>>Department of Ecological Modelling >> Personal homepage Thorsten Wiegand >> Abstracts
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.
Vegetation change in semiarid communities: simulating
probabilities and time scales
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.
Simulated plant population responses to small scale
disturbance in semi-arid shrublands
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.
Population dynamics, disturbance, and pattern evolution:
identifying the fundamental scales of organization in a model ecosystem.
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.
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.
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.
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.
Live fast, die young: estimating size-age relations
and mortality pattern of shrubs species in the semi-arid Karoo, South Africa.
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.
Rule-based assessment of suitable habitat and patch
connectivity for the Eurasian lynx in Germany
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.
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 km˛. 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.
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,
Endangered species balancing between natural and human constrains: the case of
brown bears (Ursus arctos) in northern Spain
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.
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
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
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.
Rings, Circles and
Null-Models for Point Pattern Analysis in Ecology
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.
Fragmented landscapes, road mortality and patch
connectivity: modeling dispersal for the Eurasian lynx in Germany
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.
Key-words: autocorrelated random walk, Lynx pardinus, matrix fragmentation, individual-based spatially explicit simulations, interpatch connectivity, standard of plausibility.
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
Kramer-Schadt, S.,.E. Revilla, and T. Wiegand. 2005. Biological Conservation 125: 169-182
Keywords: large carnivores, Lynx lynx L., mortality scenarios, population viability analysis, spatially explicit individual-based model, species reintroduction
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
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
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*
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
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
*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
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.
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
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.
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.
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.
Kramer-Schadt, S., E. Revilla, T. Wiegand, and V. Grimm.
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
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 Ł 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.
32:359 - 369
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
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
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.
Wiegand, T, C.V.S. Gunatilleke, I.A.U.N. Gunatilleke, and T. Okuda. Ecology88: 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
Wiegand, T, C.V.S. Gunatilleke, and I.A.U.N. Gunatilleke. 2007. The American Naturalist
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.
Keywords: biodiversity, spatial patterns, spatial statistic, species–area relationship.
Wiegand, T., J. Naves and M. Garbulsky, and N. Fernández.
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.
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
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
Heterogeneity influences spatial patterns and demographics in forest stands
Getzin, S., T. Wiegand, K. Wiegand, and F.
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.
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
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.
Journal of Arid Environments
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.
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
Small-scale spatial structure within patterns of seed dispersal
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.
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.
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
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.
Fire-mediated interactions between shrubs in a South American temperate savannah
Biganzoli, F., T. Wiegand and W.B. Batista.
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 m˛)
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.
Isabel Martínez, and Andreas Huth. 2009. The American Naturalist
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
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 Dońana 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.
Jacquemyn, H., P. Endels, O. Honnay, and
T. Wiegand, 2010 Journal of Applied Ecology 47:
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.
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
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).
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:
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
The relative effects of habitat loss and fragmentation on
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.
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.
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
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
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.
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 km˛) 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.
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,
Keywords: forest limit, individual-based model, Pinus uncinata, Pyrenees,
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.
Kramer-Schadt, S., Kaiser, T., K. Frank, and T. Wiegand. 2011. Landscape Ecology 26:501-513
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.
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.
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.
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
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
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
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
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
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 (fireţ 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.
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
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.
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:
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. early view. Ecography
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.
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
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
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.
A systematic comparison of summary characteristics for quantifying point patterns in ecology
Wiegand, T, F. He, and S.P. Hubbell. early view. Ecography
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.
Spatial distribution and interspecific associations of tree
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
Disturbance-dependent spatial distribution of sexes in a
gynodioecious understory shrub
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
Effects of topography on structuring local species assemblages in a Sri Lankan mixed dipterocarp forest
Punchi-Manage, R., S. Getzin, T. Wiegand, R. Kanagaraj,
Gunatilleke, I.A.U. N. Gunatilleke, K. Wiegand, and A. Huth.
Journal of Ecology
determinants of plant community diversity and structure, dispersal
limitation, habitat association, indicator species, multivariate regression tree,
neutral theory, Sinharaja forest, spatial
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.,
Travis, J.M.J., Turnbull, L.A., Wardle, D.A. and T. Wiegand.
Journal of Ecology
101: 101: 58–67.
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. Basic and Applied Ecology 14: 93-101
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.
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