Plant secondary metabolites play important ecological and evolutionary roles, most notably in the deterrence of natural enemies. The classical theory explaining the evolution of plant chemical diversity is that new defences arise through a pairwise co-evolutionary arms race between plants and their specialized natural enemies. However, plant species are bombarded by dozens of different herbivore taxa from disparate phylogenetic lineages that span a wide range of feeding strategies and have distinctive physiological constraints that interact differently with particular plant metabolites. How do plant defence chemicals evolve under such multiple and potentially contrasting selective pressures imposed by diverse herbivore communities? To tackle this question, we exhaustively characterized the chemical diversity and insect herbivore fauna from 31 sympatric species of Amazonian Protieae (Burseraceae) trees. Using a combination of phylogenetic, metabolomic and statistical learning tools, we show that secondary metabolites that were associated with repelling herbivores (1) were more frequent across the Protieae phylogeny and (2) were found in average higher abundance than other compounds. Our findings suggest that generalist herbivores can play an important role in shaping plant chemical diversity and support the hypothesis that chemical diversity can also arise from the cumulative outcome of multiple diffuse interactions.

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Accumulating evidence suggests that herbivorous insects influence the local composition and richness of Neotropical plant species, particularly in species‐rich genera. Species richness, phylogenetic diversity, and chemical diversity all influence the ability of insect herbivores to find and utilize their hosts. The relative impact of these components of diversity on species coexistence and plant–herbivore interactions is not well understood. We constructed 60 local communities of up to 13 species of Piper (Piperaceae) in native, mature forest at a lowland wet forest location in Costa Rica. The species composition of each community was chosen such that species richness, phylogenetic diversity, and GCMS‐based chemical diversity were varied independently among communities. We predicted that chemical diversity would most strongly affect the communities across time, with smaller effects of taxonomic and phylogenetic diversity. At 13 months after the experimental planting, we assessed survivorship of each cutting, measured total leaf area loss of the survivors, leaf area loss to generalist and specialist herbivorous insect species, and local extinction of species. Generalist and specialist herbivory decreased with increasing levels of species richness and phylogenetic diversity, respectively. Surprisingly, there was no independent effect of chemical diversity on any of the three measures of herbivore damage. Nevertheless, plots with a higher chemical and phylogenetic diversity showed decreased plant mortality and local species extinction. Overall, our results suggested that both chemical and phylogenetic similarity are important factors in the assembly and maintenance of tropical plant communities. The fact that chemical diversity influences plant mortality suggests that leaf herbivores, and possibly other plant natural enemies, could increase plant diversity via the selective mortality of similar chemotypes.

Exploring the links between secondary metabolites and leaf spectral reflectance in a diverse genus of Amazonian trees. Ecosphere 12(2): e03362.