent results for the two farms. (1) At scales beyond 1000 m we found regular distribution of communal nests. (2) At scales between 200 and 300 m the communal nests were clustered. These two results were independent from the spatial distribution of trees. The regular distribution at larges scales may be due to competition of different colonies. The clustering of nests suggests that nearby communal nests (< 300 m) function as one colony.


Land use impact on Vitellaria paradoxa C.F. Gaerten. stand structure and distribution patterns: a comparison of Biosphere Reserve of Pendjari in Atacora district in Benin. Agroforestry Systems

Djossa, B.A., Fahr, J., Wiegand, T., Ayihouénou, B.E., Kalko, E.K.V., and B. A. Sinsin. 2008. Agroforestry Systems 72: 205-220

The shea tree, Vitellaria paradoxa, is a socio-economically important tree for the rural population in parts of West Africa. Our study assessed the current status of this native tree species with regard to increasing human pressure in northern Benin. We compared distribution of adult shea trees, seedlings and saplings in farmed lands with protected areas in the Biosphere Reserve of Pendjari (BRP). At our study site near BRP, agricultural activities foster recruitment of shea trees by regularly cropping of vegetation cover. Furthermore, traditional farming practices preserve adult individuals thus permitting regular fruit harvests. Consequently, most of the tallest and largest individuals of shea trees are found in framed lands. In contrast, the highest density of juvenile trees including seedlings (dbh <5 cm) and saplings (dbh 5-10 cm) occurred within BRP. Saplings were negatively affected by farming activities. Furthermore, spatial point pattern analysis revealed differences in the spatial structure of juveniles. Juveniles showed significant aggregations at small scale (< 20 m) in BRP as well as significant and positive small-scale associations with adult trees. This contrasts with farmed lands where we did not find such spatial patterns at similar small scale but only a weak aggregation between juveniles and absence of association (attraction) of adults to juveniles. Although our analyses indicate that shea trees are rather well preserved, we conclude that the observed severe reduction of saplings in farmed lands is likely to negatively impact the long-term viability of the tree population. Therefore agroforestry practices must consider the preservation of sapling populations in farming areas for long-term conservation.

Key words: Benin, conservation, Shea tree, spatial analysis.


A spatially-explicit analysis of seedling recruitment in the terrestrial orchid Orchis purpurea.

Jacquemyn, H., R. Brys, K. Vandepitte, O. Honnay, I. Roldán-Ruiz and T. Wiegand. 2007.  New Phytologist 76: 448–459.

Seed dispersal and the subsequent recruitment of new individuals into a population are important processes affecting population dynamics, genetic diversity and spatial genetic structure of plant populations.


Spatial patterns of seedling recruitment were investigated in two populations of the terrestrial orchid Orchis purpurea using both univariate and bivariate point pattern analysis, parentage analysis and seed germination experiments. • Both adults and recruits showed a clustered spatial distribution with cluster radii of about 4-5m. Results of the parentage analysis showed that offspring-dispersal distances were slightly larger than distances obtained from the point pattern analyses. The suitability of microsites for germination differed among sites, with strong constraints in one site and almost no constraints in the other.


Our results provide a clear and coherent picture of recruitment patterns in a tuberous, perennial orchid. Seed dispersal is limited to a few meters from the mother plant, whereas the availability of suitable germination conditions may vary strongly from one site to the next. Because of a time-lag of three to four years between seed dispersal and actual recruitment and irregular flowering and fruiting patterns of adult plants, interpretation of recruitment patterns using point patterns analyses ideally should take into account the demographic properties of orchid populations.


Key-words: micro-site limitation, Orchidaceae, Orchis purpurea, seed germination, seed limitation, spatial point pattern analysis.


Analyzing the spatial structure of a Sri Lankan tree species with multiple scales of clustering.

Wiegand, T, C.V.S. Gunatilleke, I.A.U.N. Gunatilleke, and T. Okuda. Ecology 88: 3088–3102

Clustering at multiple critical scales may be common for plants since many different factors and processes may cause clustering. This is especially true for tropical rain forests where theories for explaining species coexistence and community structure rest heavily on spatial patterns. We used point-pattern analysis to analyze the spatial structure of Shorea congestiflora, a dominant species at a 25-ha forest dynamics plot in a rain forest at Sinharaja World Heritage Site (Sri Lanka), which apparently shows clustering at several scales. We developed cluster processes incorporating two critical scales of clustering for exploring the spatial structure of S. congestiflora and interpret it in relation to factors such as competition, dispersal limitation, recruitment limitation, and Janzen–Connell effects. All size classes showed consistent large-scale clustering with a cluster radius of approximately 25m. Inside the larger clusters, small-scale clusters with a radius of 8m were evident for recruits and saplings, weak for intermediates, but disappeared for adults. The pattern of all trees could be divided into two independent patterns, a random pattern (nearest neighbor distance > 8m) comprising about 12% of the trees and a nested double-cluster pattern. This finding suggests two independent recruitment and/or seed dispersal mechanisms. Saplings were several times as abundant as recruits and may accumulate several recruit generations. Recruits were only weakly associated to adults and occupied about half of the large-scale clusters, but saplings almost all. This is consistent with recruitment limitation. For about 70% (95%) of all juveniles the nearest adult was less than 26m (53m), suggesting a dispersal limitation which may also be related with the critical large-scale clustering. Our example illustrates how the use of specific and complex null hypothesis of spatial structure in point pattern analysis can help to better understand the biology of a species and to generate specific hypotheses to be further investigated in the field.


Key words: Janzen–Connell, multiple clustering, pair-correlation function, point-pattern analysis, Ripley’s K-function, Sinharaja Forest Dynamics Plot, spatial point processes  


Species associations in a heterogeneous Sri Lankan Dipterocarp forest.

Wiegand, T, C.V.S. Gunatilleke, and I.A.U.N. Gunatilleke. 2007.  The American Naturalist 170 E77–E95

High biodiversity in tropical forests has both fascinated and puzzled biologists for more than half a century. The spatial pattern of species distributions in these systems could provide significant insights into the mechanisms that maintain high species richness. We used point pattern analysis to examine the spatial distribution of 46 frequent tree species (dbh > 10cm) in a fully mapped 500m x 500m tropical forest plot in Sinharaja, Sri Lanka. We aimed to disentangle the role of plant species interactions (second-order effects) and environment (first-order effects) in determining the spatial distribution of these plant species. To characterize first-order associations (i.e., segregation and overlap), we developed a classification scheme based on Ripley’s K and nearest neighbor statistics. We subsequently used heterogeneous Poisson null models, accounting for possible environmental heterogeneity, to reveal significant small-scale interactions (i.e., repulsion and attraction). Overall, 53% of all species pairs occupied largely disjoint areas (segregation), 40% showed partial overlap, and 6% overlapped. Consequently, only 5% of all species pairs showed significant repulsion or attraction, whereas about half of the species showed significant intraspecific aggregation or regularity. Significant spatial effects disappeared within approximately 15-20 m of the focal plant and occurred most frequently within 2-4 m. While lack of significant second-order species interactions can be taken as support for the unified neutral theory, the observed pattern of first-order segregation does not support the assumption of the zero-sum dynamics of the neutral theory. The strong tendency of species to avoid each other may have supplementary effects on other processes promoting species coexistence


Keywords: coexistence, habitat association, pair-correlation function, plant-plant interactions, point pattern analysis, tropical forest.  


How individual species structure diversity in tropical forests.

Wiegand, T, C.V.S. Gunatilleke,  I.A.U.N. Gunatilleke, and A. Huth. 2007. PNAS 104:19029–19033.

A persistent challenge in ecology is to explain the high diversity of tree species in tropical forests. Although the role of species characteristics in maintaining tree diversity in tropical forests has been the subject of theory and debate for decades, spatial patterns in local diversity have not been analyzed from the viewpoint of individual species. To measure scale-dependent local diversity structures around individual species, we propose individual species–area relationships (ISAR), a spatial statistic that marries common species–area relationships with Ripley’s K to measure the expected alpha diversity in circular neighborhoods with variable radius around an arbitrary individual of a target species. We use ISAR to investigate if and at which spatial scales individual species increase in tropical forests’ local diversity (accumulators), decrease local diversity (repellers), or behave neutrally. Our analyses of data from Barro Colorado Island (Panama) and Sinharaja (Sri Lanka) reveal that individual species leave identifiable signatures on spatial diversity, but only on small spatial scales. Most species showed neutral behavior outside neighborhoods of 20 m. At short scales (<20 m), we observed, depending on the forest type, two strongly different roles of species: diversity repellers dominated at Barro Colorado Island and accumulators at Sinharaja. Nevertheless, we find that the two tropical forests lacked any key species structuring species diversity at larger scales, suggesting that ‘‘balanced’’ species–species interactions may be a characteristic of these speciesrich forests. We anticipate our analysis method will be a starting point for more powerful investigations of spatial structures in diversity to promote a better understanding of biodiversity in tropical forests.


Keywords: biodiversity, spatial patterns, spatial statistic, species–area relationship.


Animal habitat quality and ecosystem functioning: exploring seasonal patterns using NDVI.

Wiegand, T., J. Naves and M. Garbulsky, and N. Fernández. Ecological Monographs 78: 87-103

Many animal species have developed specific evolutionary adaptations to survive prolonged periods of low energy availability that characterize seasonal environments. The seasonal course of primary production, a major aspect of ecosystem functioning, should therefore be an important factor determining the habitat quality of such species. We tested this hypothesis by analyzing the relationship between habitat quality and ecosystem functioning for brown bears (Ursus arctos), a species showing hyperphagia and hibernation as evolutionary adaptation to seasonal peaks and bottlenecks in ecosystem productivity,respectively. Our unique long-term data set comprised data from two brown bear populations in northern Spain on historical presence, current presence, and reproduction. The data were classified on a grid of 5 x 5 km pixels into five classes: frequent reproduction, sporadic reproduction, frequent presence, sporadic presence, and recent extinction. We used the longterm average of the seasonal course of NDVI (normalized difference vegetation index) as a proxy for ecosystem functioning and investigated the relationship between habitat quality and ecosystem functioning with methods borrowed from statistical point-pattern analysis.

We found that brown bears indeed selected habitat with specific ecosystem functioning (i.e., the variance in all habitat classes was smaller than in the landscape overall) and the relationship between habitat quality and ecosystem functioning was ordered. First, the average distance in ecosystem functioning between two habitat classes was larger if the difference in habitat quality was larger. Second, habitat for which there was the greatest need (i.e., breeding habitat) occupied the narrowest niche regarding ecosystem functioning and showed the most pronounced seasonality. Progressively poorer classes occupied wider niches that partlyoverlapped those of better classes. This indicated that nonbreeding animals are less selective.

Our methodology provided new insight into the relationship between ecosystem functioning and habitat quality and could be widely applied to animal species living in seasonal environments. Because NDVI data are continuously collected, our methodology allows for continuous monitoring of changes in habitat quality due to global change.

Key words: brown bear; ecosystem functioning; endangered species; extinction; habitat quality; NDVI; northern Spain; point-pattern analysis; remote sensing; seasonality; Ursus arctos.



Grazing impacts in vegetated dune fields: predictions from spatial pattern analysis

Blanco, P.D., C. M. Rostagno, H. F. del Valle, A. M. Beeskow, and T. Wiegand. Rangeland Ecology and Management 61: 194-203.

This study deals with the changes induced by grazing on soil erosion processes in vegetated dune fields of Península Valdés, in the Patagonia region of Argentina. We performed a spatial analysis to assess erosion features’ patterns. Blowouts, used as main indicators of aeolian erosion processes, as well as dune crests, which are susceptible to erosion, were mapped on aerial photographs and images from Landsat 7 Enhanced Thematic Mapper Plus, in eight paddocks under two grazing conditions: lightly (0.4 sheep/ha) and heavily grazed (0.8 sheep/ha). From the mapped locations of water points, crests, and blowouts we calculated a spatial statistic (O-ring statistic), which gives the expected intensity of blowouts within the area covered by crests as function of distance away from water points. Additionally, to explore if the density of crests around water points influences the density of blowouts, we estimated the intensity of dune crests in the neighborhood of water points and compared the densities of blowouts among water points with low, medium, and high densities of crests. For the heavy grazing treatment we found highly significant (P<0.05) aggregation of blowouts around water points with peak densities threefold higher than expected at random occurring between 90 and 210 m. However, the aggregation was only weakly significant for the light grazing treatment and occurred only at distances of about 30 m away from the water point. We found that the impact of grazing on soil stability
contrasted among sites with different sensitivity to accelerated erosion. In sites with a high density of dune crests close to water points, wind erosion becomes more intense and the density of blowouts increases. A more sustainable management of these rangelands depends primarily on the conservation of the soils. Therefore, the location of water points in sites not susceptible to accelerated soil erosion would represent a better management strategy of the dune fields of Península Valdés.

Key words: blowouts, Patagonia, ring statistic, soil erosion, spatial analysis


Heterogeneity influences spatial patterns and demographics in forest stands

Getzin, S., T. Wiegand, K. Wiegand, and F. He. 2008. Journal of Ecology 96: 807-820


1 The spatial pattern of tree species retains signatures of factors and processes such as dispersal, available resource patches for establishment, competition, and demographics. Comparison of the spatial pattern of different size classes can thus help to reveal the importance and characteristics of the underlying processes. However, tree dynamics may be masked by large-scale heterogeneous site conditions, e.g., when the restricting size of regeneration sites superimposes emergent patterns.

2 Here we ask how environmental heterogeneity may influence the spatial dynamics of plant communities. We compared the spatial patterns and demographics of western hemlock in a homogeneous and a heterogeneous site of old-growth Douglas-fir forests on Vancouver Island using recent techniques of point pattern analysis. We used homogeneous and inhomogeneous K- and pair-correlation functions, and case-control studies to quantify the change in spatial distribution for different size classes of western hemlock.

