Intraspecific trait variation structures species-interaction networks : a case study with the host plant Salix hookeriana

The strength and organization of species interactions determine the structure and dynamics of ecological communities. However, interactions do not occur between species per se – interactions occur between individuals. A rising challenge for ecology is to understand how intraspecific trait variation scales up to affect community structure and dynamics. My dissertation research has taken an empirical approach to addressing this challenge. Specifically, I conducted common garden experiments with the host plant Salix hookeriana in multiple environments to study the contributions of host-plant genetic and environmental variation to functional trait variation and, in turn, the structure of ecological interactions with its associated community. In Chapter 2, I show that Salix hookeriana exhibited genetic variation in its resistance to a diverse community of herbivorous arthropods. Rather than there being a single key trait that explained herbivore responses, I found that a range of plant-growth and leaf quality traits mediated the responses of different herbivore species and feeding guilds. In Chapter 3, I show that the effects of willow genetic variation on a community of insect herbivores cascades up to affect interactions between these herbivores and their insect parasitoids. Direct and indirect genetic effects resulted in distinct compositions of multi-trophic interactions associated with each host-plant genotype. When I simulated the additive effects of genetic variation on food-web complexity, I observed a strong positive relationship, indicating that intraspecific genetic and phenotypic variation can play a key role in structuring ecological networks, which may in turn affect community persistence. In Chapter 4, I show that host plant genotypic effects on the structure of foliar arthropod and ecotomycorrhizal communities were strong, despite variability in the biotic (ant-aphid interactions) and abiotic (wind exposure) environment of a coastal dune ecosystem. Taken together, my dissertation suggests that heritable trait variation can play a key role in shaping the structure of species-interaction networks. By linking the ecological consequences of heritable trait variation to network structure, my dissertation paves the way for empirical and theoretical studies of the interplay between ecological and evolutionary processes in affecting the maintenance of biodiversity.Science, Faculty ofZoology, Department ofGraduat