The multifactorial effects of dispersal on biodiversity in environmentally forced metacommunities

Disentangling the effects of dispersal and environmental heterogeneity on biodiversity is a central goal in ecology. Although metacommunity structure can be partitioned into spatial and environmental fractions, it remains unclear whether these statistical results can be used to infer the relative importance of dispersal limitation (spatial fraction) and environmental forcing (environmental fraction). Using an environmentally forced, spatially explicit metacommunity model, we show that the distinct effects of the mean (advection) and the standard deviation (diffusion) of the dispersal kernel on biodiversity are not easily detectable via variation partitioning alone. Although increasing dispersal ultimately leads to a decrease in the spatial fraction due to reduced dispersal limitation and greater species sorting, the magnitude of the spatial fraction depends on the complex interplay between the nature of dispersal and the type of boundary conditions in the metacommunity. Indeed, metacommunities characterized by either high or low dispersal can exhibit a small spatial fraction. A case study of a marine metacommunity experiencing strong alongshore transport is consistent with these findings, as the size of the spatial fraction is not associated with dispersal. Overall, our results suggest that accounting for the nature of environmental forcing as well as the multifactorial effects of dispersal is critical for understanding how ecological and environmental processes give rise to biodiversity across spatial scales.