Differential responses of fish assemblages to environmental and spatial factors are mediated by dispersal-related traits in Neotropical streams

The organization of species metacommunities can be controlled by local factors, such as habitat quality, landscape factors, such as land use, and spatial factors, such as stream network distances. Species within a metacommunity have different traits that may be used to understand the determinants of community structure. This study aimed to understand how changes in environmental and spatial factors affected stream fish assemblages delineated based on their dispersal-related traits, such as preferential habitat use (i.e., benthic, nektobenthic, nektonic, marginal, and surface), preference for water velocity (i.e., fast, intermediate, and slow), and body size (i.e., small, medium, and large). We sampled 18 stream reaches in a dendritic network in the Upper Paraná River basin, located in the Brazilian Cerrado biome. We used variation partitioning techniques to test the relative effects of local environmental, landscape, and spatial variables on different species groups defined according to dispersal-related traits. Environmental and spatial variables weakly explained the variation in total assemblage taxonomic composition. However, based on different functional trait groups, we found that the environment had a strong relationship with assemblage composition of nektobenthic and small fish, whereas spatial variables were most strongly associated with marginal and surface fish. Large-scale spatial variables were associated with large fish and those that prefer slow water, whereas fine-scale variables were associated with small and surface-dwelling fish. These findings help improve understanding of how functional composition of stream fish assemblages may be affected by urban development, riparian conversion, and resulting changes in local environmental conditions.