3 Our comparative analyses show that biological processes interacted with spatial heterogeneity, leading to qualitatively different population dynamics at the two sites. Population structure, survival and size structure of western hemlock were different in the heterogeneous stand in such a way that compared to the homogeneous stand, seedlings were more clustered, seedling densities higher, seedling mortality lower, adult growth faster, and adult mortality higher. Under homogeneous site conditions, seedling survival was mainly abiotically determined by random arrival in small gaps with limiting light. At the heterogeneous site, seedling densities and initial survival were much higher leading to strong density-dependent mortality and selection for faster growing individuals in larger size classes. We hypothesise that the dynamics of the heterogeneous stand were faster due to asymmetric competition with disproportionate benefit to taller plants.

Synthesis Our study supports the hypothesis that successional dynamics are intensified in heterogeneous forest stands with strong spatial structures and outlines the importance of spatial heterogeneity as a determinant of plant population dynamics and pattern formation.


Key words: case-control, dispersal strategies, inhomogeneous pair-correlation function, large-scale heterogeneity, point pattern analysis, succession, western hemlock


Temporal and spatial differentiation in seedling emergence may promote species coexistence in Mediterranean fire-prone ecosystems

De Luis, M., J. Raventós, T. Wiegand, and J. C. González-Hidalgo  2008. Ecography 31: 620-629


Mediterranean ecosystems are hotspots of species richness where fire is one of the key processes influencing their structure, composition and function. Post-fire seedling emergence constitutes a crucial event in the life cycle of plants and species-specific temporal and spatial patterns of seedling emergence have been hypothesized to contribute to the high diversity in these ecosystems. Here we study the temporal and spatial patterns of seedling emergence observed for the four dominant species (Cistus albidus, Ulex parviflorus, Helianthemum marifolium, Ononis fruticosa) after an experimental fire in a Mediterranean gorse shrubland. In a first analysis we compared the timing of emergence of each species using the Kaplan-Meier method. The spatial component of seedling emergence and the spatiotemporal relationship between different cohorts of the same species were analyzed using recent techniques of spatial point pattern analyses. We found a bimodal temporal pattern of emergence. Emergence of Cistaceae species (H. marifolium and C. albidus) occurred predominantly early after the fire while Fabaceae (O. fruticosa and U. parviflorus) emerged mainly during the following autumn. Individually, all species showed an aggregated spatial pattern and, when testing for pair interactions, we found that the clusters of individual species were spatially segregated. Additionally, the clusters of individual species showed an internal spatial structure where seedlings of different cohorts were spatially segregated. Theoretical models predict that these patterns will promote species coexistence. We identified a number of mechanisms that all have the potential to contribute to the observed pattern formation. However, the potential interaction among these mechanisms are complex and not easy to predict. Our analyses take a significant step forward in studying seedling emergence in fire prone ecosystems since, to our knowledge, this is the first time that both spatial and temporal patterns of all dominant species have been studied together.


Key words:  Cistus albidus, Ulex parviflorus, Helianthemum marifolium, Ononis fruticosa, Fabaceae, Cistaceae, Mediterranean gorse shrubland, Kaplan-Meier method, O-ring statistics, point-pattern analysis, segregation hypothesis



 Long-term dynamics of a semiarid grass steppe under stochastic climate and different grazing regimes: A simulation analysis

Paruelo, J.M., S. Pütz , G. Weber, M. Bertiller, R.A. Golluscio, M. R. Aguiar, and T. Wiegand. 2008. Journal of Arid Environments 72: 2211-2231


We built a grid-based spatial explicit stochastic model that simulates grazing events and basic processes like seedling establishment, growth or mortality of the dominant species in the grass steppes of Patagonia. After evaluating the model with field data, we performed simulation experiments aimed to explore the interaction of precipitation and grazing regimes on vegetation dynamics. Grazing generated a reduction in tussock density which results in a decline in aboveground net primary production (ANPP). Both response variables presented a non-linear behavior including high temporal variability and delay effects, which may prolong for decades. There was a clear threshold in the response of the variables to stock density, though changes become evident only when a highly selective grazing scenario was used. Under high stock density conditions, precipitation use efficiency (PUE) was 82% lower than the values for non-grazed runs. The inter-annual variability of precipitation was more important than the grazing regime in explaining differences in tussock density. Simulation results highlight important issues regarding rangeland management: grazing regime might be as important as stocking density as a degradation agent, temporal lags might obscure degradation processes for decades, the definition of monitoring variables need to consider their response time constants.


Key words:  arid systems, ecosystem dynamics, grass steppes, herbivory, individual-based model, spatial explicit models


Individual movement behavior, matrix heterogeneity, and the dynamics of spatially structured populations

Revilla, E.. and T. Wiegand 2008.  PNAS 105:19120-19125.  


The dynamics of spatially structured populations is characterized by within- and between-patch processes. The available theory describes the latter with simple distance-dependent functions that
depend on landscape properties such as interpatch distance or patch size. Despite its potential role, we lack a good mechanistic understanding of how the movement of individuals between patches affects the dynamics of these populations. We used the theoretical framework provided by movement ecology to make a direct representation of the processes determining how individuals connect local populations in a spatially structured population of Iberian lynx. Interpatch processes depended on the heterogeneity
of the matrix where patches are embedded and the parameters defining individual movement behavior. They were also very sensitive to the dynamic demographic variables limiting the time moving, the within-patch dynamics of available settlement sites (both spatiotemporally heterogeneous) and the response of individuals to the perceived risk while moving. These contextdependent dynamic factors are an inherent part of the movement process, producing connectivities and dispersal kernels whose variability is affected by other demographic processes. Mechanistic representations of interpatch movements, such as the one provided by the movement-ecology framework, permit the dynamic interaction of birth–death processes and individual movement behavior, thus improving our understanding of stochastic spatially
structured populations..

Keywords: D
emography, Iberian lynx, metapopulation, population dynamics, source-sink


Small-scale spatial structure within patterns of seed dispersal

Cousens, R.D., Wiegand, T., and M. S. Taghizadeh. 2008.  Oecologia 158:437-48


A large proportion of dispersing propagules land near their maternal plant, even in species that have evolved structures which enhance dispersal. For these propagules, their post-dispersal spatial pattern is likely to reflect the overall shape and scale of the parental plant canopy and, especially in poorly dispersing species, aggregation of propagules on the plant prior to dispersal. Localised patterns within seed shadows are also likely to be affected by secondary movement after dispersal, leading to either more or less small-scale aggregation, depending on the mechanism. Our general aim was to study the smallscale spatial structure within patterns of seed dispersal of Raphanus raphanistrum L. to generate hypotheses about the sequence of processes and events leading to the spatial pattern of dispersal in this species. More specifically, we determined the sizes of small-scale structures within the seed shadows on the ground after dispersal and the extent to which these match the sizes of pre-dispersal aggregations within the parental canopy. Variation in plant size and shape was provided by four levels of inter-specific competition resulting from differing wheat crop densities. Positions of propagules were determined using a threedimensional digitizer, and the data for each plant were analysed using point pattern analysis. Not surprisingly, larger plants, growing at lower plant density, had larger seed shadows, showing an overall influence of maternal plant size. The pattern of propagules exhibited significant small-scale aggregates, with similar sizes on the plant and on the ground. There was no evidence that aggregation size was greater on the ground or increased with time, but the strength of the aggregation increased with length of time on the ground.

Keywords: architecture, spatial distribution, Raphanus raphanistrum


The impact of fire and density-dependent mortality on the spatial patterns of a pine forest in the Hulun Buir sandland, Inner Mongolia, China


Yu, H. T. Wiegand, X. Yang, and L. Ci. 2009. Forest Ecology and Management 257: 2098-2107


Wildland fire, especially surface fire, is one of the major disturbances in forest ecosystems such as the Mongolian pine forest of the Hulun Buir sandland. However, little is known about the impact of fire on the spatial patterns at the stand level and its consequences for successional dynamics. To fill this gap we mapped three plots of Mongolian pine forest comprising sites of the same age that were affected by fire in 2006 and 1994, and a site without fire. We explored the stand structure using diameter at breast height (DBH) distributions and basal area, and used point pattern analysis to quantify the observed spatial patterns. Null models included homogeneous Poisson and Thomas cluster processes for univariate analyses, toroidal shift to test for independence, and random labelling for analyzing mortality. Large stems showed at all three stands a tendency to regularity, whereas small stems were mostly clustered. However, small stems did not become more regular at plots with earlier fire, and only the “overstocked” 2006 pre-fire stand showed negative association between large and small trees. Mortality was non-random, showing clustering of dead (or surviving) trees and clear density dependence where stems with more neighbours had a higher risk to die. Fire killed mostly smaller trees and after prolonged fire-free periods the stand approached the pre-fire patterns structurally and spatially. Our study showed that surface fire causes strong thinning in later succession stages and pushes the forest into maturity. It may also enhance resistance of our forest to more severe fire under the relatively harsh environmental conditions. A novel test statistic for random labeling allowed directly testing for density dependent mortality and could be widely applied in point pattern analysis.

Keywords: disturbance, Hulun Buir sandland, Mongolian pine, pair correlation function, point pattern analysis, self-thinning, spatial pattern, stand, surface fire


Multi-generational analysis of spatial structure in the deceptive orchid Orchis mascula


Jacquemyn, H., Wiegand, T., Vandepitte, K., Brys, R., Roldán-Ruiz, I. and O. Honnay. 2009. Journal of Ecology 97:206–216


1. In long-lived, terrestrial orchids, strong aggregation of adults and recruits within populations and pronounced spatial association between recruits and adults can be expected when seed dispersal is limited, probabilities of seed germination decrease with increasing distance from mother plants and/or not all mother plants contribute to future generations. When individuals are distributed evenly across life-history stages, these processes can also be expected to result in a significant fine-scale spatial genetic structure in recruits that will persist into the adult-stage class.

2. We combined detailed spatial genetic and point pattern analyses across different generations with parentage analyses to elucidate the role of the diverse processes that might determine spatial structure in Orchis mascula.

3. Analyses of spatial point patterns showed a significant association between adults and recruits and similar clustering patterns for both. Weak, but highly significant spatial genetic structure was observed in adults and recruits, but no significant differences were observed across life stages, indicating that the spatial genetic structure present in recruits persists into the adult stage.

4. Parentage analyses highlighted relatively short seed dispersal distances (median offspring-recruitment distance: 1.55 and 1.70 m) and differential contribution of mother plants to future generations.

5. Persistence of fine-scale spatial genetic structure from seedlings into the adult stage class is consistent with the life history of O. mascula, whereas relatively large dispersal distances of both pollen and seeds compared to the fine-scale clustering of adults and seedlings suggest overlapping seed shadows and mixing of genotypes within populations as the major factors explaining the observed weak spatial genetic structure.

6. Nonetheless, comparison of the spatial association between recruits and adults with the genetic analysis of offspring-parent distances suggests that the tight clustering of recruits around adults was probably caused by decreasing probabilities of seed germination with increasing distance from mother plants.

7. Synthesis. This study shows that the approach presented here, which combines spatial genetic and spatial pattern analyses with parentage analyses, may be broadly applied to other plant species to elucidate the processes that determine spatial structure within their populations.

micro-environmental variation, parentage analysis, seed flow, spatial genetic structure, spatial point pattern analysis


Fire-mediated interactions between shrubs in a South American temperate savannah


Biganzoli, F., T. Wiegand and W.B. Batista. 2009.  Oikos 118: 1383-1395


We examined spatial patterns of fire-caused mortality and after-fire establishment of two dominant shrub species, Baccharis dracunculifolia and Eupatorium buniifolium in a humid temperate South American savannah. Our objective was to determine whether fires mediate in interactions between these two species. After a natural fire burned a large tract of savannah, we established two plots (respectively 550 and 500 m²) within which we mapped all surviving and dead shrubs as well as all individuals of shrub species that recruited in the following year. We used techniques of point-pattern analysis to test specific null hypotheses about spatial associations in the distribution, mortality, and establishment of shrubs. Results support the notions that fire mediates interactions between these two species. Fire-caused death of E. buniifolium tended to occur selectively in the vicinities of Baccharis individuals, and recruitment of B. dracunculifolia tended to be concentrated in the places of dead shrubs. These responses, however, were contingent on local abundances of shrubs which depend in part from the recent fire history. Anthropogenic perturbation of the natural fire regime would have therefore distorted the role of fire mediated interactions as drivers of the dynamics of the vegetation of this temperate savannah.

Baccharis dracunculifolia, Eupatorium buniifolium, fire, mortality, palm savannah, point pattern analysis, regeneration, shrubland, spatial pattern, topography


Recruitment in tropical tree species: revealing complex spatial pattern


Wiegand, T., Isabel Martínez, and Andreas Huth. 2009. The American Naturalist 174:  E106 - E140


Seed dispersal should leave a signature on the spatial distribution of recruits that can be quantified using sophisticated techniques of spatial pattern analysis. Here we study spatial patterns of five frugivore-dispersed tropical tree species at the Barro Colorado Island forest, Panama, to describe detailed properties of the spatial patterns of recruits and to investigate whether these patterns were produced by temporally consistent mechanisms. Our spatial point pattern analyses detected the existence of surprising spatial structures, such as double-cluster and superposition patterns, and they allowed for a detailed quantification of their properties. The spatial recruitment patterns were composed of two independent components comprising a random component and a component showing a complex spatial pattern with two critical scales of clustering. The analysis allowed an estimation of the relative contribution of scatter dispersal versus clump dispersal in effective seed dispersal for our study species. Additionally, the cluster characteristics were temporally consistent over 25 years and correlated with several species traits. We are just beginning to discover the richness of spatial patterns found at tropical forests, and we are confident that a combination of advanced point pattern analysis with field data will allow for significant advances in establishing the link between spatial patterns and processes.

