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Spatial Patterns of Tropical Tree Species Retain Little Signature of Species Interactions
8 Oct, 2007 04:30 pm
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.
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 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.
Wiegand, Thorsten, et al . 2007. Species associations in a heterogeneous Sri Lankan Dipterocarp forest. The American Naturalist, 170 E77–E95.