The field of community ecology is fundamentally concerned with the assembly and maintenance of diversity across space and time. Two of the most fundamental questions in the field, then, are 1) why do we see variation in composition and diversity across space and time, and 2) how are diversity and assemblage structures maintained? A common model for beginning to understand these questions is the idea of ecological “filters” that restrict species from a regional pool. Different kinds of filters apply different kinds of selective pressures, and because species’ traits are what allow them to pass through filters, studying the distributions and dispersion of traits within the community can help us understand how these filters act on the species pool. A variety of factors may cause communities to have traits that are overdispersed – more disparate than expected by chance – or underdispersed or clustered – more similar than expected by chance.My dissertation attempts to address these fundamental questions in communities associated with eelgrass (Zostera marina) – an herbaceous marine angiosperm (seagrass) that forms monospecific beds across nearly 40o of latitude in the northern hemisphere. Eelgrass is home to a diversity of epifaunal invertebrate mesograzers – animals that live on the leaves of the plant and feed on fouling microalgal epiphytes, as well as macroalgae and fresh and decaying eelgrass tissue. Peracarid crustaceans are one of the most abundant and diverse of the mesograzers. These crustaceans – amphipods, isopods, and their relatives – are found worldwide and are especially susceptible to predation by the diverse suite of resident and juvenile fishes that also call eelgrass beds home. My first chapter draws on data from a global experimental network to examine how the functional structure of eelgrass peracarid communities varies across space and with different ecological filters. I found that dispersion strongly increased with increasing predation and decreasing latitude – communities at low-latitude sites and those that experienced high predation intensity were more overdispersed than those at high latitudes and with low predation intensity. Ocean and epiphyte load appeared as secondary predictors; Pacific communities were more overdispersed while Atlantic communities were more clustered, and increasing epiphytes were associated with increased clustering. Together these results point to the importance of both biotic interactions and the historical legacies of distinct species pools in structuring communities. My second chapter narrows in on eelgrass beds in Northern California to investigate the role of diverse suites of predator (fish) community traits as ecological filters that drive patterns of dispersion in prey (peracarid) communities. Fish traits related to prey detection and capture selected for more overdispersed peracarid communities, particularly with respect to body size and activity level, suggesting that prey may be pushed to disparate areas of trait space to avoid consistent detection by predators across the community. I also found correlations between the trait dispersions of predator and prey communities that strengthened after accounting for the effects of habitat filters on predator dispersion, suggesting that habitat filtering effects on predator species pools may hinder their ability to affect prey community assembly. Specific predator traits may have measurable impacts on the community assembly of prey, inviting experimental tests of predator trait means on community assembly, and explicit comparisons of how the relative effects of habitat filters and intraguild competition on predators impact their ability to affect prey community assembly. Finally, my third chapter returns to the global experimental network from the first chapter to address more coarse-scale patterns of community structure beyond peracarids. This time, I examined in epifauna communities dominated by peracarids and gastropods. The abundance of these two taxa exhibited a strong latitudinal cline in turnover, with gastropods abundant at high- latitude sites, and peracarids abundant at low-latitude sites, especially in the Atlantic. This pattern appeared to be driven by greater eelgrass genetic diversity at lower latitudes, which strongly influenced both the richness and abundance of peracarids, but less so for gastropods. The two taxa exhibited functional complementarity, and so global variation in genetic diversity led to geographic variation in the distribution of functional traits across the range of eelgrass. These results add to a growing body of literature that suggests that variation in traits underlaid by genetic differences within species has important bottom-up consequences for assemblage variation and ecosystem function across broad spatial scales.