One of the central goals of community ecology is to understand the mechanisms that maintain diversity. There is now compelling evidence that negative density dependence, where small individuals grow poorly near members of their own species is pervasive among canopy tree species in the temperate zone (overdispersion). Disturbance and enemies are also well documented diversity maintenance mechanisms. What remains unknown, however, is how these three mechanisms interact. We simultaneously examined the interaction among negative density dependence, disturbance via canopy gap formation, fire, and herbivory in a fully factorial 16-year experiment in the geographic center of the Eastern Deciduous Forest Biome. We used spatial point pattern analysis to look for the spatial signature of negative density dependence among woody plants to test the hypotheses: 1. Negative density dependence maintains diversity. 2. Disturbance negates negative density dependence. 3. Vertebrate herbivores strengthen negative density dependence. Specifically, we used replicated point pattern analysis and pooled Besag's L distribution with bootstrapped 95% confidence intervals and a bootstrapped 95% acceptance interval surrounding complete spatial random to test for the presence or absence of overdispersion, the spatial signature of negative density dependence.
Results/Conclusions
We found that negative density dependence alone is not sufficient to maintain diversity in this system. Plants in plots that were undisturbed were significantly overdispersed, indicative of strong negative density dependence. Disturbance via canopy gaps resulted in a significantly more random spatial pattern than when undisturbed. Disturbance simultaneously resulted in a 66% average increase in diversity (10 species +- 1 standard error) when combined with the removal of large vertebrate herbivores. Indeed, where negative density dependence was present (i.e. plants were significantly overdispersed in terms of Besag's L) species richness was on average significantly lower (6 species +- 0.68 standard error) than when forests were disturbed by canopy gaps. Furthermore, when vertebrate herbivores were excluded, negative density dependence was weakened indicating that vertebrate herbivores strengthen negative density dependence. Our findings suggest that while negative density dependence is occurring in temperate deciduous forests it may not be the largest contributor to the maintenance of diversity.