Arthur R. Cooper

Compared to taxonomic approaches, traits‐based approaches are often implemented to gain a more mechanistic understanding of how biotic communities respond to changing environmental conditions, including landscape stressors. Studies evaluating trait–environment relationships have increased in prevalence over the last 2 decades, but such analyses have rarely been conducted across continental extents in which component regions exhibit substantial spatial variation in stressors and natural environmental conditions. Consequently, the role of biogeographic context in analyses of trait–environment relationships remains under studied. Here, we evaluated the generalisability of trait–environment relationships across the conterminous U.S.A. to advance our understanding of the functional biogeography of fluvial fishes. We sought to: (1) identify trait–environment relationships that were consistent regardless of regional differences in natural environmental conditions; and (2) address whether the multivariate structure of trait–environment relationships was comparable across regions. We used RLQ and fourth‐corner analyses to identify significant trait–environment relationships across nine large ecoregions comprising the conterminous U.S.A. These statistical approaches provided a framework to analyse co‐variation among 16 functional traits (e.g., trophic ecology, habitat preference) and 17 environmental variables representing 597 stream fish species whose abundances had been recorded in 45,183 stream reaches between 1990 and 2019. Trait–environment relationships varied among ecoregions such that few trait–environment relationships were consistent. Although some trait–environment relationships were significant in multiple ecoregions, the strength and multivariate structure of such relationships varied. These findings suggest that the functional traits most responsive to changing environmental conditions are determined by distinct biogeographic processes. Our findings add to a growing consensus that functional traits‐based approaches are context dependent. Understanding the complex relationships between functional traits and environmental conditions across large geographic regions can inform ecological theory and improve conservation efforts at broad spatial extents.