Keywords: BCI tropical forest, dispersal, point pattern analysis, pair-correlation function, multiple clustering, spatial statistics


Spatial pattern of adult trees and the mammal-generated seed rain in the Iberian pear.


Fedriani, J.M. T. Wiegand, and M. Delibes. 2010. Ecography 33: 545-555


The degree to which plant individuals are aggregated or dispersed co-determines how a species uses resources, how it is used as a resource, and how it reproduces. Quantifying such spatial patterns, however, faces several methodological issues that can be overcame by using Spatial Point Pattern Analyses. We used SPPA to assess the distribution of P. bourgaeana adult trees and their seeds (within fecal samples) dispersed by three mammals (badger, fox, and wild boar) within a 72-ha plot across a range of spatial scales. Pyrus bourgaeana trees in our study plot (n = 75) were clearly aggregated with a critical spatial scale of about 25 m, and approximately nine randomly distributed tree clusters were identified. As expected from their marking behaviors, the spatial patterns of fecal deposition varied widely among mammal species. Whereas badger feces and dispersed seeds were clearly clustered at small spatial scales (<10 m), boar and fox feces were relatively scattered across the plot. A toroidal shift null model testing for independence indicated that boars tended to deliver seeds to the vicinity of adult trees and thus could contribute to the maintenance and enlargement of existing tree clusters. Badgers delivered feces and seeds in a highly clumped pattern but unlike boars, away from P. bourgaeana neighborhoods; thus, they are more likely to create new tree clusters than boars. The strong tree aggregation is likely to be the result of one or several non-exclusive processes, such as the spatial patterning of seed delivery by dispersers and seedling establishment beneath mother trees. In turn, the distinctive distribution of P. bourgaeana in Doñana appeared to interact with the foraging behavior of its mammalian seed dispersers, leading to neighbourhood-specific dispersal patterns and fruit-removal rates. Our study exemplifies how a detailed description of the properties of patterns generates testable hypotheses concerning the ecology of zoochorous. Pyrus bourgaeana dispersers were unique and complementary in their spatial patterning of seed delivery, which likely confers resilience to their overall service and suggests lack of redundancy and expendability of any one species.




Evidence for the spatial segregation hypothesis:  a test with nine-year survivorship data in a Mediterranean fire-prone shrubland


Raventós, J., T. Wiegand, and M. De Luis, M. 2010. Ecology  91:2110-2120


A current focus of ecology is the investigation of spatial effects on population and community dynamics; however, spatio-temporal theory remains largely untested by empirical observations or experimental studies. For example, the segregation hypothesis predicts that intraspecific aggregation should increase the importance of intraspecific competition relative to interspecific competition, thereby enhancing local coexistence in plant communities. We applied recent methods of point pattern analysis to analyze a unique long-term data set on fully mapped seedling emergence and subsequent survival in a Mediterranean gorse shrubland after experimental fires and simulated torrential rainfall events. Our overall aim was to test if the observed spatial patterns were consistent with the segregation hypothesis during the entire community dynamics from early seedling emergence to the establishment of a mature community, i.e., we explored if the observed initial segregation did indeed prevent interspecific competition from becoming dominant. We used random labelling as null model and specific test statistics to evaluate different biological effects of the spatial interactions that determine mortality.

We found that mortality was clearly not random. Comparison of the probability of mortality in dependence on the distance to conspecific and to heterospecific plants showed that mortality was controlled almost entirely by intraspecific interactions, which is consistent with the segregation hypothesis. Dead plants were aggregated and segregated from surviving plants, indicating two-sided scramble competition. Spatial interactions were density-dependent and changed their sign over the course of time from positive to negative when plants grew to maturity. The simulated torrential rainfall events and subsequent erosion caused non-specific mortality of seedlings, but did not reduce the prevalence of intraspecific competition. Our results provide support for the hypothesis that the spatial distribution of plants may profoundly affect competition and can be an important determinant in the coexistence of species and biodiversity.

Keywords: Facilitation, Mediterranean shrubland, fire, pair correlation function, scramble and contest competition, spatial point pattern analysis, random mortality hypothesis


Evaluating management interventions in small populations of the rare Primula vulgaris using spatio-temporal analyses of point patterns


Jacquemyn, H., P. Endels, O. Honnay,  and T. Wiegand, 2010 Journal of Applied Ecology 47: 431–440


1. In high-intensity agricultural landscapes, small landscape elements such as hedgerows, ditch banks, and rows of pollard trees may represent the last safeguard of many plant and animal species, some of them being rare or even threatened with extinction. However, due to their small size and low habitat quality, long-term population survival cannot be ascertained and often active management is needed to maintain viable populations of species forced to survive in these small landscape elements.

2. Population models are needed to assess the threats to species at risk and to evaluate alternative management actions. Here, we present a methodology to evaluate management interventions using spatio-temporal analyses of point patterns. We apply this method to populations of the rare primrose Primula vulgaris that occur along ditch bank habitats in Flanders, Belgium.

3. The effects of ditch bank clearing on the establishment success of seedlings was investigated by comparing spatial patterns of seedling recruitment, survival and mortality between populations that were grazed and populations that were severely disturbed by mechanical clearing of ditch banks followed by annual mowing. A total of 884 seedlings were mapped and monitored during 4 consecutive years (1999-2002).

4. In all populations, plants showed significant clustering, but in cleared sites only seedlings were significantly clustered around adults. Spatial patterns of mortality varied according to the management intervention. In grazed sites, mortality was almost random, whereas in cleared sites we found clear evidence for strong negative density-dependent mortality. There was no evidence that the presence of adults affected survival of recruits in any of the sites studied.

5. Synthesis and applications. This study shows that the analysis of spatial point patterns contributes to our understanding of the population dynamics of plant species occurring in different environments. The approach can be broadly applied to other plant species to elucidate the processes that determine the number of individuals that establish and persist into later life stages and will help conservation managers to refine management strategies intended to conserve or restore plant populations.

Keywords: competition, O-ring statistics, pair correlation function, point pattern analysis, Primula, random mortality hypothesis, recruitment


Species associations in an old-growth temperate forest in north-eastern China

Wang, X., T. Wiegand, Z. Hao, B. Li, J. Ye, and J. Zhang. 2010, Journal of Ecology 98: 674–686


1. Studying the spatial pattern of plants may provide significant insights into processes and mechanisms that maintain species richness. We used data from a fully mapped 25-ha temperate forest plot at Changbaishan (CBS), north-eastern China, to conduct a community-wide assessment of the type and frequency of intra- and interspecific spatial association patterns. We analysed complex scale effects in the patterning of large trees of 15 common species. First we tested for overall spatial patterning at 6-m, 30-m and 50-m neighbourhoods and classified the types of bivariate association patterns at these spatial scales (analysis 1). We then explored selectively small-scale (0-20 m) association patterns conditioning on the larger-scale pattern (analysis 2) and tested selectively for positive large-scale (50-250 m) association patterns (analysis 3).

2. Analysis 1 provided ample evidence for non-random spatial patterning, and the type and frequency of spatial association patterns changed with scale. Trees of most species pairs co-occurred less than expected by chance and positive associations were rare in local neighbourhoods. Analysis 2 revealed a separation of scales where significant small-scale interactions fade away at distances of 10-15 m. One third of all species pairs showed significant and mostly negative bivariate small-scale association, which occurred more often than expected by chance between species sharing attributes such as family, fruit type and habitat association. This suggests the occurrence of competitive interactions. Analysis 3 showed that only 8% of all species pairs co-occurred at large scales.

3. Comparison of our results with an analogous study conducted in the species-rich tropical forest at Sinharaja, Sri Lanka, revealed several structural similarities including the dominance of segregation and partial overlap in the overall patterning (analysis 1) and the separation of scales (analysis 2). However, species pairs at CBS showed considerably more significant negative small-scale associations (31% vs. 6% at Sinharaja).

4. Synthesis. The techniques presented here allow for a detailed analysis of the complex spatial association in species-rich forests and have the potential to reveal indicative patterns that may allow researchers to discriminate between competing hypotheses of community assemblage and dynamics. However, this will require comparative studies involving a large number of plots.

Key-words: Changbaishan (CBS), coexistence, pair correlation function, point pattern analysis, spatial segregation, species association, temperate forest



Spatial associations among tree species in a temperate forest community in North-western Spain

Martínez, I., T. Wiegand, F. Glez. Taboada, and J. R. Obeso. 2010. Forest Ecology and Management 260: 456-465


Analyzing spatial patterns in plant communities may provide insights in the importance of different processes for community assembly and dynamics. We applied techniques of spatial point pattern analysis to data from a fully mapped plot of a temperate forest community (Corylus avellana, Crataegus monogyna, Fagus sylvatica, Ilex aquifolium and Taxus baccata) in north-western Spain to conduct a community-wide assessment of the type and frequency of intra- and interspecific spatial association patterns. We first explored the overall intra- and inter-specific patterning, and then classified the types of association patterns at various neighbourhoods. By conditioning on the larger-scale pattern we then explored small-scale (0-15 m) intraspecific and interspecific patterns. Association patterns varied from strong positive association at small scales to, as a by-product, repulsion at intermediate scales. Surprisingly, there were no negative associations at small scales, but trees were arranged in multi-species clumps, up to 2.5 m in diameter and comprising a few individuals. Ilex and Corylus, the understorey species, were frequently involved in the clumps, showing positive small-scale association with the other tree species. Our analyses highlighted that animal mediated seed dispersal, interspecific facilitation and perturbation processes may operate successively to shape tree distributional patterns, although their relative importance vary among species. Given the complexity of the patterns described and the current threats to some of the species studied, directed experiments in the field are needed to further elucidate some of the hypotheses derived.. Spatial associations among tree species in a temperate forest community in North-western Spain. Forest Ecology and Management

Key-words: Fagus sylvatica, Fleshy-fruited trees, Nearest neighbour function, Pair-correlation function, Spatial point patterns, Temperate deciduous forest.



The relative effects of habitat loss and fragmentation on
population genetic variation in the red-cockaded
woodpecker (Picoides borealis)

Bruggeman, D. J., T. Wiegand, and N. Fernández. 2010. Molecular Ecology 19: 3679–3691


The relative influence of habitat loss, fragmentation and matrix heterogeneity on the viability of populations is a critical area of conservation research that remains unresolved. Using simulation modelling, we provide an analysis of the influence both patch size and patch isolation have on abundance, effective population size (Ne) and FST. An individual-based, spatially explicit population model based on 15 years of field work on the red-cockaded woodpecker (Picoides borealis) was applied to different landscape configurations. The variation in landscape patterns was summarized using spatial statistics based on O-ring statistics. By regressing demographic and genetics attributes that emerged across the landscape treatments against proportion of total habitat and O-ring statistics, we show that O-ring statistics provides an explicit link between population processes, habitat area, and critical thresholds of fragmentation that affect those processes. Spatial distances among land cover classes that affect biological processes translated into critical scales at which the measures of landscape structure correlated best with genetic indices. Therefore our study infers pattern from process, which contrasts with past studies of landscape genetics. We found that population genetic structure was more strongly affected by fragmentation than population size, which suggests that examining only population size may limit recognition of fragmentation effects that erode genetic variation. If effective population size is used to set recovery goals for endangered species, then habitat fragmentation effects may be sufficiently strong to prevent evaluation of recovery based on the ratio of census:effective population size alone.

Key-words: effective population size, fragmentation per se, FST, individual-based model, landscape
genetics, O-ring statistics, Picoides borealis, spatially explicit population model


Analyzing the spatial structure of Broughtonia cubensis (Orchidaceae) populations in the dry forests of Guanahacabibes, Cuba.

Raventós, J., E. Mujica, T. Wiegand, and A. Bonet. 2011. Biotropica 43: 173–182.


A current key issue in ecology is the role of spatial effects on population and community dynamics. In this paper, we tested several hypotheses related to spatial structures and coexistence of epiphytic tropical orchid species with special emphasis on the endemic species Broughtonia cubensis. More specifically, we explored the spatial structure of orchid-host plant communities at three different levels of organization (occupied vs. non occupied host trees, trees with B. cubensis vs. other orchids, and reproductive vs. non reproductive B. cubensis plants). We mapped all potential host trees and orchids at three 20 m × 20 m plots and applied techniques of spatial point pattern analysis such as mark connection and mark correlation functions to evaluate departures from randomized communities. We found spatial aggregation of trees with epiphytic orchids and segregation between trees with and without epiphytic orchids, and that there was an intraspecific spatial aggregation of B. cubensis in relation to the other seven epiphytic orchid species. Furthermore, we found spatial aggregation of reproductive B. cubensis individuals and segregation between reproductive and non-reproductive individuals on their phorophytes. Thus, orchid-host plant communities show hierarchical spatial structuring with aggregation and segregation at different levels of organization. Our results point to an enhancement of local species in the coexistence of tropical epiphytic orchid communities, by reducing competition through niche differentiation.


epiphyte ecology; mark correlation functions; orchid; point pattern analysis

Assessing spatiotemporal predator–prey patterns in heterogeneous habitats

Birkhofer, K., S. Scheu, and T. Wiegand.  2010.  Basic and Applied Ecology 11:486-494


Disentangling the contribution of biotic interactions (density-dependent) and environmental heterogeneity (density-independent) to the formation of spatial patterns between predators and prey is crucial for a better understanding of food-web interactions. Most techniques for the analysis of spatial patterns assume that abiotic processes influence the distribution of individuals with similar intensity at all locations of a study area (stationarity). This simplification may result in a spurious description of predator–prey associations in environmentally heterogeneous habitats. In a spatially explicit way we sampled ground-active linyphiid and lycosid spiders and their Collembola prey along a forest-meadow gradient and analysed the change in spatial relationships with time. We used techniques of point pattern analysis and pair-correlation functions to summarize spatial patterns. To disentangle the contribution of biotic interactions and environmental heterogeneity on pattern formation we compared observed functions with those arising from null models either assuming environmental homogeneity or accounting for habitat heterogeneity. All taxa were aggregated at the three sampling periods if habitat homogeneity was assumed, but only linyphiid spiders were still clustered after accounting for environmental heterogeneity. A similarly contrasting result was present for the spatial relationship between predators and their prey, with association under the assumption of homogeneity, but strong repulsion that intensified with time if accounting for environmental heterogeneity. Results from additional bivariate null models under which either predator or prey locations were fixed, suggest that Collembola showed lower activity density in more suitable, but predator-rich habitats. Biotic interactions were important drivers of the spatial distribution of ground-active predators and their decomposer prey in the analysed forest floor food-web. However, these structuring forces remain hidden when using simple spatial models that ignore environmental heterogeneity. Therefore, for understanding predator–prey interactions in spatially complex habitats, such as grasslands and forests, spatial models considering habitat heterogeneity are indispensible.


