Scale-sensitive stream slope drives nested fish trait-based diversity

The stream slope influences the diversity of stream fish, as it is an important riverscape characteristic related to flow energy, substrate size, and channel morphology. These conditions limit the movement and local persistence of species across the network, affecting local and regional species composition. Therefore, we can predict that streams with steep channels and harsh flow conditions would select a restricted set of species and ecomorphological traits, whereas those with gentle slopes would allow more species to coexist. This would cause a nested pattern in which the species and trait composition of streams with steep slopes (e.g., high swimming capacity) are a subset of those with gentle slopes (e.g., varied swimming capacities). We evaluated the influence of stream slope on a fish metacommunity, investigating the response of trait-based diversity to the stream slope for 21 fish assemblages sampled in two subtropical watersheds of Brazil. We found that trait-based alpha diversity was negatively related to slope. Moreover, stream sites with steep channels showed a subset of species and traits present in streams with gentle slopes. Finally, the spatial scale at which slope was measured had different effects on fish diversity, and the strongest effect was observed when the slope was estimated using the entire channel. Thus, we suggest caution in choosing the spatial scale, as the local slope may not be a good representation of the riverscape characteristics that drive the taxonomic and trait diversity of local fish. The stream slope is an important driver of fish diversity in streams by filtering species traits and determining species occurrence, while nestedness is a relevant pattern emerging from differences in stream gradient among sites.