Separating the effect of mechanisms shaping species-abundance distributions at multiple scales in a subtropical forest

Species abundance distributions (SADs) play an important role in the current dispute over mechanisms shaping community assembly. Niche theory assumes differential occurrence of species in different habitats while neutral theory emphasizes stochastic events and dispersal. The previous tests of niche and neutral models shaping SADs lead to the claim that SADs are not informative for inferring underlying processes. Using spatial statistical models in a fully mapped 24-ha subtropical forest in China, we first demonstrate that one can not distinguish between the effect of habitat heterogeneity and dispersal limitation on SADs by inspecting whether the observed SADs fall within 95% confidence intervals of the simulated SADs. Subsequently, we demonstrate that SADs can be used to detect mechanisms shaping SADS by comparing alternative process-based models using model selection techniques. We found that dispersal limitation explain SADs at smaller spatial scales, while the combination of niche and dispersal limitation explain SADs at larger scales. These processes are linked with the degree of conspecific aggregation, informing further attempts to refine and parameterize the statistical theory of sampling SADs.