Araneae; Collembola; Density-dependence; Food-web; Habitat heterogeneity; Ideal free distribution; Intraguild interactions; Pair-correlation function; Spatial distribution; Spatial pattern


Size dominance regulates tree spacing more than competition within height classes in tropical Cameroon.

Getzin, S., M. Worbes, T. Wiegand and  K. Wiegand. 2011. Journal of Tropical Ecology 27: 93-102


Does competition prevail in large size classes of trees in tropical forests? This question is fundamental to our understanding of the demography and dynamics occurring in rain forests. We investigated this question based on an undisturbed late-secondary forest on a 1-ha plot in central Cameroon. Trees were stem-mapped and classified into three size classes: understorey, midstorey, and overstorey. The diameter at breast height and yearly biomass increment were determined as measures of plant growth and performance. Spatial statistics such as pair- and mark-correlation functions were used to detect scale-dependent patterns that could be caused by competition within and between the three size classes. The results revealed a random pattern and spatially uncorrelated measures of plant growth of overstorey trees. This suggests that competitive effects are of minor importance in the large size class of overstorey trees. Likewise, only weak evidence for competition between trees was found within the two lower size classes. However, negative distance correlations were found between the different size classes. We suggest that competition within height classes was relatively low due to the diversity of species with their variable niche differentiations and phenotypic plasticity that may compensate for competitive effects.


Cameroon, diversity, mark-correlation function, pair-correlation function, spatial patterns, tropical forest


Conserving pelagic habitats: seascape modelling of an oceanic top predator

Louzao,M. D. Pinaud, C. Peron, K. Delord, T. Wiegand, and H. Weimerskirch. 2011. Journal of Applied Ecology 48: 121–132

1. Currently pelagic ecosystems are changing significantly due to multiple threats. An important management policy is to establish Marine Protected Areas (MPAs), until now overlooked due to the difficulty of declaring ‘high seas’ protected areas, obtaining long-term distribution data on indicator species and the dynamic nature of these ecosystems.
2. Within this framework, we developed predictive habitat suitability models of an oceanic predator, the vulnerable wandering albatross Diomedea exulans, in the highly dynamic Southern Ocean. Based on a long-term tracking database (1998-2008), we estimated three quantitative ecological indices that complementarily describe the hierarchical habitat use of the species at multiple spatial scales: where the species (1) spent more time (the seascape, based on the time spent per area), (2) searched for prey (the foraging habitat, based on zones of increased foraging intensity using first passage time) and (3) fed (the feeding habitat, based on prey capture data).
3. Predictive habitat models reasonably matched the observed distribution patterns and described albatross multi-scale habitat use as a hierarchical arrangement: albatrosses foraged over topographic features in subtropical waters, nested within the wider seascape due to the constraint imposed by the colony effect, whereas feeding occurred nested over the continental shelf and seamounts in areas of low oceanographic variability within the Polar Frontal Zone.
4. Within the current oceanographic conditions, the location of key pelagic habitats for albatrosses breeding in the southern Indian Ocean encompassed certain topographic features such as pelagic areas surrounding main breeding sites, seamounts and submarine mountain ranges. The placement of these pelagic hotspots depends on the current sea surface temperature conditions.
5. Synthesis and applications. The present study provides two key conservation and management tools. First, we provide the first map to support the development of a prospective network of priority conservation zones across the southern Indian Ocean based on habitat predictions of an oceanic indicator species. This could be used not only to support conservation of top predators but also the underlying biodiversity associated with pelagic key habitats. Secondly, the developed habitat modelling procedure is widely applicable and could be used to track changes in species distribution in both marine and terrestrial environments within the current global change scenario.


First passage time, habitat modelling, indicator species, network of marine protected areas, prey capture data, Southern Ocean, time spent per unit area, wide-ranging predators, Wandering Albatross.

Assessing habitat suitability for tiger in the fragmented Terai Arc Landscape of India and Nepal

Kanagaraj, R., T. Wiegand, S. Kramer-Schadt, M. Anwar, and  S.P. Goyal. Ecography, 34: 970-981


Tiger (Panthera tigris) populations have declined dramatically in the Terai Arc Landscape (TAL; India and Nepal), and remaining populations are highly fragmented and endangered. As part of a research program to aid tiger management by identifying critical areas for conservation, we aimed to (i) identify the factors which affect the distribution of tigers in the TAL; (ii) explore the role of spatial scale in habitat selection; (iii) map potentially suitable habitats; and (iv) assess the quality of potential corridors linking suitable habitats. We used an approach based on presence and pseudo-absence data, combining ecological niche factor analysis and generalized linear models. We used an information-theoretic approach to compare our data on tiger presence with different hypotheses on tiger habitat selection (i.e., protective habitat, prey species, human disturbance), and spatial scales. All hypotheses yielded models with high prediction accuracy (>79%). The most parsimonious model included variables characterizing habitat suitability of the 2 main prey species. More detailed assessment of potentially suitable areas using an extended source-sink approach suggested that most of the habitats outside the protected areas were attractive sink-like habitats (i.e., they suffered high levels of human disturbance in otherwise good habitats). Overall, 24% (c.18,500 km²) of the study area was predicted as suitable (probability cut-off P > 0.5), approximately 7% of which is under protection. Our models showed that protecting the remaining concentrations of tigers requires focusing management efforts on specific areas outside the currently protected areas. These are characterized by good natural suitability; however, they suffer from a high level of human disturbance. Our models underscore the importance of minimizing human disturbances in these areas to avoid that they act as attractive sinks but act as corridors between existing subpopulations.


Elucidating demographic processes underlying tree line patterns: a novel approach to model selection for individual-based models using Bayesian methods and MCMC.  

Martínez, I., T. Wiegand, J. J. Camarero, E. Batllori, and E. Gutiérrez 2011. American Naturalist 177: E136-E152


Alpine tree-line ecotones are characterized by marked changes at small spatial scales that may result in a variety of physiognomies. A set of alternative individual-based models was tested with data from four contrasting Pinus uncinata ecotones in the central Spanish Pyrenees to reveal the minimal subset of processes required for tree-line formation. A Bayesian approach combined with Markov chain Monte Carlo methods was employed to obtain the posterior distribution of model parameters, allowing the use of model selection procedures. The main features of real tree lines emerged only in models considering nonlinear responses in individual rates of growth or mortality with respect to the altitudinal gradient. Variation in treeline physiognomy reflected mainly changes in the relative importance of these nonlinear responses, while other processes, such as dispersal limitation and facilitation, played a secondary role. Different nonlinear responses also determined the presence or absence of krummholz, in agreement with recent findings highlighting a different response of diffuse and abrupt or krummholz tree lines to climate change. The method presented here can be widely applied in individual-based simulation models and will turn model selection and evaluation in this type of models into a more transparent, effective,
and efficient exercise.


Keywords: forest limit, individual-based model, Pinus uncinata, Pyrenees,
spatially explicit population model.



Statistical inference for stochastic simulations models - theory and application  

Hartig, F. J. Calabrese, B. Reineking, T. Wiegand, and A. Huth. 2011. Ecology Letters 14: 816-827


Statistical models are the traditional choice to test scientific theories when observations, processes or boundary conditions are subject to stochasticity. Many important systems in ecology and biology, however, are difficult to capture with statistical models. Stochastic simulation models offer an alternative, but they were hitherto associated with a major disadvantage: their likelihood functions can usually not be calculated explicitly, and thus it is difficult to couple them to well-established statistical theory such as maximum likelihood and Bayesian statistics. A number of new methods, among them Approximate Bayesian Computing and Pattern-Oriented Modelling, bypass this limitation. These methods share three main principles: aggregation of simulated and observed data via summary statistics, likelihood approximation based on the summary statistics, and efficient sampling. We discuss principles as well as advantages and caveats of these methods, and demonstrate their potential for integrating stochastic simulation models into a unified framework for statistical modelling.

Keywords: Bayesian statistics, indirect inference, intractable likelihood, inverse modelling, likelihood approximation, likelihood-free inference, maximum likelihood, model selection, parameter estimation, stochastic simulation.



Analyzing the effect of stepping stones on target patch colonisation in structured landscapes for Eurasian lynx

Kramer-Schadt, S., Kaiser, T., K. Frank, and T. Wiegand. 2011. Landscape Ecology 26:501-513


With habitat loss and fragmentation having become two of the major threats to the viability of species, the question of how to manage landscapes for species conservation has attracted much attention. In this context, the planning of stepping stones has been proposed to increase connectivity in fragmented landscapes. We present a simulation study with a neutral landscape approach to assess the effects of steppingstones on colonization success. To that end, we used a spatially explicit, calibrated population model of the European lynx (Lynx lynx) coupled with structured landscapes, in which we could control the landscape parameters of dispersal habitat coverage and contagion, as well as the number and size of stepping stones available for breeding. In general, we found that colonization success increased with increasing habitat coverage but decreased with increasing habitat contagion, while the introduction of stepping stones had significant effects in critical situations. Especially at low to medium dispersal habitat coverage and high disperser mortality, stepping stones had a positive effect on colonization success when they were large enough to produce new dispersers, but negative effects when they were small and located in a way that dispersers would be distracted from more suitable breeding habitat patches. The latter clearly constituted a shading effect and argues for a thorough consideration of the trade-offs related to stepping stone size and location when implementing stepping stones as a conservation measure, especially when the number of individuals of conservation concern is low.

Keywords: Stepping stones, Connectivity, Habitat loss, Habitat fragmentation, Simulation model, Neutral landscapes, Eurasian lynx, Lynx lynx, Conservation, Management



Frugivore behavior determines plant distribution: a spatially-explicit analysis of a plant-disperser interaction.   

Rodríguez-Pérez, J., T. Wiegand, and L. Santamaria. 2012. Ecography 35: 113-123.


The spatial distributions of plants (and other primarily sessile organisms) depend on the interplay between their ecological requirements and the spatial template set before, during, and after the dispersal process. In the specific case of animal-dispersed seeds, the spatial characteristics of animal behaviour during the dispersal process are likely to leave a lasting imprint on plant distribution. Here, we hypothesize that the activity patterns of the frugivorous lizard Podarcis lilfordi directly influence the spatial distribution of the fleshy-fruited shrub Daphne rodriguezii. To evaluate this hypothesis, we first analysed lizard activity, following radio-tracked lizards during the plant’s fruiting period, and identified its main determinants at several spatial scales of habitat aggregation (from 12.5 to 150 m). We hypothesised that lizard activity depends on differential habitat features explaining its territory use plus habitat preferences associated with each movement bout. In a second step, the most important determinants of lizard activity plus the variables describing habitat structure were used to predict the presence of adult and juvenile plants. Predictability of lizard activity (based on AUC and Pearson regression coefficients) was higher at broad spatial-scales of habitat aggregation (75 m). The two best predictors of lizard activity were the habitat features of, and the distance to, the core area (defined as the area enclosing the 0.50 cumulative probability of lizard locations). Plant presence was best predicted by models based on a combination of lizard activity and habitat features at local spatial scales (1.5 m). Best models included habitat features and lizard activity for adult plants, and local-scale habitat features, the proximity of adult plants and lizard activity for juveniles. In both cases, most plants (50 – 60%) were located at “optimal sites” (both favourable for lizards and with adequate habitat features), whereas a small fraction of them (3-10 %) were located at dispersal-limited sites (i.e. with adequate habitat features but suboptimal for lizards). Our results thus suggest that the interplay between lizard activity and local habitat features determines the spatial patterns of juvenile-plant presence and leaves a lasting signature on adult-plant distribution.

