Disentangling Environmental, Spatial and Niche Influences on Fish Beta Diversity Along the Sandy Beaches of an Amazonian River

Beta diversity is an important descriptor of community structuring and it plays a crucial role for understanding the patterns and processes shaping biodiversity at multiple spatial extents. We investigated the influence of local, spatial, landscape factors and niche characteristics on fish beta diversity along the Acre River in Acre, Brazil. Fish were sampled at 31 sites along the riverbank, each spaced 10 km apart (measured fluvial distance) from June to September 2017. We quantified a set of environmental variables to represent the physical and limnological characteristics of each site, landscape variables describing the surrounding land‐use composition and spatial structure was defined by fluvial distances among sampling sites. We calculated total beta diversity, the local contribution to beta diversity (LCBD), LCBD based on abundance (LCBD ab ) and presence‐absence data (LCBD pa ), as well as the individual species contribution to beta diversity (SCBD). Species niche breadth (generalist and specialist species) and niche position (marginal and non‐marginal species) were estimated using the Outlying Mean Index (OMI). We used beta regression models to assess the influence of local environmental, spatial and landscape variables on LCBD ab and LCBD pa , whereas the influence of niche breadth and niche position was evaluated on SCBD. Ecologically different beaches occur in the upper river, influencing local patterns of fish diversity. Distance negatively influenced LCBD ab values, while depth of the area had a positive effect. For LCBD pa , only distance had an influence, which was negative. Niche position negatively influenced SCBD, whereas niche breadth had a positive effect. These results indicate that generalist and non‐marginal species contribute disproportionately to fish beta diversity, highlighting the importance of conserving predominant habitat types within the river. Our findings suggest that indices considering the ecological uniqueness of individual sites can guide biodiversity conservation efforts, protecting both local and riverine diversity by maintaining connectivity between habitats and species' adaptations.