Understanding how abiotic heterogeneity shapes variation in complex, multi-trophic networks is an exciting new frontier in ecology. One ubiquitous source of abiotic variation that could shape network structure is soil resource availability, which can alter traits and interactions of species across trophic levels. Multiple theories address the relationship between resources and trophic interactions: the Resource Availability Hypothesis (RAH) suggests that resource availability shapes plant traits and, consequently, their quality to herbivores; in turn, the Slow Growth, High Mortality (SGHM) and High Performance, High Mortality Hypotheses (HPHM) relate plant quality to interactions between herbivores and natural enemies. The objective of this work is to unify these theories by exploring how soil resource variability drives trophic interactions, and extend predictions to emergent properties of ecological networks. Specifically, we asked 1) are plants in lower-resource soils more resistant to herbivores; 2) do lower-resource soils support more specialized plant-herbivore interactions; and 3) do these changes ramify to the third trophic level? To address these questions we sampled the diverse, dominant, and heretofore understudied assemblage of larval Lepidoptera and parasitoid enemies associated with California chaparral, across a natural soil fertility mosaic of serpentine (low resource) and non-serpentine (higher resource) soils. Using a transect-based design we collected caterpillars from 150 Ceanothus, Arctostaphylos, Quercus, and Adenostoma shrubs in serpentine and non-serpentine soils, rearing ~80 species of Lepidoptera and the ~40 species of parasitoid wasp and fly they host. Finally, we built interaction networks between plants, herbivores, and parasitoids to explore structural changes across soil context.
Results of this study suggest that shrubs on low-resource soils are generally more resistant to herbivores than neighboring conspecific and congeneric plants. This more resistant plant community in turn supports more specialized plant-herbivore interactions, due to a shift in realized diet breadth by generalist herbivores: generalist species consume fewer host genera in low-resource soil patches, effectively becoming more specialized and creating a more specialized, or modular, plant-herbivore network. Differences in plant-herbivore network structure are paralleled at the third trophic level, with more specialized herbivore-enemy networks in low-resource serpentine soils. Together, these results suggest that effects of abiotic resource availability are not limited to traits and composition of organismal communities, but can ramify to alter how links are distributed between interacting species across multiple trophic levels.