Environmental heterogeneity and dispersal limitation explain different aspects of β‐diversity in Neotropical fish assemblages

Abstract From a metacommunity perspective, patterns of β‐diversity emerge from the interplay between spatial and environmental processes. For obligate aquatic dispersers, river network constraints can be reflected in diversity patterns. Here, we aimed to determine the effects of environmental heterogeneity and spatial connectivity on β‐diversity components. We sampled fish assemblages in 21 sites across a longitudinal gradient. Beta‐diversity for taxonomic and functional composition among sites was decomposed into its turnover and nestedness components. We investigated whether environment was more important to functional β‐diversity and spatial factors had a higher contribution to dissimilarity in species composition. Further, we tested whether environmentally homogeneous sites with higher spatial connectivity showed lower compositional changes. We found that spatial factors were more important for taxonomic β‐diversity, whereas environment explained functional dissimilarity among habitats. Although environment and spatial factors contribute to total β‐diversity, they explained different components: while environment explained a higher portion of turnover, spatial factors were related to nestedness. The effects of mere spatial isolation and directional connectivity differed for species and functional β‐diversity and for the components of β‐diversity. Including directionality in spatial connectivity enabled the explanation of a higher proportion of variation of total species β‐diversity and its turnover. Our results suggest that niche and spatial processes may influence differently taxonomic and functional β‐diversity components. Thus, habitat filtering was the primary mechanism affecting functional diversity and species turnover, whereas spatial connectivity drove species nestedness.