Intraspecific phytochemical variation as mosaics of defense in tropical forests
AuthorGlassmire, Andrea Elizabeth
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This dissertation research contributes to understanding the causes of phytochemical variation in natural ecosystems and the effects of that variation on herbivores. Phytochemically mediated interactions are common in biotic communities, and include important associations between plants and herbivorous insects. Variation in chemistry within a host plant species has been a focus of plant defense theory because increases in concentrations or complexity of phytochemical mixtures can influence the development, diversity and evolution of herbivores by creating mosaics of defense that regulate the densities of herbivores across a geographic landscape. To test hypotheses relevant to plant defense theory, I examined determinants of phytochemical variation and whether intraspecific variation creates mosaics of chemical defense for associated arthropod communities. Here, I review theory on plant defense and summarize clear knowledge gaps then I propose the Soil Mosaic Hypothesis, which posits that multi-trophic diversification can result from soil heterogeneity across the geographic landscape. The empirical data presented in this dissertation include three studies that address the causes and consequences of plant defense for a tropical shrub, Piper (Piperaceae) and associated insect communities. First, I compared the magnitude of effects of resource availability versus herbivore-induced defenses on variation in defensive phytochemistry. Next, I documented patterns of phytochemical variation on community and population structure of specialist herbivores using one host plant species, P. kelleyi and the specialist geometrid caterpillars on that plant. Finally, I established paired P. kelleyi cuttings at different canopy heights across an elevational gradient to manipulate heterogeneity in the quantity and quality of light availability. For this final study, phytochemical variation was quantified and the effects of light on chemical defense were measured as a function of herbivory and development in specialist and generalist herbivores. I found that abiotic and biotic factors interact to influence the phytochemical profile of plants. In particular, light heterogeneity significantly contributed to changes in phytochemistry across small spatial scales and had cascading effects on specialist herbivores. Overall, subtle differences in phytochemical variation across the geographic landscape have consequences on the performance, diversity, and evolution of specialist herbivores and were undoubtedly part of the impressive diversification of herbivorous insects.