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...

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.

Little is known about the evolution, diversity, and functional significance of secondary metabolites in reproductive plant parts, particularly fruits and seeds of plants in natural ecosystems. We compared the concentration and diversity of amides among six tissue types of Piper reticulatum: leaves, roots, flowers, unripe fruit pulp, ripe fruit pulp, and seeds. This represents the first detailed description of amides in P. reticulatum, and we identified 10 major and 3 minor compounds using GC/MS and NMR analysis. We also detected 30 additional unidentified minor amide components, many of which were restricted to one or a few plant parts. Seeds had the highest concentrations and the highest diversity of amides. Fruit pulp had intermediate concentrations and diversity that decreased with ripening. Leaves and roots had intermediate concentrations, but the lowest chemical diversity. In addition, to investigate the potential importance of amide concentration and diversity in plant defense, we measured leaf herbivory and seed damage in natural populations, and examined the relationships between amide occurrence and plant damage. We found no correlations between leaf damage and amide diversity or concentration, and no correlation between seed damage and amide concentration. The only relationship we detected was a negative correlation between seed damage and amide diversity. Together, our results provide evidence that there are strong selection pressures for fruit and seed defense independent of selection in vegetative tissues, and suggest a key role for chemical diversity in fruit-frugivore interactions.

The known prenylated benzoic acid derivative 3-geranyl-4-hydroxy-5-(3″,3″-dimethylallyl)benzoic acid (1) and two new chromane natural products were isolated from the methanolic extract of the leaves of Piper kelleyi Tepe (Piperaceae), a midcanopy tropical shrub that grows in lower montane rain forests in Ecuador and Peru. Structure determination using 1D and 2D NMR analysis led to the structure of the chromene 2 and to the reassignment of the structure of cumanensic acid as 4, an isomeric chromene previously isolated from Piper gaudichaudianum. The structure and relative configuration of new chromane 3 was determined using 1D and 2D NMR spectroscopic analysis and was found to be racemic by ECD spectropolarimetry. The biological activity of 1-3 was evaluated against a lab colony of the generalist caterpillar Spodoptera exigua (Noctuidae), and low concentrations of 2 and 3 were found to significantly reduce fitness. Further consideration of the biosynthetic relationship of the three compounds led to the proposal that 1 is converted to 2 via an oxidative process, whereas 3 is produced through hetero-[4+2] dimerization of a quinone methide derived from the chromene 2.