An experimental test of dispersal limitation of species diversity in eelgrass meadows

Biogenic habitats, including eelgrass, support productive and diverse species assemblages in spatially heterogeneous seascapes. Eelgrass-associated faunal diversity depends not only on conditions within each eelgrass meadow but also on their probability of dispersing among eelgrass meadows in a seascape in which meadows are separated by soft sediments, kelp or rocky habitats. Experiments are required to establish dispersal limitation in communities. Here, we experimentally tested the hypothesis that dispersal limitation and edge effects influence local faunal diversity in eelgrass meadows. We manipulated habitat spacing and exposure of standardized substrate and quantified invertebrate colonization. We predicted that invertebrate species richness would decline with increasing distance from the meadow edge and that mobility traits would covary with species’ responses to distance. We also predicted that eelgrass edges would harbor communities distinct from the interior of the meadow and that these differences would covary with species’ trophic traits. We found that species richness decreased with increasing distance from meadows and this pattern was not explained by reduced abundance or filtering of species based on traits, consistent with dispersal limitation of diversity. Edges did not affect total community richness or abundance, and the only trait that varied with meadow-position was average body size, which was larger within the meadow interior. These results suggest that even short-distance dispersal limitation can structure the eelgrass invertebrate community but that communities of similar richness, trait profiles and abundance will colonize all available habitat in the absence of dispersal barriers.