Relative Effects of Local- and Landscape-Scale Environmental Factors on Stream Fish Assemblages: Evidence from Ohio and Idaho, USA

Despite increased attention to spatial considerations in watersheds, the relative influence of abiotic factors on stream fish assemblages across spatial scales and in the context of contrasting human footprints is not fully understood. This study explored the relative influences and spatial relationships of abiotic environmental factors at landscape (i.e., watershed) and local (i.e., reach) scales on characteristics of stream fish assemblages including species richness (S), diversity (H’ and 1/D), productivity (density – no. m^-2 and biomass – g m^-2), and the representation of major feeding guilds (percent top carnivores and percent benthic insectivores) in two geographic regions (northern Idaho and Ohio, USA). Using a partial constrained ordination approach, we found that multiscalar environmental variables explained a similar magnitude of the overall variation in fish assemblage characteristics across both study regions (Ohio – 68.3% and Idaho – 69.6%) although the relative effects of spatially-explicit factors differed. Landscape-scale factors accounted for the greatest amount of variation (36.2%) in fish assemblages in Ohio watersheds, with human-dominated land-use and land-cover factors driving the relationship.  In contrast, a mixture of landscape-scale (17.2%) and local-scale (24.9%) factors accounted for variation in fish assemblages from Idaho streams.  Additionally, relative spatial position and spatially-structured environmental variables accounted for substantially more variation in fish assemblages in ID than in OH.  Taken as a whole, our results suggest that the relative importance of watershed- and reach-level environmental factors, as well as the contribution of spatial influences, were different in each of our study areas, supporting regionally-specific conservation and management strategies. The contrasting results between geographic regions encourages additional research efforts that address the potential role that different levels of anthropogenic disturbance may play in constraining fish assemblages across multiple spatial scales.