Direct and indirect effects of host food quality on host life history, host susceptibility to parasitism, and parasitoid life history

Abstract

Ecological communities are complex, comprising species and environmental factors that are so entangled in their effects on one another that ecologists and evolutionary biologists will forever be mystified by how they are assembled and function. Against such complexity, we have come to understand that autotrophic resources can have large cascading impacts on higher trophic levels via direct interactions (the effect of one species or environmental factor on another). However, because of indirect interactions (between species or environmental factors that are mediated through direct interactions with other species or factors), the relationships and dynamics we expect in natural communities are often not observed. In this thesis, I investigate how food resources directly impact consumer life histories and how this direct interaction indirectly impacts tertiary consumers in an experimental resource-host-parasitoid community. First, I refine an artificial resource system, and develop a technique to transfer seed beetle (host) eggs laid on natural legume seeds to artificial seeds which set the stage for the two highly replicated experimental life history assays in this thesis. In the first study, I ask whether variation in host life history traits in response to food quality is consistent within and across stages of host development. Using a cross-sectional experimental approach combined with a stage-structured population model to estimate instar specific host vital rates, I show that food quality effects on host vital rates, growth and development are not consistent throughout ontogeny, suggesting host food quality may cascade to impact host susceptibility to parasitism and parasitoid life histories. In the second study, I ask whether host food quality indirectly cascades to impact host susceptibility to parasitism and parasitoid life histories. Using a similar cross-sectional approach, I show host food quality indirectly impacts host susceptibility to parasitism but has little to no effect on parasitoid life histories. Overall, my research shows that, despite large effects on host life histories, host food quality effects are markedly reduced in parasitoids in this system, emphasizing the need to consider specific species life histories when characterizing resource-host-parasitoid community relationships and dynamics, and whether or not resources cascade to impact higher trophic consumers.