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    Canopy of lowland hill dipterocarp forest in Sinharaja taken from the top of a lowland hill - Sinharaja

   (about 800m asl). It shows different species in different stages of leaf flushing (light green) and early

   fruiting (pinkish - red) stages but none in the picture in bloom. Source: Nimal Gunatilleke, Universität



Spatial patterns of tropical tree species retain little signature of species interactions

A persistent challenge in ecology is to explain the high species diversity of tropical forests. One approach to facing this challenge is to analyze the spatial patterns of explicit maps of individual tree locations, which should retain a signal of processes promoting coexistence. Previous research suggest strong positive or negative associations e.g., as a result of competitive or facilitative interactions. To our surprise, we have discovered in our data set collected at a 25ha plot of tropical forest in Sri Lanka that the majority of all species pairs showed segregated spatial patterns, or only a partial overlap, and not more than 6% of the species pairs showed significant associations.

We summarize our findings in an article just published in the leading journal The American Naturalist. Our analyses are based on a fully mapped 25ha plot of tropical rain forest at the Sinharaja UNESCO World Heritage Site in Sri Lanka where all individuals ≥ 1 cm diameter at breast height have been mapped, measured, and identified to species. This plot is part of the network of the Center for Tropical Forest Science (CTFS; http://www.ctfs.si.edu) and has several series of closely related congeneri and partially sympatric species; the majority of these are endemic to Sri Lanka.

To paint an overall picture of the association between tree species we analyzed the spatial patterns of several thousand species pairs. In a first step, we explored how species pairs were distributed relative to each other. In more than half of all cases we found segregated patterns meaning that the two species were basically distributed in disjunctive patches within the plot. Only in about 6% of all cases we found a co-occurrence pattern. It thus looks as if the species were distributed in this forest in a way that minimizes the opportunities of individual trees of different species to come into close contact with each other.

In a second step, we addressed the question whether the spatial pattern of species pairs showed a signature of significant positive or negative association. To this end we used advanced spatial-statistical methods, so-called point-pattern analyses, which are used, for example, to study the spatial structure of galaxies. However, the problem of studying spatial association between species is that habitat association may confound the effect of plant-plant interactions because both can produce locally elevated or reduced species co-occurrence. The breakthrough of our analysis is that we found a way to disentangle these two effects. This allowed us to look at our data in a novel way.

Previous studies showed that growth and survival of trees depend quite strongly on their neighbors. We therefore expected strong signatures of positive or negative interactions between species which such as those caused by competition or facilitation. However, to our surprise we found that only approximately 6% of the 2070 species pairs showed significant associations. This apparent contradiction raises the question why such non-neutral processes, which should also operate at Sinharaja, did not leave a detectable signature in the spatial pattern. We hypothesize that non-neutral processes affecting tree performance equilibrate and produce in most cases neutral bivariate patterns in the spatial distribution of trees. This can be interpreted as a strong argument in favor of neutral theory. A second complementary hypothesis is that trees of two given species just “meet” too seldom to produce specific associations and that therefore the set of species neighbors encountered by individuals of a given species is quite variable and not predictable for the individual.

Our finding of lack of species associations is a novel pattern and a step towards an understanding of the complexities of the origin and maintenance of species richness in tropical forests.

Press Release 04/2008
New method for measuring biodiversity
How individual species help structure the biodiversity of tropical rainforests

Leipzig. German and Sri Lankan researchers have developed a new method for measuring the impacts of species on local biodiversity. It makes it possible to determine whether a certain species promotes or suppresses species diversity. The new method extends a procedure familiar to biologists that involves investigating species numbers in relation to area (the species-area relationship, or SAR), by adding sophisticated statistical methods so that it can be used to describe the role of individual species in their impact on biodiversity. This individualised method (‘individual species-area relationship’, or ISAR) makes it easier to identify key species. “We are effectively looking at diversity in the ecosystem through the glasses of the individual species,” says co-author Dr Andreas Huth of the UFZ. This means that in future it will be easier to understand the role of individual species in ecosystems and to implement targeted protection measures for key species. In addition, the method can be used to investigate better the ecological consequences of changes in land use.

The researchers used their new method to evaluate unique data from two tropical rainforests in Sri Lanka and Panama that are part of a network coordinated by the Center for Tropical Forest Science (CTFS; http://www.ctfs.si.edu). Within this network, every single tree with a trunk bigger than a pencil has been mapped and monitored for years on about a dozen selected sample plots, some as large as 50 hectares, in tropical rainforests around the globe. The researchers compared in their study around 40 000 larger trees in the tropical rainforest on Barro Colorado Island, Panama, with those in the Sinharaja World Heritage Site in Sri Lanka. To their surprise, more than two third of all species did not leave identifiable signatures on spatial diversity. The other tree species had an impact on local biodiversity only in their immediate surroundings, within a radius of up to 20 metres, but not on a large scale. These findings support the much-debated ‘neutral theory’, according to which species characteristics are unimportant for certain community attributes and play only a subsidiary role in the stability and diversity of ecosystems. The study reveals that the two tropical forests lacked any key species structuring species diversity at larger scales, suggesting that ‘‘balanced’’ species–species interactions may be a characteristic of these species rich forests. Dr Thorsten Wiegand says, “Biodiversity researchers have not been able to agree on which processes permit a high level of species diversity to emerge, and which processes keep these complicated systems stable”.

On the research plot in Panama there were lots of ‘repellent’ species. By contrast, the one in Sri Lanka is dominated by ‘attractor’ species, i.e. species that promote biodiversity. “It is not yet known why these two tropical rainforests are so different in this regard” say Savitri Gunatilleke and her husband Nimal, both professors at the University of Peradeniya, “but our method is a leap forward in an understanding of the complexities of the origin and maintenance of species richness in tropical forests”. After all, the method was being used for the first time. “We first used the new method in tropical rainforests, but it is universally applicable and can be used for plants in all ecosystems,” says Dr Andreas Huth of the UFZ. The new method closes a gap between rather more crude descriptions of biodiversity (for the whole ecosystem) and extremely detailed analyses (interactions between individual species). In future then, the models already developed at the UFZ for sustainable forestry management will also be optimised.


  Canopy of the seasonally moist tropical forest at Barro Colorado Island (BCI),

  Panama, with bright yellow flowering trees of the golden guayacan, Tabebuia

  guayacan (Seem.) Hemsl. Source: Smithsonian Tropical Research Institute,

  Center for Tropical Forest Science.


    Modified: 12.03.2008   Resp.: Thorsten Wiegand     webmaster