Background/Question/Methods   Environmental factors at multiple spatial scales are known to influence stream fish assemblages. However, the relative influence of environmental characteristics across spatial scales is poorly understood. In particular, the potential disruptive effects of anthropogenic alterations likely alter fundamental scalar relationships between fish and their watershed environments. To explore the impact of anthropogenic disturbances on environmental drivers of stream fish assemblages across spatial scales, we assessed the relative importance of factors at the regional, landscape (i.e., watershed), and local (i.e., reach) scales on species richness, diversity (H' and 1/D), abundance (/m2), and biomass (g/m2) in 16 watersheds in Idaho and 16 watersheds in Ohio. We used the Akaike's Information Criterion (AICc) and information-theoretical model selection to evaluate multiple regression models related to the above question.

Results/Conclusions   Our results suggest that human-related impacts at the watershed scale drove patterns of diversity and species richness in more developed landscapes, as represented by our OH streams. In ID, where anthropogenic stressors were markedly less pronounced, regional and reach scale factors, were more important for determining fish assemblage characteristics. Specifically, in both regions (ID and OH), we found that patterns in assemblage richness and diversity were largely driven by variables describing stream size (e.g., drainage area and stream order). Regional factors including mean annual precipitation and latitude, in addition to local factors (e.g., geomorphic characteristics) explained a substantial portion of the variation observed in assemblage richness and diversity. In contrast, watershed-scale variables, especially pertaining to urban and exurban land use, were the predominant predictors of OH fish assemblage characteristics. In conclusion, our results offer evidence that human-induced changes to the physical environment may significantly alter the influence of scale-related factors considered critical in shaping stream fish communities.