Unravelling patterns of taxonomic and functional diversity of Amazon stream fish

Environmental filtering and dispersal limitation are essential processes affecting the variability of ecological communities. However, their relative contributions are highly debated and remain largely unknown in several systems, such as the hyperdiverse Amazon Basin. We determined the relative role of local, catchment, spatial, and biogeographical variables on the taxonomic and functional alpha and beta diversity of stream fish. We sampled 54 streams across six river basins in the Amazon. For each stream, we obtained 35 local habitat variables and 11 climate-catchment variables. Watercourse distances and basin identity were converted into spatial and biogeographical variables, respectively. We found that taxonomic alpha diversity responds both to spatial and local predictors, whereas its functional counterpart was mainly associated with local variables. Biogeography was the main factor structuring taxonomic and functional beta diversity, with a secondary contribution of catchment and local variables. Locally, substrate type and isolation determined the number and relative abundance of species and traits. The shared variation between biogeographical and environmental variables, especially altitude and slope, were strongly associated with beta diversity patterns, indicating a joint role of habitat filtering and dispersal limitation. Our results show the need to include both spatial and environmental variables in studies of stream communities at large spatial scales, as they are related to distinct processes that regulate community structure. We also encourage future studies to account for the responses of multiple facets of biodiversity to different drivers, as they provide essential and complementary information for community ecology and biogeography.