Environmental filters of freshwater fish community assembly along elevation and latitudinal gradients

Abstract Aim The aim was to identify drivers of historical freshwater fish community assembly by testing for interactions between functional traits and two climatic gradients, namely elevation and latitude. Many studies conclude that environmental filtering is the dominant process of community‐wide trait convergence at high elevations and high latitudes, but a full understanding of which specific filters cause trait convergence is lacking. Location Great Plains–Rocky Mountains, USA. Time period 1960–2018. Major taxa studied Freshwater fish. Methods Using 2,539 stream fish community surveys documenting the native distributions of 51 species, we evaluated trait–environment relationships (trade‐offs) and trait–dispersion relationships (standardized effect sizes from null models) along elevation and latitudinal gradients in four functional trait categories. We tested whether elevation and latitude exhibited similar trait–environment and trait–dispersion relationships using four functional trait categories (life history, habitat, trophic and locomotion). We predicted that elevation and latitude would have similar relationships owing to shared environmental filters of cold stress and dispersal constraints. Results Life history was the only trait category to display evidence of increased environmental filtering along both gradients, with trait dispersion more constrained at high elevations and high latitudes. Trade‐offs revealed that periodic strategies (high fecundity, low juvenile survivorship) decreased with elevation and increased with latitude, whereas equilibrium strategies (low fecundity, high juvenile survivorship) declined along both gradients. Despite the observation of predictable trade‐offs for habitat, trophic and locomotion trait strategies, there was little evidence for environmental filtering of these traits at high elevations and high latitudes. Main conclusions Elevation and latitude appear to be environmental filters of freshwater fish community assembly when mediated through life‐history strategies, which is likely to be attributable to thermal constraints on growth and reproduction. Our results support the role of environmental filtering in trait convergence patterns along climatic gradients, and we identify cold stress as one specific filter of community assembly.