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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 …