When Soils Cascade: Effects of Soil Resource Availability on Interactions between Plants,Herbivores, and their Natural Enemie

Soil resource heterogeneity is understudied as a source of variation in herbivore community size and structure across natural landscapes. This project extends predictions of resource availability hypotheses to understand how soil variation shapes herbivore communities. In particular, we explore how soil type mediates bottom-up effects of plant quality and top-down processes of parasitism and predation to jointly shape herbivore assemblages and structure of plant-herbivore ecological networks.;We use four dominant shrub genera (Adenostoma, Arctostaphylos, Ceanothus, Quercus) that grow across a natural mosaic of low (serpentine) and high resource (non-serpentine) soils to explore variation in plant resistance, and ultimately determine whether such differences scale up to alter abundance and diversity of herbivores. We show that growing on serpentine soils increases plant resistance, as measured by herbivore performance, both within and across host plant species. We then collect over 2,300 larval Lepidoptera of 80+ species over two years (2014/2015) to show that these differences in plant resistance are associated with less abundant and diverse herbivore assemblages across natural landscapes.;Next, we explore whether soil type not only shapes herbivore abundance and diversity, but also the structure of their interactions with host plants. To do this, we used our 2,300 herbivore observations to build plant-herbivore ecological networks for each soil type. We then quantified modularity, a network structural property considered important to stability and persistence of antagonistic networks, for each soil network. We show that generalist herbivore species narrow their diet breadth in more resistant serpentine plant communities, increasing modularity of the plant-herbivore network on low-resource soils. These results are consistent across both years of study, suggesting that greater modularity -- and thus, greater stability -- may be an invariant property of plant-herbivore networks on low resource soil types.;We then asked whether soil type influences the strength of parasitism or predation experienced by herbivores. Soil-mediated plant resistance may influence top-down effects by prolonging herbivore growth, and thereby increasing temporal vulnerability to enemies, or by altering herbivore abundance on plants, which can shape density-dependent predation. We quantified the strength of parasitism by rearing 353 parasitoids from the field-collected larvae mentioned above. We measured bird predation by constructing exclusion cages around 48 Ceanothus and Adenostoma plants across soil types, and measuring change in herbivore abundance between caged and control plants. We found that soil type influenced parasitism and predation rates, but that effects were contingent on host plant genus and on diet breadth of the herbivore species.;In sum, this project finds pervasive effects of soil type on both bottom-up and top-down processes shaping herbivore communities. We find a strong association between serpentine soil and plant resistance, with ramifying effects on herbivore abundance, diversity, and network modularity. We also find effects of soil type on the strength of parasitism and predation, though these appear highly context-dependent. Together, these findings suggest that soil resource variation may be an important, yet underappreciated driver of herbivore communities across heterogeneous landscapes. In addition, this study documents new host plant associations (36 plant genera) and larval morphologies (16 species), contributing important natural history information to our understanding of larval Lepidoptera in the California chaparral.