What are the ecological causes and consequences of variation in phytochemical diversity within and between plant taxa? Despite decades of natural products discovery by organic chemists and research by chemical ecologists, our understanding of phytochemically mediated ecological processes in natural communities has been restricted to studies of either broad classes of compounds or a small number of well-characterized molecules. Until now, no studies have assessed the ecological causes or consequences of rigorously quantified phytochemical diversity across taxa in natural systems. Consequently, hypotheses that attempt to explain variation in phytochemical diversity among plants remain largely untested. We use spectral data from crude plant extracts to characterize phytochemical diversity in a suite of co-occurring plants in the tropical genus Piper (Piperaceae). In combination with 20 years of data focused on Piper-associated insects, we find that phytochemical diversity has a direct and positive effect on the diversity of herbivores but also reduces overall herbivore damage. Elevated chemical diversity is associated with more specialized assemblages of herbivores, and the cascading positive effect of phytochemistry on herbivore enemies is stronger as herbivore diet breadth narrows. These results are consistent with traditional hypotheses that predict positive associations between plant chemical diversity, insect herbivore diversity, and trophic specialization. It is clear from these results that high phytochemical diversity not only enhances the diversity of plant-associated insects but also contributes to the ecological predominance of specialized insect herbivores.he Anthropocene has been characterized by huge losses of biodiversity caused by rapid global change, including habitat loss, fragmentation, invasive species, and climate change. Ecologists struggle to understand not only the consequences of diversity loss but also how to quantify ecologically relevant dimensions of diversity, including genetic, taxonomic, and functional diversity. Although it has been difficult to measure, phytochemical diversity (i.e., richness and abundance of plant compounds) is a key axis of functional diversity (1) that affects associated trophic levels and is likely driving other aspects of biodiversity (2-4). Variation in phytochemical or metabolic diversity in plants, which is further downstream than genomic, transcriptomic, or proteomic diversity (5, 6), potentially reflects variation in response to a diversity of natural enemies, including specialist and generalist insect herbivores (7,8). Furthermore, phytochemistry is one of the most relevant traits to measure when determining functional roles of plants in natural and managed communities (9).Considering the importance of phytochemical diversity for numerous natural processes, it is not surprising that a broad range of ecological and evolutionary hypotheses has been proposed to explain their role in interactions between plants and herbivores. From a coevolutionary perspecti...

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Summary Chemically mediated plant–herbivore interactions contribute to the diversity of terrestrial communities and the diversification of plants and insects. While our understanding of the processes affecting community structure and evolutionary diversification has grown, few studies have investigated how trait variation shapes genetic and species diversity simultaneously in a tropical ecosystem.We investigated secondary metabolite variation among subpopulations of a single plant species, Piper kelleyi (Piperaceae), using high‐performance liquid chromatography (HPLC), to understand associations between plant phytochemistry and host‐specialized caterpillars in the genus Eois (Geometridae: Larentiinae) and associated parasitoid wasps and flies. In addition, we used a genotyping‐by‐sequencing approach to examine the genetic structure of one abundant caterpillar species, Eois encina, in relation to host phytochemical variation.We found substantive concentration differences among three major secondary metabolites, and these differences in chemistry predicted caterpillar and parasitoid community structure among host plant populations. Furthermore, E. encina populations located at high elevations were genetically different from other populations. They fed on plants containing high concentrations of prenylated benzoic acid.Thus, phytochemistry potentially shapes caterpillar and wasp community composition and geographic variation in species interactions, both of which can contribute to diversification of plants and insects.

Prominent models of herbivore regulation focus on predators, low plant biomass, or poor resource quality as factors that limit herbivore populations. We examined predictions of these tritrophic models for herbivores on the understory shrub, Piper cenocladum, which is defended by mutualistic ants (Pheidole bicornis) and three amide secondary metabolites. To examine sources of variation in P. cenocladum amide content and to compare the effects of amides vs. ants on herbivores, we used three linked experiments in the field and in shadehouses. We manipulated light, nutrient availability, and presence of symbionts for experimental plant fragments and shrubs and then quantified leaf amide concentration. We also examined relationships between amide content and damage by the three most common groups of folivores on P. cenocladum: generalist orthopterans, specialist coleopterans, and specialist lepidopterans. For all experiments, enhanced resources and absence of symbionts caused higher levels of amides. These increased chemical defenses had strong effects on generalist herbivores in this system, while the ant predators were more effective at depressing herbivory by specialists. The negative effects of amides on specialist lepidopterans were small, suggesting that these herbivores are adapted to chemical defenses in their host plant. It is possible that our results are part of a more general trend where top-down effects are stronger against specialist herbivores, while chemical defenses are more effective against generalists. We concluded that different models of herbivore regulation were supported by components of the P. cenocladum arthropod community, depending on resource availability and on the portion of the web examined.

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