Keywords: Habitat preference, dispersal limitation, spatial scales, seed shadow, seed dispersal


Marked point pattern analysis on genetic paternity data for uncertainty assessment of pollen dispersal kernels

Niggemann, M., T. Wiegand, J.J. Robledo-Arnuncio, and R. Bialozyt. 2012. Journal of  Ecology 100: 264-276


1. Obtaining accurate estimates of the pollen dispersal kernel is central to a wide range of ecological studies. Assessing their statistical uncertainty is as important, but rarely considered.
2. We developed a new method of marked point processes for nonparametric estimation of dispersal kernels based on data of genetic paternity analysis that does not require assumptions about the shape of the dispersal distribution. This allows for construction of Monte Carlo simulation envelopes of a given null model, such as random mating, and for uncertainty assessment of the observed dispersal kernel.
3. We applied our method to characterize spatial patterns of pollen flow in an isolated population of Populus nigra in Central Germany and to assess the associated statistical uncertainty in estimates of within-population dispersal kernels. We compared our nonparametric within-population kernel estimate to that of established methods of parametric kernel fitting including (i) a general mating model, (ii) a simplified mating model using categorical paternity data, and (iii) least-squares regression of the nonparametric kernel estimate.
4. Our analysis showed a significant departure from the random mating null model. We found a highly significant excess of mating events at short distances (< 400 m) and a weakly significant shortage of mating events at larger distances (1500 - 2000 m). Simulation envelopes of the null model were very wide at larger distances (> 2000 m), indicating large uncertainty on the detailed shape of the kernel’s tail.
5. Results of the point pattern analysis were consistent with kernel fits using published maximum-likelihood mating models. Model selection revealed that two-component pollen dispersal kernels were the most parsimonious functions.
6. Synthesis: Our approach of nonparametric kernel estimation could be widely applied for dispersal data from genetic paternity analysis and complements traditional kernel estimation by providing a nonparametric kernel estimate and effective methods for an uncertainty assessment in kernel estimation. Our results indicate that statistical model fitting may substantially underestimate the uncertainty in kernel estimation, especially at larger distances.


dispersal, gene flow, dispersal kernel, mark correlation function, paternity analysis, plant, point pattern analysis, Populus nigra, spatial statistics



Spatial patterns of tree species richness in two temperate forests

Wang, X., T. Wiegand, A. Wolf, R. Howe, S. Davies, and Z. Hao. 2011. Journal of  Ecology 99:1382-1393

1. The relative contribution of external vs. internal clustering mechanisms for determining community structure and its manifestations has been the subject of a continuous debate, but few attempts have been made to examine their single and joint effects in a compound process model.
2. In this study, we tested four a priori hypotheses on the relative importance of habitat heterogeneity (topography and soil) and internal clustering mechanisms such as dispersal limitation on the species–area relationship (SAR) in two fully mapped 25-ha plots of temperate forests in the Changbaishan (CBS) Nature Reserve, China, and the Chequamegon-Nicolet National Forest in Wisconsin, USA.
3. We used the distance decay curve to test the generality of the results obtained for the SAR. To find out if the relative importance of internal and external clustering mechanisms changed with life stage we conducted separate analyses for small, large, and all trees.
4. Model selection favoured the most complex hypothesis that assumed an influence of both habitat heterogeneity and internal clustering on SAR and the distance decay curve. For the CBS plot, which shows weak topographic structuring, models were consistent with data only if soil factors were included into assessment of habitat heterogeneity. At the Wabikon plot we could not test soil variables, but inclusion of topographic variables substantially improved the fit of the distance decay curve.
5. In general, the results of the SAR agreed with those of the distance decay curve, but the latter was sensitive to positive habitat-mediated species associations. The SAR, but not distance decay, distinguished among competing hypotheses for the community of large trees at CBS, where species exhibited only weak clustering.
6. Contrary to our expectations, we did not find substantial differences in the relative importance of internal and external clustering mechanisms with life stage.
7. Synthesis. Our analysis of spatial community structure for two relatively diverse temperate forests revealed that the factors governing spatial community structure may not substantially differ from those in highly diverse tropical forests. This result adds to our understanding of the ecological processes underlying the spatial diversity structure in natural forest communities.

Key-words: Aggregation, Changbaishan, determinants of plant community diversity and structure, habitat heterogeneity, Possion processes, Wabikon, temperate forest, Thomas processes



Tropical tree species assemblages in topographical habitats change in time and with life stage 

Kanagaraj, R., T. Wiegand, Liza S Comita and A. Huth.  2011.  Journal of  Ecology 99:1441-1452

1. Recent studies have documented shifts in habitat associations of single tropical tree species from one life stage to the next. However, the community-level consequences of such shifts have not been investigated, and it is not clear whether they would amplify, neutralize or completely alter habitat structuring during the transitions to the adult community.
2. We compared habitat-driven species assemblages at three life stages (i.e. recruitment, juvenile and reproductive stages) and six censuses for tree and shrub species in a fully censused 50-ha plot of Panamanian lowland forest. Habitat types were determined using multivariate regression trees that group areas with similar species composition (i.e. species assemblages) according to their topographical characteristics.
3. Three topographical variables (a topographical wetness index, slope and elevation) were major determinants of species assemblages. When analysing individuals of all life stages together, we found a distinct and temporally consistent structuring of the plot into four dominant habitat types (low and high plateaus, slope and swamp) which was consistent with previous classifications. Basically, the same habitat structuring emerged for the juvenile communities of individual censuses.
However, recruits showed a weak and temporally inconsistent habitat structuring.
4. A notable homogenization in species assemblages occurred during the transition from juvenile to reproductive, through both a reduction in the number of species assemblages (in 3 censuses, one large reproductive assemblage covered 93% of the plot, and in others, an additional slope habitat emerged) and a reduction in the classification error. Overall, habitat structuring became noisier and weaker over the 25 years of the study.
5. Synthesis. Our results suggest that mortality processes during the transition from recruits to juveniles must enhance the signal of habitat structuring. However, during the transition to the reproductive stage, species may have lost the advantage of being in the habitat with which they had become associated, or the quality of habitat changed during their life span because of larger climatic changes. The homogeneous assemblages of the reproductive stage could be interpreted as support for neutral theories, but further research is required to unravel the mechanisms behind these intriguing observations.

Key-words: environmental heterogeneity, habitat preference, multivariate regression tree
analysis, plant population and community dynamics, regeneration niche, species assemblages,
topography, tropical forest diversity



Individual species-area relationships and spatial patterns of species diversity in a Great Basin, semi-arid shrubland 

Rayburn, A.P. and  T. Wiegand 2012.  Ecography 35:341-347

Traditional biodiversity metrics operate at the level of a plant community but do not capture spatial variation in diversity from a ‘plant’s-eye view’ of a community. Recently-developed statistics consider the spatial patterns of plants as well as the number and distribution of species in local plant neighborhoods to quantitatively assess multispecies spatial patterns from a ‘plant’s-eye view’. We used one such statistic, the individual species-area relationship (ISAR), to assess spatial patterns of species diversity in a Great Basin (USA) semi-arid shrubland through an analysis of a spatial dataset on shrub species and locations. In conjunction with appropriate null models, the ISAR blends species-area relationships with second-order spatial statistics to measure the expected species richness in local neighborhoods of variable size around the individuals of a focal species within a community. We found that, contrary to a previous analysis using more traditional methods, the community was well-mixed with a typical shrub surrounded on average by 4.9 shrub neighbors of 2.1 species at a neighborhood scale of 1.0 m. We also found statistically significant fine-scale variation in diversity patterns, such that neighborhoods of two species were more diverse than expected by a heterogeneous Poisson null model that accounted for larger-scale habitat heterogeneity. However, this effect was caused by intraspecific aggregation of these species and was not due to positive interspecific association. Contrary to previous findings in other semi-arid shrublands, our analysis suggests that the spatial pattern of the shrub community was not significantly structured by interspecific facilitation. This result supports growing evidence for balanced species patterns of adult plants in multi species communities. Our approach may be used in other communities to describe complex multispecies spatial patterns, quantify species-specific associations with diversity patterns, and to generate hypotheses regarding relationships between patterns and community-structuring processes.

Key-words: spatial species diversity, ISAR, plant spatial patterns, semi-arid shrublands


Non-random distribution of orchids reflects different mycorrhizal association patterns in a hybrid zone of three Orchis species

Jacquemyn, H., R Brys, O. Honnay, I. Roldán-Ruiz, B. Lievens, and T. Wiegand. 2012. New Phytologist 193: 454-464


  • Nonrandom species–species associations may arise from a range of factors, including localized dispersal, intra- and interspecific interactions and heterogeneous environmental conditions. Because seed germination and establishment in orchids are critically dependent upon the availability of suitable mycorrhizal fungi, species–species associations in orchids may reflect associations with mycorrhizal fungi.

  • To test this hypothesis, we examined spatial association patterns, mycorrhizal associations and germination success in a hybrid zone containing three species of the genus Orchis (Orchis anthropophora, Orchis militaris and Orchis purpurea).

  • Hybridization occurred predominantly between O. purpurea and O. militaris. The spatial distribution patterns of most pure species and hybrids were independent from each other, except that of O. purpurea and its hybrids. The fungal community composition of established individuals differed significantly between pure species, but not between hybrids and O. purpurea. Seed germination experiments using pure seeds showed that the highest number of protocorms were found in regions where adult individuals were most abundant. In the case of hybrid seeds, germination was restricted to areas where the mother plant was most abundant.

  • Overall, these results suggest that the observed nonrandom spatial distribution of both pure and hybrid plants is dependent on the contingencies of the spatial distribution of suitable mycorrhizal fungi.

Key-words: hybridization;mycorrhizal fungi;Orchidaceae;seed germination;spatial point pattern analysis


A resprouter herb reduces negative density-dependent effects among neighboring seeders after fire

Raventós, J., T. Wiegand, F. T. Maestre, and M. De Luis. 2012. Acta Oecologica 38: 17-23


Plant communities are often composed of species belonging to different functional groups, but relatively few studies to date have explicitly linked their spatial structure to the outcome of the interaction among them. We investigated if mortality of seeder species during their establishment after fire is influenced by the proximity of the resprouter herb Brachypodium retusum. The study was conducted in a Mediterranean shrubland (00 390 W; 38 430 N), 40 km northwest of Alicante (Spain) with Ulex parviflorus, Cistus albidus, Helianthemum marifolium, and Ononis fruticosa as dominant obligate seeder species and a herbaceous layer is dominated by the resprouter B. retusum. We followed the fate of mapped seedlings and the biomass of B. retusum one, two, three and nine years after an experimental fire. We used point pattern analyses to evaluate the spatial pattern of mortality of seeder species at these years in relation to the biomass of B. retusum. We hypothesize that B. retusum may initially have a positive impact on seeder survival. We implemented this hypothesis as a point process model that maintains the overall number of dead seeder plants, but seeder survival varied proportionally to the biomass of B. retusum in its neighborhood. We then contrasted this hypothesis with a previous analysis based on a random mortality hypothesis. Our data were consistent with the hypothesis that proximity of B. retusum reduced the mortality of seeder plants at their establishment phase (i.e., 2 yrs after fire). However, we found no evidence that B. retusum influenced seeder mortality when plants grow to maturity. We also found that, under the more stressful conditions (fireþ erosion scenario), B. retusum had a lower impact on the performance of seeder species. Our results suggest that B. retusum may reduce negative density dependent effects among neighboring seeder plants during the first years after fire.


Key-words: Functional groups, Pair correlation functions, Uni- and bivariate spatial, point pattern analysis, Rainfall, Erosion, Survivorship, Facilitation, Competition




Understanding the long-term spatial dynamics of a semiarid grass-shrub steppe through inverse parameterization for simulation models

Cipriotti, P.A., M.R.Aguiar, T, Wiegand, and J. M. Paruelo. Oikos 121: 848- 861


Desertification threatens 70% of all dry lands worldwide by diminishing the provision of economic and ecosystem services. However, since long-term vegetation dynamics of semiarid ecosystems are difficult to study, the opportunities to evaluate desertification and degradation properly are limited. In this study, we tailored, calibrated and tested a spatially-explicit simulation model (DINVEG) to describe the long-term dynamics of dominant grass and shrub species in the semiarid Patagonian steppe. We used inverse techniques to identify parameterizations that yield model outputs in agreement with detailed field data, and we performed sensitivity analyses to reveal the main drivers of long-term vegetation dynamics. Whereas many parameterizations (10–45%) matched single field observations (e.g. grass and shrub cover, species-specific density, aboveground net primary production [ANPP]), only a few parameterizations (0.05%) yielded simultaneous match of all field observations. Sensitivity analysis pointed to demographic constraints for shrubs and grasses in the emergence and recruitment phase, respectively, which contributed to balanced shrub-grass abundances in the long run. Vegetation dynamics of simulations that matched all field observations were characterized by a stochastic equilibrium. The soil water content in the top layer (0–10 cm) during the emergence period was the strongest predictor of shrub densities and population growth rates and of growth rates of grasses. Grasses controlled the shrub demography because of the resource overlap of grasses with juvenile shrubs (i.e. water content in the top layer). In agreement with field observations, ecosystem function buffered the strong variability in precipitation (a simulated CV in ANPP of 16% vs CV in precipitation of 33%). Our results show that seedling emergence and recruitment are critical processes for long-term vegetation dynamics in this steppe. The methods presented here could be widely applied when data for direct parameterization of individual-based models are lacking, but data corresponding to model outputs are available. Our modeling methodology can reduce the need for long-term data sets when answering questions regarding community dynamics.


Dispersal limitation and spatial scale affect model based projections of Pinus uncinata response to climate change in the Pyrenees

Martínez, I., F. González-Taboada, T. Wiegand, J. J. Camarero, and E. Gutiérrez. 2012. Global Change Biology 18: 1714–1724


Species Distribution Models (SDMs) were employed to assess the potential impact of climate change on the distribution of Pinus uncinata in the Pyrenees, where it is the dominant tree species in subalpine forest and alpine tree lines. Predicting forest response to climate change is a challenging task in mountain regions but also a conservation priority. We examined the potential impact of spatial scale on SDM projections by conducting all analyses at four spatial resolutions. We further examined the potential effect of dispersal constraints by applying a threshold distance of maximal advancement derived from a spatially explicit, individual-based simulation model of tree line dynamics. Under current conditions, SDMs including climatic factors related to stress or growth limitation performed best. These models were then employed to project P. uncinata distribution under two emission scenarios, using data generated from several regional climate models. At the end of this century, P. uncinata is expected to migrate northward and upward, occupying habitat currently inhabited by alpine plant species. However, consideration of dispersal limitation and/or changing the spatial resolution of the analysis modified the assessment of climate change impact on mountain ecosystems, especially in the case of estimates of colonization and extinction at the regional scale. Our study highlights the need to improve the characterization of biological processes within SDMs, as well as to consider simultaneously different scales when assessing potential habitat loss under future climate conditions.

Key-words: climate change, dispersal limitation, Pinus uncinata, Pyrenees, Species Distribution Models (SDMs).


