Relative effects of local- and landscape-scale environmental factors on stream fish assemblages: evidence from Idaho and Ohio, USA

Despite increased attention to spatial considerations in catchments, the importance of both environmental factors and spatial context in understanding stream fish assemblages is not fully resolved. This study explored the relative influences and spatial relationships of environmental factors at landscape (i.e., catchment) and local (i.e., reach) scales on characteristics of stream fish assemblages including species richness (S), diversity (H', even-ness, and 1/D), density and biomass, and composition (% top carnivores and % benthic insectivores). We conducted this research in two geographic regions (northern Idaho and Ohio, USA) characterized by distinct environmental attributes. Using a partial constrained ordination approach, we found that pure spatial factors explained 26 % of fish assemblage variation in Ohio (OH) and 18 % in Idaho (ID). Shared (spatially-structured) environmental variables explained more variation in fish assemblages in ID (28.6 %) than in OH (20.7 %). The influence of pure (non-spatial) environmental characteristics accounted for nearly 24 % of the variation observed in fish assemblages in OH catchments, whereas pure environmental factors accounted for 31 % of assemblage variation in ID. Within the pure environmental component, the influence of local, landscape, and joint (i.e., combined effects of environmental variables from both spatial scales) effects accounted for relatively equal amounts of assemblage variation for both study regions. However, local-scale variables were slightly more important in ID, whereas joint influences were more important in OH. Taken as a whole, our results suggest that a complex suite of spatial and environmental factors influenced fish assemblages but that the relative importance of these components differed in each of our study areas. The strong influence of spatial patterns in our results reveals the importance of integrating spatial context into studies predicting fish assemblage characteristics. The contrasting findings between geographic regions encourage additional research efforts that address regional variability in multi-scale environmental influences on fish assemblages.