Fish assemblages on two continents

Aim: Fish assemblages –whether defined by taxonomy or functional traits—respond to regional and local habitat variation. We sampled rivers of Mongolia and the western United States (US) to determine the scale at which habitat could predict fish assemblage variation, classified by taxonomy or functional traits. Our hypothesis was that fish assemblages could be predicted using valley-scale hydrogeomorphology and reach-scale hydrology. We further predicted that if valley-scale variables explained high variation in fish assemblages then reach-scale variables would explain additional dimensions.

Location: Mongolia, United States

Methods: We evaluated reach- and valley-scale hydrogeomorphology of rivers in the US and Mongolia in each of three ecoregions, grassland, forest, and endorheic. Fishes were collected using backpack electrofisher following standard protocols.

Results: Ordinations resulted in distinct assemblage patterns that corresponded with habitat variables at both valley- and reach-scales. Hydrogeomorphology differed for Mongolia and US rivers and likely contributed to different patterns that explained fish assemblage variation classified by taxonomy vs. traits. Ecoregions differed in factors contributing to fish assemblage patterns, likely a result of differences in hydrogeomorphology and historical influences, as well as effects of introduced species in the US.

Main Conclusions: We found that fish assemblages were structured by hydrogeomorphic processes occurring at valley- and reach-scales, and that variables predicting fish assemblages vary with scale, ecoregion, and continent. We found a common pattern where if valley-scale variables provided high explanation of fish assemblages, then reach-scale variables frequently explained more ordination dimensions than valley-scale variables. This implies that reach-scale hydrology variables are always strong predictors of fish assemblage variation, and valley-scale geomorphology variables are sometimes strong predictors. We found evidence that introduced species or anthropogenic impacts modified our analyses predicting fish assemblage variation of Mongolia and US mountain steppe rivers. Although anthropogenic impacts were substantially higher for western US rivers than for Mongolia rivers, we were unable to detect strong differences in our ability to predict fish assemblage variation from reach- and valley-scale habitat variables.