For almost two decades, ecological studies have addressed the importance of plant species diversity for associated animal diversity and the functioning of ecosystems. Recently, a burgeoning focus of research in ecology is on how population-level diversity scales up to affect patterns and processes at the community- and ecosystem-level. In this dissertation, I present results from a series of common garden experiments in which I manipulated genotypic diversity of tall goldenrod (Solidago altissima) to address a suite of questions about how intraspecific variation in a dominant old-field plant species shapes communities of associated arthropods and ecosystem processes. In these studies, I found that host-plant genotypic diversity had non-additive effects on insect herbivore and predator diversity and that incorporating temporal dynamics into community genetics studies is essential for predicting how different community members perceive and respond to genetically variable host-plant traits. I found that variation among host-plant genotypes had strong effects on the diversity and composition of foliage-based arthropods, but only weak effects on litter-based microarthropods. Additionally, I found strong effects of instraspecific genetic variation in goldenrod functional traits on primary productivity, litter quality, decomposition rate, nitrogen release, and community invasibility. Together, my results indicate that within-species variation is an important, but all to [sic] often overlooked, influence on the structure and dynamics of communities and ecosystems
