Spatial scale, reservoirs and nonnative species influence the homogenization and differentiation of Great Plains—Rocky Mountain fish faunas

Reservoir construction and the introduction of nonnative species are major anthropogenic drivers of biotic change in freshwater ecosystems. To understand the influence of these drivers, we quantified the degree to which fish faunas have either homogenized or differentiated at multiple spatial scales across the Great Plains—Rocky Mountain continuum (Wyoming, USA), given that homogenization processes are scale-dependent. Homogenization was most prevalent at the largest scale, with an average increase in similarity of 6.8% among river basins. At an intermediate scale, sub-basins with reservoirs had homogenized faunas in comparison to sub-basins without reservoirs, which were more differentiated. Differentiation was dominant at the smallest scale with a 7.8% average decrease in similarity among individual sampling sites. Reservoirs had only localized homogenization impacts along stream systems, and homogenization was greater for streams connected to large reservoirs. Large-sized streams appeared to trend towards homogenization, whereas small and medium streams trended towards differentiation. Reservoirs altered fish faunas from historical conditions, but did not result in cross-stream homogenization because of the idiosyncratic nature of reservoir introductions that reflect environmental gradients and socio-economic factors. Our results provide insight into how spatial scale, reservoirs, and nonnative species interact to influence the degree of homogenization and differentiation.