Testing the independent species’ arrangement assertion made by theories of stochastic geometry of biodiversity
Wiegand, T., A. Huth,  S. Getzin, X. Wang,  Z. Hao,  S. Gunatilleke, and N. Gunatilleke.  Proceedings B  279: 3312-3320

The assertion that the spatial location of different species is independent of each other is fundamental in major ecological theories such as neutral theory that describes a stochastic geometry of biodiversity. However, this assertion has rarely been tested. Here we use techniques of spatial point pattern analysis to conduct a comprehensive test of the independence assertion by analysing data from three large forest plots with different species richness: a species-rich tropical forest at Barro Colorado Island (Panama), a tropical forest in Sinharaja (Sri Lanka), and a temperate forest in Changbaishan (China). We hypothesize that stochastic dilution effects owing to increasing species richness overpower signals of species associations, thereby yielding approximate species independence. Indeed, the proportion of species pairs showing: (i) no significant interspecific association increased with species richness, (ii) segregation decreased with species richness, and (iii) small-scale interspecific interaction decreased with species richness. This suggests that independence may indeed be a good approximation in the limit of very species-rich communities. Our findings are a step towards a better understanding of factors governing species-rich communities and we propose a hypothesis to explain why species placement in species-rich communities approximates independence.

Key-words: environmental heterogeneity, forests, neutral theory, point pattern analysis, spatial pattern, species interactions

Spatial variation in below-ground seed germination and divergent mycorrhizal associations correlate with spatial segregation of three co-occurring orchid species

Jacquemyn, H., R Brys,  B. Lievens, and T. Wiegand. 2012. Journal of Ecology 100: 1328-1337


1. Understanding the many factors that affect community composition and coexistence of species
in natural environments is one of the central goals in ecology. As germination and establishment of
seedlings in orchids are crucially dependent on mycorrhizal availability, the diversity and spatial distribution of orchid mycorrhizal fungi are likely factors that contribute to orchid coexistence.
2. In this study, we combined molecular identification techniques with seed germination experiments
and spatial point pattern analyses to investigate to what extent differences in mycorrhizal
association patterns affected spatial variation in seed germination and the above-ground distribution
of three co-occurring terrestrial orchid species (Anacamptis morio, Gymnadenia conopsea and
Orchis mascula).
3. The three species associated with a different set of mycorrhizal fungi, except A. morio and G. conopsea, which shared one fungal associate. The number of fungal lineages detected associating with
each species also differed between species, being highest in G. conopsea, which associated with five
lineages, and restricted to a single lineage in O. mascula.
4. Seed germination experiments showed that below-ground seed germination was restricted to
locations where orchid individuals were most abundant, and quickly declined with increasing distance
from the nearest above-ground congener. Spatial point pattern analyses indicated significant
fine-scale spatial clustering that was highest in O. mascula and lowest in G. conopsea.Moreover, the
spatial distributions of the three species were independent from each other, except for A. morio and
G. conopsea.
5. Synthesis. Our results support previous findings that co-occurring orchid species tend to use different
mycorrhizal partners. Although the possibility that variation in local environmental conditions
affected seed germination could not be completely ruled out, our results suggest that the
presence of specific mycorrhizal fungi contributed, at least partly, to the spatial distribution and
coexistence of the investigated orchid species.

Key-words: abundance, determinants of plant community diversity and structure, germination,
mycorrhiza, Orchidaceae, point pattern analysis, species interactions


Foraging in a changing environment: habitat shifts of an oceanic predator over the last half century

Louzao, M; O. Aumont,  T. Hothorn,  T. Wiegand, and H. Weimerskirch. 2013  Ecography 36: 57-67.


Spatial modelling studies stress the importance of predicting future species distribution in changing environments, but it is also important to establish historical distribution ranges of species to provide baseline conditions for understanding distribution shifts. We focused on pelagic ecosystems, the largest ecosystem on Earth. Based on boosting algorithms, we reconstructed the foraging patterns of an oceanic predator, wandering albatross Diomedea exulans, in the highly dynamic Southern Ocean over the last half century. To access the unobserved past oceanographic conditions, we used simulations of the OPA-PISCES oceanic model for the 1958–2001 period. Firstly, we validated the simulated oceanographic variables (sea surface temperature and height, wind speed and chlorophyll a) for the 1998–2001 period with remotely sensed oceanographic data, which were highly correlated, except chlorophyll a. Secondly, we developed two habitat models (based on simulated and observed oceanographic variables) describing the foraging probability of albatrosses. We detected no statistically significant differences between the two models and predictions of both models matched the observed distribution patterns reasonably. Finally, we projected the most likely historical key pelagic habitats of albatross for the 1958 to 2001 period and characterised recurrent, occasional and unfavourable foraging areas in a decadal basis based on average predictions and their standard deviations. Our findings 1) provided a historical baseline (1958–1968) of recurrent, occasional and unfavourable foraging habitats, 2) evidenced a progressive habitat shift the following decades driven by a propagation of sea surface height from SE South Africa towards Antarctica from 1958 to 2001 and 3) measured habitat change rates of wandering albatross over the last half century. To our knowledge, our study provides the first quantitative long-term assessment of the spatial response of a marine top predator to changing pelagic habitats of the Southern Ocean and highlighted the oceanographic mechanisms involved, offering new insights on future effects of climate change on the pelagic realm


Palms, peccaries and perturbations: widespread effects of small-scale disturbance in tropical forests

Queenborough, S.A., M.R. Metz, T. Wiegand, and R. Valencia. 2012. BMC Ecology 12:3. 



Background: Disturbance is an important process structuring ecosystems worldwide and has long been thought to be a significant driver of diversity and dynamics. In forests, most studies of disturbance have focused on large scale disturbance such as hurricanes or tree-falls. However, smaller sub-canopy disturbances could also have significant impacts on community structure. One such sub-canopy disturbance in tropical forests is abscising leaves of large arborescent palm (Arececeae) trees. These leaves can weigh up to 15 kg and cause physical damage and mortality to juvenile plants. Previous studies examining this question suffered from the use of static data at small spatial scales. Here we use data from a large permanent forest plot combined with dynamic data on the survival and growth of > 66,000 individuals over a seven-year period to address whether falling palm fronds do impact neighboring seedling and sapling communities, or whether there is an interaction between the palms and peccaries rooting for fallen palm fruit in the same area as falling leaves. We tested the wider generalisation of these hypotheses by comparing seedling and sapling survival under fruiting and non-fruiting trees in another family, the Myristicaceae.

Results: We found a spatially-restricted but significant effect of large arborescent fruiting palms on the spatial structure, population dynamics and species diversity of neighbouring sapling and seedling communities. However, these effects were not found around slightly smaller non-fruiting palm trees, suggesting it is seed predators such as peccaries rather than falling leaves that impact on the communities around palm trees. Conversely, this hypothesis was not supported in data from other edible species, such as those in the family Myristicaceae.

Conclusions: Given the abundance of arborescent palm trees in Amazonian forests, it is reasonable to conclude that their presence does have a significant, if spatially-restricted, impact on juvenile plants, most likely on the survival and growth of seedlings and saplings damaged by foraging peccaries. Given the abundance of fruit produced by each palm, the widespread effects of these small-scale disturbances appear, over long time-scales, to cause directional changes in community structure at larger scales.


Individual-based movement models reveals sex-biased effects of landscape fragmentation on animal movement

Anadón, J.D., Wiegand, T. and A. Giménez.2012.. Ecosphere 3: 64. 



Animal movement and behavior may respond to habitat modification or fragmentation in non trivial ways, thereby strongly conditioning the fate of populations. This study aims to understand movement patterns of non-dispersal animals in both natural and altered landscapes, using the endangered terrestrial tortoise Testudo graeca as example. We used individual-based simulation models representing competing hypotheses on tortoise movement. Model parameterization and selection was based on radiotracking data and an inverse approach that is able to deal with observation uncertainty, individual variability, and process stochasticity. We find that land use intensification had a strong impact on the movement and behavior of non-dispersing individuals of T. graeca. In natural landscapes, males and females showed a similar movement and behavior profile with a strong home behavior component, and little individual variability. However, in altered landscapes, movement and behavior greatly varied among individuals, particularly in females, and males and females showed different movement patterns. Females showed a wide range of movement patterns, from strong home behavior to an unbounded movement. Our study shows that population or movement models that assume single behavioral states for animals inhabiting different landscape structures can be strongly misleading and, furthermore, that the impact of landscape modification on movement and behavioral patterns can be strongly sex-biased. Flexible, individual-based movement models coupled with inverse parameterization and model selection approaches proved useful in understanding the mechanisms controlling animal movement patterns.

Key words: animal movement, fragmentation, individual-based model, landscape, model parameterization, model selection, random walk, spur-thighed tortoise, stochastic simulation model, Testudo graeca



Adult proximity and frugivore's activity structure the spatial pattern in an endangered plant

Rodríguez-Pérez, J., T. Wiegand, and A. Traveset. 2012. Functional Ecology  26:1221–1229



1. Seed dispersers play a key role in shaping the spatial patterns of plant populations. After their disappearance, we expect a substantial increase in plant aggregation, which can ultimately cascade into high plant competition.
2. We used data of fully mapped distribution of four populations of the shrub Daphne rodriguezii from Menorca Island (Balearic Islands, W Mediterranean Sea), one in which D. rodriguezii coexists with its only disperser, the lizard Podarcis lilfordi, and three populations where this lizard went extinct long ago.
3. We used spatial point-pattern analysis to better understand the lizard's role on the spatial distribution of D. rodriguezii. To this end, we used specific point-process models that represented our main hypotheses on the impact of (i) habitat suitability, (ii) disperser activity, (iii) proximity of adults on the distribution of juvenile and adult shrubs and (iv) the impact of adult proximity on juvenile survival.
4. Plants were aggregated in all populations, and aggregation at short distances was stronger in populations without lizards. The observed spatial pattern of juveniles was better explained by a combination of hypotheses (ii) and (iii). The density of juveniles below adults was similar in all populations, but density far from adults was higher in the population with dispersers. In populations without dispersers, juveniles suffered higher mortality below adults.
5. Overall, our results suggest that the effects of lizard extinction were subtle. In the absence of lizards' dispersal, plants self-replace in favourable locations, but a decline in density, is evidenced near adults attributed to higher competition. Lizards, which often defecate under fruiting plants, create also a tight juvenile–adult association, but allow juveniles to escape from the immediate proximity of adults attributed to longer-distance movements. Our study shows how the analysis of the spatial pattern of plants can leads to detailed hypotheses on the underlying mechanisms structuring plant populations.
Key words: habitat suitability;heterogeneous Poisson process;point-pattern analysis;seed dispersal;spatial distribution;univariate and bivariate cluster process


A systematic comparison of summary characteristics for quantifying point patterns in ecology

Wiegand, T, F. He, and S.P. Hubbell. 2013. Ecography. 36: 92-103



Many functional summary characteristics such as Ripley's K function have been used in ecology to describe the spatial structure of point patterns to aid understanding of the underlying processes. However, their use is poorly guided in ecology because little is understood how well single summary characteristics, or a combination of them, capture the spatial structure of real world patterns. Here, we systematically tested the performance of combinations of eight summary characteristics [i.e. pair correlation function g(r), K-function K(r), the proportion E(r) of points with no neighbor at distance r, the nearest neighbor distribution function D(r), the spherical contact distribution Hs(r), the kth nearest-neighbor distribution functions Dk(r), the mean distance nn(k) to the kth neighbor, and the intensity function λ(x)]. To this end we used point pattern data covering a wide range of spatial structures including simulated (stationary) as well as real, possibly non-stationary, patterns on tree species in a tropical forest in Panamá. To measure the information contained in a given combination of summary characteristics we used simulated annealing to reconstruct the observed patterns based only on the limited information provided by this combination and assessed how well other characteristics of the observed pattern were recovered.

We found that the number of summary characteristics required to capture the spatial structure of stationary patterns varied between one (for patterns with near random structures) and three (for patterns with complex cluster and superposition structures), but with a robust ranking g(r), Dk(r), and Hs(r) that was largely independent on pattern idiosyncrasies. Stationary summary characteristics [with ranking g(r), Dk(r), Hs(r), E(r)] captured small- to intermediate scale properties of non-stationary patterns, but for describing large-scale spatial structures the intensity function was required. Our finding revealed that the current practice in ecology of using only one or two summary characteristics bears danger that essential characteristics of more complex patterns would not be detected. The technique of pattern reconstruction presented here has wide applications in ecology.


Spatial distribution and interspecific associations of tree
species in a tropical seasonal rain forest of China

Lan, G., S. Getzin, T. Wiegand, Y. Hu, H. Zhu,G. Xie, M. Cao.2012. PLoS ONE 7(9): e46074.



Studying the spatial pattern and interspecific associations of plant species may provide valuable insights into processes and mechanisms that maintain species coexistence. Point pattern analysis was used to analyze the spatial distribution patterns of twenty dominant tree species, their interspecific spatial associations and changes across life stages in a 20-ha permanent plot of seasonal tropical rainforest in Xishuangbanna, China, to test mechanisms maintaining species coexistence. Torustranslation tests were used to quantify positive or negative associations of the species to topographic habitats. The results showed: (1) fourteen of the twenty tree species were negatively (or positively) associated with one or two of the topographic variables, which evidences that the niche contributes to the spatial pattern of these species. (2) Most saplings of the study species showed a significantly clumped distribution at small scales (0–10 m) which was lost at larger scales (10– 30 m). (3) The degree of spatial clumping deceases from saplings, to poles, to adults indicates that density-dependent mortality of the offspring is ubiquitous in species. (4) It is notable that a high number of positive small-scale interactions were found among the twenty species. For saplings, 42.6% of all combinations of species pairs showed positive associations at neighborhood scales up to five meters, but only 38.4% were negative. For poles and adults, positive associations at these distances still made up 45.5% and 29.5%, respectively. In conclusion, there is considerable evidence for the presence of
positive interactions among the tree species, which suggests that species herd protection may occur in our plot. In addition, niche assembly and limited dispersal (likely) contribute to the spatial patterns of tree species in the tropical seasonal rain forest in Xishuangbanna, China.


