The advantage of the extremes: tree seedlings at intermediate abundance in a tropical forest have the highest richness of aboveground enemies and suffer the most damage

1. Tropical forest tree diversity has been hypothesized to be maintained via the attraction of density responsive and species-specific enemies. Tests of this hypothesis usually assume a linear relationship between enemy pressure (amount of damage and enemy richness) and seedling or tree density. However, enemy pressure is likely to change nonlinearly with local seedling abundance and community scale tree abundance if enemies are characterized by nonlinear functional responses. 2. We examined the abiotic and biotic factors associated with richness of above-ground enemies and foliar damage found in tree seedlings in a tropical forest in Puerto Rico. Rather than identify specific enemies targeting these seedlings, we used damage morphotypes, a paleo-ecological method, to derive a proxy for enemy species richness. 3. We found that the relationships between local and (conspecific seedling density) community scale (conspecific basal area of adult trees) abundance and both richness of above-ground enemies and foliar damage were hump-shaped. Seedlings of tree species existing at intermediate levels of abundance, at both local and community scales, suffered more damage and experienced pressure from a greater diversity of enemies than those existing at high or low densities. 4. We hypothesized that greater damage at intermediate abundance level could arise from a rich mixture of generalist and specialist enemies targeting seedlings of intermediate abundance tree species. Consistent with this hypothesis, we found that generalist enemies were more diverse on species at rare or intermediate abundance relative to common tree species. However, specialist enemies showed no significant trend across tree species abundance at either the local or community scales. 5. Synthesis. Our results suggest that interspecific variation in tree species abundance leads to differences in the magnitude and type of damage tropical tree seedlings suffer. This variation leads to a nonlinear, hump-shaped relationship between species abundance and enemy damage, highlighting fruitful directions for further development of species coexistence theory.