Host-plant genetic variation structures gall-parasitoid networks
Theory predicts that a modular network structure (i.e. groups of species that frequently interact with each other) and increased connectance (i.e number of links between species over the number of possible links) promotes food web stability and persistence. However, interactions within food webs ultimately occur between individuals, not species, and it remains unclear how intra-species variation contributes to the structure of ecological networks. In this study, we examined how genetic variation within a dominant host-plant, Salix hookeriana, influenced gall-parasitoid networks. We surveyed 26 unique willow genotypes growing in a common garden for their galls and associated parasitoids to build a quantitative food web of gall-parasitoid interactions for each genotype. Additionally, we measured gall morphology, as well as a suite of host-plant architectural, physiological, and chemical traits to elucidate the underlying mechanisms by which willow genetic variation affects gall-parasitoid assemblages.
We found that willow genotypes varied more than three-fold in the diversity and abundance of both galls and parasitoids, resulting in variation in gall-parasitoid network connectance. Moreover, differences in gall and parasitoid community composition led to distinct network clusters, or modules, among host-plant genotypes. Gall community composition and gall size were associated with willow productivity, leaf water content, and leaf nutrient content, whereas the composition of parasitoids were related to the size and identity of galls, as well as plant architectural complexity. Together, our results indicate that genetic variation plays a key role in the assembly of ecological networks and that incorporating individual variation will further the understanding of factors governing the structure and dynamics of food webs.