Explaining the mechanisms responsible for the maintenance of biodiversity is a fundamental goal of ecology. Two prominent mechanisms hypothesized to maintain local species diversity are intraspecific competition and antagonistic interactions with specialized natural enemies. A key criterion of these diversity-maintenance mechanisms is specificity: stable coexistence can arise from species-specific negative interactions, but not from homogenizing interactions with generalists. In this study, we examined variation in foliar chemical defenses among co-occurring tree species to gain insights into the degree of enemy specialization and elucidate the role of specialized enemies in determining conspecific negative density dependence (CNDD) and local diversity in a temperate oak-hickory forest. We collected leaves from 220 saplings across 22 tree species in a large, stem-mapped forest dynamics plot and analyzed leaf chemical extractions with nuclear magnetic resonance (NMR). We compared broad-spectrum chemical diversity to estimate the dissimilarity of chemical profiles across and within species as a proxy of the specialization of natural enemies. We then compared the dissimilarity of species’ chemical profiles to the strength of CNDD and neighborhood diversity across species to test the hypotheses that specialized enemies increase 1) CNDD in sapling recruitment and 2) tree species diversity at the scale of local neighborhoods.
Results/Conclusions
We found significant interspecific differences in foliar chemical profiles between two clusters of species. The two species clusters show some phylogenetic patterns, but contain widely separated monophyletic and paraphyletic groups. This clustering suggests specialization of natural enemies is likely across tree species in this temperate forest. Mean interspecific chemical dissimilarity increased with the strength of species’ CNDD, consistent with the hypothesis that more chemically distinct species experience stronger CNDD due to antagonistic interactions with specialized enemies. At the scale of local tree neighborhoods, rarified-species richness and Shannon’s diversity increased with mean inter- and intraspecific chemical dissimilarity, consistent with the hypothesis that specialized natural enemies increase local species diversity by limiting densities of conspecifics. Our findings suggest an important role for specialized natural enemies in determining variation in density-dependent recruitment across species and spatial patterns of local biodiversity within temperate forests.