Connectivity and vagility determine beta diversity and nestedness in North American and European freshwater fish

Aim This study assesses the extent to which biogeographic patterns in freshwater fish beta diversity and nestedness are due to dispersal limitation, from differences in dispersal opportunity across geographic areas and in dispersal ability across species. Location Europe and the Atlantic and Pacific realms of North America ( NA ), east and west of the Continental Divide. Methods The effects of glaciation, realm shape, connectivity, current climate and vagility on regional‐level beta diversity and nestedness were investigated. Turnover and nestedness‐resultant dissimilarity components of beta diversity and the nestedness metric based on overlap and decreasing fill ( NODF ) were calculated from regional species lists and the contributions of turnover and segregation to nestedness structure quantified. Results Geographic distance was a stronger predictor of beta diversity than climatic and environmental heterogeneity distances. Species range shapes varied with postglacial colonization direction, being more extensive in an east–west direction in Europe than in Atlantic NA . Turnover increased with declining connectivity, in unglaciated areas, and in non‐migratory species. Species were significantly less nested than expected because of high turnover and within‐realm heterogeneity in regional faunas. Deviations from nestedness were greater in unglaciated areas and in migratory species. Non‐migrants, but not migrants, exhibited coincident range boundaries. Main conclusions Spatial trends in beta diversity and nestedness in freshwater fish in NA and Europe result primarily from differences in postglacial recolonization opportunity across realms and in dispersal ability across species. Multiple metrics are necessary to identify the processes determining the spatial structure of species assemblages.