The Influence of Spatial Extent and Distance on Macroinvertebrate‐Environment Relationships in Conterminous <scp>USA</scp> Stream Sites

ABSTRACT Aim The relationships between environmental variables and macroinvertebrate assemblages depend on spatial extent, but the relative importance of local and regional factors remains unclear. Understanding these scale‐dependent relationships is crucial for improving biodiversity assessments and ecosystem management. We hypothesise that climate variables drive macroinvertebrate composition at broad spatial extents, whereas local water quality and habitat conditions are more influential at smaller spatial extents. Location A total of 5,788 streams across nine ecoregions in the conterminous United States, sampled by the United States Environmental Protection Agency (EPA). Time Period Samples collected in the years 2008, 2009, 2013, 2014, 2018, and 2019. Major Taxa Studied Freshwater macroinvertebrates, including 992 genera. Methods We analysed macroinvertebrate abundance data alongside 25 environmental variables categorised into four types: physical‐habitat structure, water quality, watershed characteristics, and climate. We applied Multivariate Multiscale Codependency Analysis (MMCA) at both national and ecoregion‐specific scales to determine which environmental variables were most strongly associated with assemblage composition at different spatial extents. Results Assemblage‐environment relationships varied with spatial extents. Climate and watershed variables were the dominant drivers at broader spatial extents, whereas local water quality and physical habitat structure were more influential at smaller inter‐site distances. Additionally, the environmental variables most strongly associated with macroinvertebrate composition varied across ecoregions, suggesting that regional factors such as land use and historical environmental conditions shape biotic responses. Main Conclusions Our findings underscore the importance of spatial scale in ecological studies and conservation planning. Recognising how key environmental predictors change with spatial extent enhances biodiversity assessments, informs targeted conservation strategies, and improves ecosystem management. The observed regional variation in assemblage‐environment relationships highlights the need for regionally tailored management approaches to maintain and restore aquatic biodiversity.