Disturbance-dependent spatial distribution of sexes in a gynodioecious understory shrub
Castilla, A.R. T. Wiegand, C. Alonso, and C.M. Herrera. 2012.  Basic and Applied Ecology 13: 405–413



Several ecological conditions and processes occurring naturally in plant populations may lead to spatial aggregation of sexes within populations of sexually polymorphic species. In addition, ecological disturbances such as forest management or fire could also affect the spatial distribution of sexes within populations. Spatial aggregation of sexes can have important consequences for the fitness of the individuals in sex-biased patches through increased pollen limitation and/or variation in the male fitness
of hermaphrodites. Therefore, spatial aggregation of sexes could be relevant for the maintenance of the sexual polymorphism in plant species. Here, we used point pattern analysis to study the spatial distribution of female and hermaphrodite individuals within a single population of the gynodioecious understory shrub Daphne laureola, inhabiting a young pine reforestation and the adjacent natural undisturbed area. In the undisturbed area, we found that hermaphrodites were distributed randomly whereas females were spatially aggregated at short distances. Such aggregation could result from narrower ecological amplitude, cytonuclear determination of sex and limited dispersal and/or increased cloning in females. In the reforested area, females were slightly more abundant and the two sexes were randomly distributed among all plants. Recolonization processes after certain forest disturbances could thus favour the establishment of females and alter the spatial distribution of sexes in this understory



Effects of topography on structuring local species assemblages in a Sri Lankan mixed dipterocarp forest

Punchi-Manage, R., S. Getzin, T. Wiegand, R. Kanagaraj, C.V.S. Gunatilleke, I.A.U. N. Gunatilleke, K. Wiegand, and A. Huth.  2013. Journal of Ecology 101: 149–160


1. One of the primary goals in community ecology is to determine the relative importance of processes and mechanisms that control biodiversity. Here, we examined habitat-driven species assemblages and species distribution patterns as well as their temporal variations for three life stages of two censuses of a 25-ha mixed dipterocarp forest at Sinharaja (Sri Lanka).
2. Our general objective was to find out whether the species assemblages and associated habitat types changed with life stage, spatial scale and species attributes. We also analyse whether the habitat types were related to certain indicator species. Habitat types were determined with multivariate regression tree analyses driven by topographic variables.
3. We found species assemblages associated with five distinct habitat types that appeared consistently for all life stages of the two censuses. These habitats were related to ridge-valley gradients and a pronounced contrast in south-west versus north-east aspect. Habitat-driven structuring was weak at the recruit stage but strong in the juvenile and adult stages. The species assemblage variance explained by topographic variables for different life stages ranged between 10% for recruits and 23% for juveniles.
4. The species assemblages determined for different spatial scales (10, 20, 50 m) showed similar habitat partitioning, but the variance explained by the topographic variables increased in all life stages with spatial scale. This could be due to the homogenizing effect of topographic variables at the larger scales and unaccounted environmental variation at the smaller scales. The number of indicator
species identified in the two censuses was higher in the juvenile stage than in the adult stage,and nearly all indicator species in the adult stage were also indicator species in the juvenile stage.
5. Synthesis. Our study showed that approximately 75% of the variance in local species composition is unexplained. This may be due to spatially structured processes such as dispersal limitation, unaccounted biotic and abiotic environmental variables, and stochastic effects, but only 25% were due to topographic habitat association. Although the pronounced ridge-valley gradient and contrast of
south-west versus north-east aspect created consistent habitats, our results suggest that local species assemblages at Sinharaja forest are jointly shaped by neutral and niche processes.

determinants of plant community diversity and structure, dispersal limitation, habitat association, indicator species, multivariate regression tree, neutral theory, Sinharaja forest, spatial
scale, topography


Identification of 100 fundamental ecological questions

Sutherland, W. J., Freckleton, R.P., Godfray, H.C.J., Beissinger, S.R., Benton, T., Cameron, D.D., Carmel, Y., Coomes, D.A., Coulson, T., Emmerson, M.C., Hails, R.S., Hays, G.C., Hodgson, D. J., Hutchings, M. J., Johnson, D., Jones, J. P.G., Keeling, M.J., Kokko, H., Kunin, W.E., Lambin, X., Lewis, O.T., Malhi, Y., Mieszkowska, N., Milner-Gulland, E.J., Norris, K., Phillimore, A.B., Purves, D.W., Reid, J.M., Reuman, D.C., Thompson, K., Travis, J.M.J., Turnbull, L.A., Wardle, D.A. and T. Wiegand. 2013 Journal of Ecology 101: 101: 58–67.


1. Fundamental ecological research is both intrinsically interesting and provides the basic knowledge required to answer applied questions of importance to the management of the natural world. The 100th anniversary of the British Ecological Society in 2013 is an opportune moment to reflect on the current status of ecology as a science and look forward to high-light priorities for future work.
2. To do this, we identified 100 important questions of fundamental importance in pure ecology. We elicited questions from ecologists working across a wide range of systems and disciplines. The 754 questions submitted (listed in the online appendix) from 388 participants were narrowed down to the final 100 through a process of discussion, rewording and repeated rounds of voting. This was done during a two-day workshop and thereafter.
3. The questions reflect many of the important current conceptual and technical pre-occupations of ecology. For example, many questions concerned the dynamics of environmental change and complex ecosystem interactions, as well as the interaction between ecology and evolution.
4. The questions reveal a dynamic science with novel subfields emerging. For example, a group of questions was dedicated to disease and micro-organisms and another on human impacts and global change reflecting the emergence of new subdisciplines that would not have been foreseen a few decades ago.
5. The list also contained a number of questions that have perplexed ecologists for decades and are still seen as crucial to answer, such as the link between population dynamics and life-history evolution.
6. Synthesis. These 100 questions identified reflect the state of ecology today. Using them as an agenda for further research would lead to a substantial enhancement in understanding of the discipline, with practical relevance for the conservation of biodiversity and ecosystem function.

Key-words:  community ecology, ecology, ecosystems, evolutionary ecology, population ecology,
research priorities



How can we bring together empiricists and modellers in functional biodiversity research?

Jeltsch,F., N Blaum, U. Brose, J D. Chipperfield, Y. Clough, N. Farwig, K. Geissler, C. H. Graham, V.r Grimm, T. Hickler, A. Huth, F. May, K. M. Meyer, J. Pagel, B. Reineking, M. C. Rillik, K. Shea, F. M. Schurr, . Schröder, K. Tielbörger, L. Weiss, K. Wiegand, T. Wiegand, C. Wirth, D. Zurell. 2013. Basic and Applied Ecology 14: 93-101


Improving our understanding of biodiversity and ecosystem functioning and our capacity to inform ecosystem management requires an integrated framework for functional biodiversity research (FBR). However, adequate integration among empirical approaches (monitoring and experimental) and modelling has rarely been achieved in FBR. We offer an appraisal of the issues involved and chart a course towards enhanced integration. A major element of this path is the joint orientation towards the continuous refinement of a theoretical framework for FBR that links theory testing and generalization with applied research oriented towards the conservation of biodiversity and ecosystem functioning. We further emphasize existing decision-making frameworks as suitable instruments to practically merge these different aims of FBR and bring them into application. This integrated framework requires joint research planning, and should improve communication and stimulate collaboration between odellers and empiricists, thereby overcoming existing reservations and prejudices. The implementation of this integrative research agenda for FBR requires an adaptation in most national and international funding schemes in order to accommodate such joint teams and their more complex structures and data needs.

Keywords:  Biodiversity theory; Biodiversity experiments; Conservation management; Decision-making; Ecosystem functions and services; Forecasting; Functional traits; Global change; Monitoring programmes; Interdisciplinarity



Understanding species persistence for defining conservation actions: A management landscape for jaguars in the Atlantic Forest

De Angelo, C., A. Paviolo, T. Wiegand, R. Kanagaraj and M. S. Di Bitetti. 2013. Biological Conservation 159: 422–433


Habitat models constitute useful instruments for understanding species-habitat interactions and can constitute helpful conservation tools. The Upper Paraná Atlantic Forest (UPAF) of South America still holds the world’s southernmost jaguar (Panthera onca) population. Our aims were: (i) to test several a priori hypotheses on the factors affecting jaguar persistence in this region, (ii) to map habitat suitability and identify areas with potentially conflicting habitat conditions, and (iii) to identify priority areas for management and improve the conservation initiatives for jaguars and the UPAF. Following an information-theoretic approach, we used presence records of jaguars and pseudo-absences in generalized linear models. We structured hypotheses into two groups which demand different management actions: land cover and human persecution. The best model of each group was used to develop a two-dimensional habitat model. Jaguar persistence was favoured by current and historical native forest cover, and hindered by human land uses. Protection favoured jaguar presence whereas human accessibility and high human population density had negative effects. The two-dimensional model suggests that <8% (20,670 km2) of the landscape represents potential core areas for jaguars (good land-cover characteristics and low human persecution) and 11.8% (32,563 km2) stands as potentially attractive sinks where good land-cover conditions conflict with high human persecution. Reduction of human persecution is urgently needed to increase the core areas for jaguars in this region, but improvement of land-cover conditions is important for sustaining the connectivity among jaguar populations that seem to be isolated in different areas of the UPAF.

Keywords:  Attractive sink, Human persecution, Land-cover conditions, Panthera onca, Two-dimensional habitat model, Upper Paraná Atlantic Forest.


Spatiotemporal patterns of seedling-adult associations in a temperate forest community

Martínez, I., F. Glonzález-Taboada, T. Wiegand, and J. R. Obeso. 2013. Forest Ecology and Management 296: 74-80



The spatial patterns of seedling recruitment were examined in a temperate deciduous forest stand of NW Spain. The emergence and survival of individual seedlings were sampled during two recruitment seasons for the five dominant tree species (Corylus avellana, Crataegus monogyna, Fagus sylvatica, Ilex aquifolium and Taxus baccata). Point pattern analyses based on the mark correlation functions and the independent marking null model were used to explore the relationship between seedling density and the location of individual adults of the same and of different species. Overall, we found that negative or null patterns of association dominated at intermediate to large scales in our study site. Surprisingly, there were almost no positive associations at small scales, except for some pairs of fleshy-fruited species. At the same time, the massive recruitment of F. sylvatica following a mast event was accompanied by positive associations at larger scales. Spatial changes in seedling abundance were demonstrated to depend not only on the distribution of conspecific adult trees, but to lay a spatial signature of the location of adults from other species. The temporal persistence of some of these patterns and changes associated to varying production highlight the need for a community approach to study tree recruitment. 

Keywords:  Biotic interactions; Fagus sylvatica; Fleshy-fruited trees; Mark correlation function; Seedling emergence; Spatial point patterns


Phylogenetic and functional area relationships in two temperate forests

Wang, X, N. G. Swenson, T. Wiegand, A. Wolf, R. Howe, Y. Zhao, X. Bai, D. Xing, and Z. Hao. 2013. Ecography Ecography 36: 883–893



Phylogenetic diversity (PD, the diversity of lineages) and functional diversity (FD, the diversity of functional traits or groups in a biological community) reflect important yet poorly understood attributes of species assemblages. Until recently, few studies have examined the spatial variation of PD and FD in natural communities. Yet the relationships between PD and FD and area (termed PDAR and FDAR), like the analogous species–area relationship (SAR), have received less attention and may provide insights into the mechanisms that shape the composition and dynamics of ecological communities. In this study, we used four spatial point process models to evaluate the likely roles of the random placement of species, habitat filtering, dispersal limitation, and the combined effects of habitat filtering and dispersal limitation in producing observed PDARs and FDARs in two large, fully mapped temperate forest research plots in northeast China and in north-central USA. We found that the dispersal limitation hypothesis provided a good approximation of the accumulation of PD and FD with increasing area, as it did for the species area curves. PDAR and FDAR patterns were highly correlated with the SAR. We interpret this as evidence that species interactions, which are often influenced by phylogenetic and functional similarity, may be relatively unimportant in structuring temperate forest tree assemblages at this scale. However, the scale-dependent departures of the PDAR and FDAR that emerged for the dispersal limitation hypothesis agree with operation of competitive exclusion at small scales and habitat filtering at larger scales. Our analysis illustrates how emergent community patterns in fully mapped temperate forest plots can be influenced by multiple underlying processes at different spatial scales.


Spatio-temporal analysis of the effects of hurricane Ivan on two contrasting epiphytic orchid species in Guanahacabibes, Cuba

Wiegand, T., J. Raventós,  E. Mujica, E. González. and A. Bonet. 2013. Biotropica 45: 441–449.



Hurricanes represent the dominant type of disturbance in many tropical coastal forests. Here, we focus on mortality of epiphytic orchids caused by hurricane Ivan in the Guanahacabibes National Park (Cuba) and subsequent population recovery. We analyzed different aspects of hurricane damage on two contrasting epiphytic orchids, Broughtonia cubensis and Dendrophylax lindenii, as observed in three plots of coastal vegetation and in three plots of semi-deciduous forest, respectively. First, we quantified the damage to host trees and orchids and explored if hurricane damage depended on height, size, or identity of the host tree. Second, we used mark connection and mark correlation functions to conduct a detailed analysis of small-scale spatial patterns in hurricane damage for host trees and orchids. Finally, we analyzed the degree of recovery after Ivan during the 6 yr following the storm. Damage of B. cubensis host trees was independent of height and size, but Ivan severely affected larger and higher host trees of D. lindenii. Spatial analysis revealed non-random structure in damage that differed between species. Broughtonia cubensis exhibited small-scale spatial correlation in the proportion of damaged orchids, whereas D. lindenii did not. Dendrophylax lindenii showed ‘patchy’ damage patterns, correlated with height, but B. cubensis did not. The relative growth rate of B. cubensis for the 5–17 mo following Ivan was only moderately reduced and fully recovered in subsequent years, whereas that of D. lindenii was severely reduced the first year and did not fully recover thereafter. We hypothesize that differences in the host, vegetation type, and the traits of the two orchids contribute to the different responses to the hurricane.

