Community assembly in subtidal epifaunal communities in the Gulf of Maine: A phylogenetic and functional trait approach

Abstract

How organisms assemble to form a community is a central question in ecology, with commonly inferred mechanisms including environmental filtering, competitive exclusion, and random processes. In community phylogenetics, phylogenetic clustering is often interpreted as evidence of environmental filtering, where closely related species with similar traits are favored under particular environmental conditions, whereas phylogenetic overdispersion may indicate competitive exclusion, as coexisting species are more distantly related than expected by chance. Community phylogenetic and functional trait analyses have been used to identify the relative importance of these processes, but most studies have focused on terrestrial angiosperm systems consisting of a single taxonomic group (e.g., a single plant class). Here, community phylogenetic and functional trait analyses are applied to multiphyletic subtidal epibenthic invertebrate assemblages at eight sites in the Gulf of Maine to investigate the relative importance of deterministic or stochastic forces in structuring these communities. At local sites, some communities were phylogenetically overdispersed, suggesting community assembly consistent with competitive exclusion. Five traits exhibited phylogenetic signal, indicating that phylogeny can serve as a proxy for ecological similarity among species for these traits. Functional trait diversity was overdispersed for one site and clustered for another, suggesting local assembly via competitive exclusion and environmental filtering, respectively. Regionally, taxonomic beta diversity was less than expected by chance but phylogenetic and functional trait beta diversity were greater than expected, indicating that species are not dispersal limited and that different environmental conditions among sites select for different species. These results suggest that subtidal epibenthic invertebrate assemblages in the Gulf of Maine are structured by deterministic forces and that different assembly processes operate across spatial scales: Locally, competitive exclusion appears to be important, whereas regionally environmental filtering may play a role.