Does variation in the strength of negative density dependence explain differences in species diversity among tropical forests?

Background/Question/Methods

Understanding how high levels of diversity are maintained in tropical forests remains a challenge for ecologists.  One mechanism proposed to explain species coexistence in these diverse communities is negative density dependence, i.e. high densities of conspecific neighbors result in lower growth rates, survival and reproductive success.  While evidence of negative density dependence has been found for a number of tropical tree species, cross-site studies that compare  the relative strength of negative density dependence across tropical forests have not until now been attempted. Using data from a network of forest dynamics plot coordinated by the Smithsonian’s Center for Tropical Forest Science, we asked whether the strength of negative density dependence varies among tropical forests, and if so, whether that variation can explain differences in tree species diversity among sites.

Using data on sapling survival in 12 fully mapped tropical forest dynamics plots, we employed a hierarchical Bayesian approach to determine the strength of negative density dependence for sapling survival at the community-level for each site. We then tested whether the strength of negative density dependence varied significantly among sites, and whether that variation could explain differences in species richness among forests. 

Results/Conclusions

We found evidence of negative density dependence at all of our tropical forest sites:  the probability of survival for an individual sapling was negatively related to the density of conspecific neighbors for some species in all of these forests.  The overall community-wide effect of conspecific neighbors on sapling survival also tended to be negative, suggesting that negative density dependence is widespread in tropical tree communities. However, the proportion of species experiencing significant negative density dependence and the community-level strength of negative density dependence varied among sites.  These results have implications for species coexistence in tropical tree communities and for variation in species diversity among forests within the tropics.