Relevance of spatial factors is scale-dependent in shaping stream fish assemblages

Understanding the main processes shaping the assemblage of fish species of basin headwaters is still challenging. We aimed to assess the effect of spatial factors at two different scales and the effect of environmental factors and dams on the stream fish assemblage in the Upper Paraná Basin. We performed a partial redundancy analysis (pRDA) to compute the effect of broad and fine-scale spatial and environmental factors on the fish assemblage. The model explained 25% of the total variation of fish assemblage in the region (environmental plus spatial effects), with 5% of the variation of fish composition explained purely by environment, 10% purely by broad-scale spatial variables, and 3% purely by fine-scale spatial variables. Dams did not explain the stream fish assemblage when taking into account the spatial range studied. Our results show the relevance of isolation in the spatial configuration of the entire drainage basin to understand the primary processes shaping stream fish assemblages. At the same time, local environmental filters seem not to be strong enough to generate species sorting at broad scales in the basin, although they become more relevant at finer scales. Therefore, different processes are coinciding in different intensities and at different scales, which explains the stream fish assemblage in the region.