Variation in soil resources is a ubiquitous form of environmental heterogeneity, with strong direct and indirect effects on organismal traits and species interactions. 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) do lower-resource soils support depauperate herbivore communities; 2) do these herbivore communities experience greater parasitism rates; and 3) how does soil context alter modularity (network-level specialization) of the tri-trophic plant-caterpillar-parasitoid ecological network. 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 larvae in the lab until emergence of an adult moth or parasitoid. Finally, we built interaction networks between plants, herbivores, and parasitoids to explore structural changes across soil context.
Results/Conclusions Results of this study suggest that herbivore communities are less abundant and diverse in low-resource soil contexts. This result held across two interspecific shrub pairs as well as an intraspecific comparison, suggesting that low-resource soils may affect herbivore communities similarly on both evolutionary and ecological time scales. When we examined parasitism, we found that herbivores experienced equal parasitism rates across soils, despite differences in larval abundance. Finally, we found that low resource serpentine soils supported tri-trophic networks of more modular structure than higher resource non-serpentine soils, due to reduced partner diversity of generalist species in serpentine. Because modular antagonistic networks are considered more stable to perturbation, this result indicates that low resource soils may support more robust interaction webs. Together, these results suggest that abiotic resource availability shapes not only composition of trophic communities, but how links are distributed between interacting species. By scaling these processes up to entire trophic webs we can make predictions of how community-level properties, such as resilence and stability, may vary across heterogeneous landscapes.