Beyond a single patch: local and regional processes explain diversity patterns in a seagrass epifaunal metacommunity

Abstract Ecological communities are jointly structured by dispersal, density-independent responses to environmental conditions, and density-dependent biotic interactions. Metacommunity ecology provides a framework for understanding how these processes combine to determine community composition among local sites that are regionally connected through dispersal. In 17 temperate seagrass meadows in British Columbia, Canada, we tested the hypothesis that eelgrass (Zostera marina L.) faunal assemblages are dispersal limited, such that meadows separated by greater spatial distances host distinct epifaunal invertebrate fauna. We used hierarchical joint species distribution modelling to understand the relative contribution of environmental conditions, spatial distance between meadows, and unexplained variation on species distributions. We found that modelled species compositional similarity did not decay as a function of spatial distance, and sites in the same region were no more similar to each other than sites farther away. These results suggest that dispersal limitation of these invertebrates does not strongly influence diversity over 1000 km of coastline. Abiotic environmental conditions such as temperature, nitrate levels, and salinity partially explained regional patterns of abundance, but their relative importance varied across taxa. We found co-occurrence patterns between taxa that could not be explained by shared responses to the environment, suggesting the possibility of interspecific interactions influencing community structure. Our results support findings from previous studies that seagrass faunal communities tend to be somewhat homogenized across regional spatial scales even up to 1000 km. Minor variations in community structure across sites could be attributed to the abiotic environment, or intraspecific interactions at the site or meadow-level spatial scale. Our results give refined hypotheses about the specific ecological mechanisms driving community structure at various spatial scales, which in turn can inform targeted experiments, monitoring, and management decisions for maintaining diversity in this important ecosystem.