Dana M. Infante

Disentangling the effects of natural factors and human disturbances on freshwater systems is essential for understanding the distributions and composition of biological communities and their relationship with physicochemical and biological factors. As the spatial extent of ecological investigations increases from local to global scales, efforts to account for the increasing influence of natural factors become more important. This article synthesizes the current knowledge and commonly used approaches for disentangling these effects on aquatic systems. New understanding has been facilitated by the availability of large-scale geospatial landscape databases that facilitate regional analyses and classifications in conjunction with novel approaches to identify reference conditions and statistical partitioning analyses. This synthesis begins with a summary of how natural factors and human disturbances interactively affect aquatic systems. It then provides an overview of why it is essential to separate the effects of natural factors and human disturbances and a description of examples of landscape databases that make the separation of natural and human factors feasible. It last synthesizes currently-used common approaches for separating the effects of natural factors from human disturbances. Our synthesis assembles representative approaches to disentangling human disturbances in one place to provide new insights that stimulate integrated uses of multiple approaches and the development of new approaches so that management actions can be taken to protect and restore aquatic ecosystem health.

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.