An appreciation for intraspecific variation is central to the study of evolutionary biology, however community ecologists have often assumed equivalence among conspecific organisms. In recent years, intraspecific genetic variation, particularly in plants, has received more attention, and has been linked to variation in plant-associated biotic assemblages and ecosystem function. However, in many cases, the phenotypic variation associated with influential genetic variation is unknown. In particular, which traits represent ecological filters that can shape assembly of associated biota is poorly understood for many plant species. The research presented here addresses this gap in knowledge through observational studies and experiments conducted using three focal plant species. I begin with a brief summary of relevant theory and the state of knowledge regarding the effect of plant traits on arthropod and microbial communities. In Chapter One, I ask how variation in alfalfa (Medicago sativa) affects the larval performance and oviposition preference of the Melissa blue butterfly (Lycaeides melissa), and if among patch variation in plant suitability can explain regional colonization patterns of alfalfa by the Melissa blue. In Chapter Two, I ask how intraspecific, and intra-individual variation among redwood trees (Sequoia sempervirens) can affect foliar fungal communities. In Chapter Three, I determine the relative influence of host-associated variation, abiotic conditions, and interspecific microbial interactions for shaping fungal endophyte communities in spotted locoweed (Astragalus lentiginosus). Finally, in Chapter Four, I return to alfalfa to ask which plant traits affect the diversity of arthropods and foliar fungi at small spatial scales. Each chapter provides evidence for the filtering effect of plant intraspecific variation on associated biotic communities. I conclude with a brief statement regarding the benefits of considering intraspecific variation in studies of community ecology and, more generally, the inferential opportunity hidden within ecological complexity
