The relative importance of abiotic, biotic, and spatial factors in structuring the stream macroinvertebrate metacommunity in a temperate rainforest

(1) We investigated how local abiotic and biotic variables versus regional spatial and dispersal variables influenced the structure of a stream macroinvertebrate metacommunity (multiple assemblages linked by dispersal), and whether this varied by dispersal mode. (2) We collected 143 benthic macroinvertebrate samples and associated abiotic and biotic data related to local habitat and species interactions across three river basins on the Olympic Peninsula (Washington, USA). We assessed macroinvertebrate assemblage composition in relation to environmental and spatial variables using mixed effects linear regression and partial redundancy analysis. (3) We found that water temperature, substrate grain size, bankfull width, and salmonid density were the local factors significantly associated with macroinvertebrate assemblage composition. Local variables explained 19.7% of the variation in taxonomic composition, while overland spatial distance explained 7.4% (p = 0.014). Local variables were more important for aerially dispersing taxa than for aquatically dispersing taxa. (4) Our results indicated that local abiotic and biotic variables acted together with dispersal and overland distance to shape the macroinvertebrate metacommunity composition across river basins. The relative importance of local habitat variables and spatial processes depended on the dispersal mode, with evidence of dispersal limitation for aquatic dispersers. (5) Our study underscores the role of dispersal in influencing the effect of the local habitat on metacommunity organization, even at small spatial scales. We emphasize the need to apply metacommunity theory to stream management, such as incorporating distance between reaches and organismal dispersal traits in bioassessment models.