Keywords:  Biodiversity; Cuba; epiphytic orchids; hurricanes; mark correlation function; phorophytes; relative growth rate


Spatial analysis of ignition point patterns and the probability of fire occurrence in the western area of Neuquén province, Argentina

Mundo, I.A, T. Wiegand, R. Kanagaraj and T. Kitzberger. 2013. Journal of Environmental Management 123: 77-87



Fire management requires an understanding of the spatial characteristics of fire ignition patterns and how anthropogenic and natural factors influence ignition patterns across space. In this study we take advantage of a recent fire ignition database (855 points) to conduct a comprehensive analysis of the spatial pattern of fire ignitions in the western area of Neuquén province (57,649 km2), Argentina, for the 1992e2008 period. The objectives of our study were to better understand the spatial pattern and the environmental drivers of the fire ignitions, with the ultimate aim of supporting fire management. We conducted our analyses on three different levels: statistical “habitat” modelling of fire ignition (natural, anthropogenic, and all causes) based on an information theoretic approach to test several competing hypotheses on environmental drivers (i.e. topographic, climatic, anthropogenic, land cover, and their combinations); spatial point pattern analysis to quantify additional spatial autocorrelation in the ignition patterns; and quantification of potential spatial associations between fires of different causes relative to towns using a novel implementation of the independence null model. Anthropogenic fire ignitions were best predicted by the most complex habitat model including all groups of variables, whereas natural ignitions were best predicted by topographic, climatic and land-cover variables. The spatial pattern of all ignitions showed considerable clustering at intermediate distances (<40 km) not captured by the probability of fire ignitions predicted by the habitat model. There was a strong (linear) and highly significant increase in the density of fire ignitions with decreasing distance to towns (<5 km), but fire ignitions of natural and anthropogenic causes were statistically independent. A two-dimensional habitat model that quantifies differences between ignition probabilities of natural and anthropogenic causes allows fire managers to delineate target areas for consideration of major preventive treatments, strategic placement of fuel treatments, and forecasting of fire ignition. The techniques presented here can be widely applied to situations where a spatial point pattern is jointly influenced by extrinsic environmental factors and intrinsic point interactions. 

Keywords:  Habitat modelling, Natural and anthropogenic fire ignitions, Pair correlation function, Patagonia, Point pattern analysis, Wildfire management strategies



Integrating movement ecology with biodiversity research - exploring new avenues to address spatiotemporal biodiversity dynamics

Jeltsch F, Bonte D, Pe’er G, Reineking B, Leimgruber P Balkenhohl N, Schröder B, Buchmann C, Mueller T, Blaum N, Zurell D, Böhning-Gaese K, Wiegand T, Eccard J, Hofer H, Reeg J, Eggers U, and S. Bauer. 2013. Movement Ecology 1: 6



Movement of organisms is one of the key mechanisms shaping biodiversity, e.g. the distribution of genes, individuals and species in space and time. Recent technological and conceptual advances have improved our ability to assess the causes and consequences of individual movement, and led to the emergence of the new field of ‘movement ecology’. Here, we outline how movement ecology can contribute to the broad field of biodiversity research, i.e. the study of processes and patterns of life among and across different scales, from genes to ecosystems, and we propose a conceptual framework linking these hitherto largely separated fields of research. Our framework builds on the concept of
movement ecology for individuals, and demonstrates its importance for linking individual organismal movement with biodiversity. First, organismal movements can provide ‘mobile links’ between habitats or ecosystems, thereby connecting resources, genes, and processes among otherwise separate locations. Understanding these mobile links and their impact on biodiversity will be facilitated by movement ecology, because mobile links can be created by different modes of movement (i.e., foraging, dispersal, migration) that relate to different spatiotemporal scales and have differential effects on biodiversity. Second, organismal movements can also mediate coexistence in communities, through ‘equalizing’ and ‘stabilizing’ mechanisms. This novel integrated framework provides a conceptual starting point for a better understanding of biodiversity dynamics in light of individual movement and space-use behavior across spatiotemporal scales. By illustrating this framework with examples, we argue that the integration of movement ecology and biodiversity research will also enhance our ability to conserve diversity at the genetic, species, and ecosystem levels.

Keywords:  Mobile links, Species coexistence, Community dynamics, Biodiversity conservation, Long distance movement, Landscape genetics, Individual based modeling


The relative importance of Janzen-Connell effects in influencing the spatial patterns at the Gutianshan subtropical forest

Zhu, Y., S. Getzin, T. Wiegand, H. Ren, K. Ma.  2013.   PLoS ONE 8(9): e74560



The Janzen-Connell hypothesis is among the most important theories put forward to explain species coexistence in species rich-communities. However, the relative importance of Janzen-Connell effects with respect to other prominent mechanisms of community assembly, such as dispersal limitation, self-thinning due to competition, or habitat association, is largely unresolved. Here we use data from a 24-ha Gutianshan subtropical forest to address it. First we tested for significant associations of adults, juveniles, and saplings with environmental variables. Second we evaluated if aggregation decreased with life stage. In a third analysis we approximately factored out the effect of habitat association and comprehensively analyzed the spatial associations of intraspecific adults and offspring (saplings, juveniles) of 46 common species at continuous neighborhood distances. We found i) that, except for one, all species were associated with at least one environmental variable during at least one of their life stages, but the frequency of significant habitat associations declined with increasing life stage; ii) a decline in aggregation with increasing life stage that was strongest from juveniles to adults; and iii) intraspecific adult-offspring associations were dominated by positive relationships at neighborhood distances up to 10 m. Our results suggest that Janzen-Connell effects were not the dominant mechanisms in structuring the spatial patterns of established trees in the subtropical Gutianshan forest. The spatial patterns may rather reflect the joint effects of size-dependent self-thinning, dispersal limitation and habitat associations. Our findings contribute to a more comprehensive understanding of the relative importance of Janzen-Connell effects in influencing plant community structure under strong topographic heterogeneity.



Hierarchical mechanisms of spatially contagious seed dispersal in complex seed-disperser networks

Fedriani, J.M., and T. Wiegand. in press.  Ecology



Intra and interspecific spatially contagious seed dispersal has far-reaching implications for plant recruitment, distribution, and community assemblage. However, logistical and analytical limitations have curtailed our understanding concerning the mechanisms and resulting spatial patterns of contagious seed dispersal in most systems and, especially, in complex seed disperser networks. We investigated mechanisms of seed aggregation using techniques of spatial point pattern analysis and extensive datasets on mutispecific endozoochorous seed rain generated by five frugivorous mammals in three Mediterranean shrublands over two seasons. Our novel analytical approach revealed three hierarchical and complementary mechanisms of seed aggregation acting at different levels (fecal samples, seeds, pairs of seed species) and spatial scales. First, the three local guilds of frugivores tended to deliver their feces highly aggregated at small and intermediate spatial scales, and the overall pattern of fecal delivery could be described well by a nested double-cluster Thomas process. Second, once the strong observed fecal aggregation was accounted for, the distribution of mammal feces containing seeds was clustered within the pattern of all feces (i.e., with and without seeds), and the density of fecal samples containing seeds was higher than expected around other feces containing seeds in two out of the three studied seed-dispersed networks. Finally, at a finer level, mark-correlation analyses revealed that for some plant species pairs the number of dispersed seeds was positively associated either at small or large spatial scales. Despite the relative invariant patterning of nested double-clustering, some attributes of endozoochorous seed rain (e.g., intensity, scales of aggregation) were variable among study sites due to changes in the ecological context in which seeds and their dispersers interact. Our investigation disentangles for the first time the hierarchy of synergic mechanisms of spatially contagious seed dispersal at a range of spatial scales in complex seed-disperser networks, providing thus a robust and widely applicable framework for future studies. 




Relative importance of spatial processes and topography on structuring species assemblages in a Sri Lankan dipterocarp forest

Punchi-Manage, R., T. Wiegand, S. Getzin, K. Wiegand, C.V.S. Gunatilleke, I.A.U. N. Gunatilleke. in press.  Ecology



Niche and neutral theories emphasize different processes that contribute to the maintenance of species diversity and should leave different spatial structures in species assemblages. In this study we used variation partitioning in combination with distance based Moran’s eigenvector maps and habitat variables to determine the relative importance of the effects of pure habitat, pure spatial, and spatially structured habitat processes on the spatial distribution of tree species composition and richness in a 25-ha tropical rainforest of Sinharaja/Sri Lanka. We analyzed the contribution of those components at three spatial scales (10 m, 20 m, 50 m) for all trees and the three life stages: recruits, juveniles and adults. At the 10 m scale 80% of the variation in species composition remained unexplained for recruits and adults, but only 55% for juveniles. With increasingly broader scales these figures were strongly reduced, mainly by an increasing contribution of the spatially structured habitat component which explained 4-30%, 20-47%, and 8-35% for recruits, juveniles, and adults, respectively. The pure spatial component was most important at the 20 m scale and reached 20%, 32%, and 23% for recruits, juveniles and adults, respectively. The spatially structured habitat component described variability at broader scales than the pure spatial component. Our results suggest that stochastic processes and spatially structuring processes of community dynamics, such as dispersal limitation and habitat association, contributed jointly to explain species composition and richness at the Sinharaja forest, but their relative importance changed with scale and life stage. Species assembly at the local scale was more strongly impacted by stochasticity, whereas the signal of habitat was stronger at the 50 m scale where plant-scale stochasticity is averaged out. Recent research points to an emerging consensus on the relative contribution of stochasticity, habitat and spatial processes in governing community assembly, but how these components change with life stage and how this is influenced by sample size remains to be explored. 



Using individual-based movement models to assess inter-patch connectivity for large carnivores in fragmented landscapes

Kanagaraj, R. T. Wiegand, S. Kramer-Schadt, S.P. Goyal. early online.  Biological Conservation



Most rare and endangered large carnivores such as tiger (Panthera tigris) exist in human-dominated landscapes as small, fragmented and isolated populations across their range. Connectivity between the remaining populations in the habitat fragments is essential for their long-term persistence and focus of management initiatives. We describe an individual-based, spatially explicit model of tiger movement behavior based on previously developed habitat models to (i) quantify inter-patch connectivity among major (protected) habitat patches in the Terai Arc Landscape of India and Nepal and (ii) investigate the effect of potential management initiatives, e.g. restoring corridors, on enhancing connectivity among fragmented protected habitats. Connectivity was not solely a function of distance between patches, but an outcome of the interplay between movement behavior and landscape composition, with asymmetric connectivity explained by canalizing or diffusing effects of the landscape, and depending on the landscape context of the starting patch. Patch connectivity was mostly determined by autocorrelation in tiger movement, the daily movement capacity, landscape structure, and the amount of matrix habitat. Several habitat patches were likely to be island-like and already effectively isolated. However, simulating scenarios of corridor restoration showed that most habitat patches in India and between India and Nepal could recover connectivity, which may mitigate negative genetic consequences of small population size and effective isolation on tiger populations in this landscape. Combining habitat models with individual-based models is a powerful and robust approach that could be widely applied to delineate dispersal corridors of large carnivores and quantify patch connectivity even if data are scarce. 


Keywords:  Carnivore conservation; Fragmented landscapes; Inter-patch connectivity; Movement model; Panthera tigris; Spatially explicit individual-based model; Terai Arc Landscape; Tiger



Quantifying spatial phylogenetic structures of fully stem-mapped plant communities

Shen, G., T. Wiegand, X. Mi, and F. He. early online. Methods in Ecology and Evolution


1. Analysis of the phylogenetic similarity of co-occurring species at different spatial scales is increasingly used for decoding community assembly rules. Here, we integrated the analysis of phylobetadiversity and marked point pattern analysis to yield a new metric, the phylogenetic mark correlation function, kd(r), to quantify spatial phylogenetic structure of fully stem-mapped communities
2. kd(r) is defined as the expected phylogenetic distance of two heterospecifics separated by spatial distance r, and normalized with the expected phylogenetic distance of two heterospecifics taken randomly from a study area. It measures spatial phylogenetic turnover relative to spatial species turnover and is closely related with the spatially explicit Simpson index.
3. We used simulated fully stem-mapped plant communities with known spatial phylogenetic structures to assess type I and II errors of the phylogenetic mark correlation function kd(r) under a null model of random phylogenetic spatial structure, and to test the ability of the kd(r) to detect scale-dependent signals of phylogenetic spatial structure. We also compared the performance of the kd(r) with two existing measures of phylobetadiversity that have been previously used to analyse fully stem-mapped plots. Finally, we explored the spatial phylogenetic structure of a 24-ha fully stem-mapped subtropical forest in China.
4. Simulation tests showed that the new metric yielded correct type I and type II errors and accurately detected the spatial scales at which various processes (e.g. habitat filtering and competition) were invoked to generate spatial phylogenetic structures. The power of the kd(r) was not affected by a phylogenetic signal in species abundance and different topologies of the phylogenetic tree.
5. Replacing phylogenetic distance by functional distance allows for application of the kd(r) to estimate spatial correlations in functional community structure. Thus, the kd(r) allows trait and phylogenetic structure to be analysed in the same framework. The phylogenetic mark correlation function is a powerful and accurate tool for revealing scale-dependent phylogenetic/functional footprints in community assemblages and allows ecologists to keep up with the increasingly available data of fully stem-mapped plots, functional traits and community phylogenies.


Keywords:   community assembly rules, phylogenetic mark correlation function, point pattern analysis, phylobetadiversity, scale dependence, spatial phylogenetic community pattern, type I and II errors



    Modified: 18.11.2013   Resp.: Thorsten Wiegand     webmaster