Density dependence generates spatial overdispersion of conspecific adult tropical trees

Background/Question/Methods

For multiple species to coexist, it is necessary that as species increase in abundance, they suffer from Conspecific Negative Density Dependence (CNDD). For trees, there is ample evidence for such negative effects of abundance on the survival and frequency of juveniles, typically on short spatial scales. Despite this short distance “repulsion” of juveniles, adult trees are generally spatially clumped, and there is little evidence that CNDD acting on juveniles propagates to the adult stage, casting doubt on its importance for coexistence.However, evidence on aggregation were previously obtained by comparing the spatial distribution of adults to a random placement across the landscape. We re-examine the spatial distributions of adult trees in the Barro Colorado Island (BCI) tropical forest plot, comparing them to a calibrated neutral model of stochastic birth-death and limited dispersal, using estimated dispersal kernels.

Results/Conclusions

When compared to a model of limited dispersal, the adults of almost all tree species are strongly and significantly overdispersed on scales of at least 100 meters, while several habitat and gap specialists are more aggregated. We further demonstrate using simulations that such strong overdispersion can only be the result of CNDD being substantially stronger than Heterospecific Negative Density Dependence (HNDD).These results suggest that the CNDD commonly found in juveniles propagates to adults, strongly influencing their spatial distribution. Furthermore, the result of CNDD > > HNDD implies large niche differences between species, with the potential to stabilize coexistence. The large scale overdispersion of many species also implies that CNDD increases compositional turnover up to substantial scales. Finally, our results demonstrate the power of using the limited dispersal null to detect biotic interactions and habitat associations.