LB 9-78 - The relative importance of biotic and abiotic factors in structuring stream invertebrate assemblages in a temperate rainforest

Aquatic invertebrates are essential to stream ecosystems, as they trophically connect detritus and primary producers to vertebrate consumers. Benthic invertebrates are highly sensitive to the environment, so discerning how specific environmental components impact these assemblages could help to explain differences between natural variability among streams and anthropogenic impacts. In this study, we addressed the following overarching research questions: i) what are the spatial patterns of taxonomic and functional diversity of benthic invertebrates, and ii) what environmental factors are associated with these patterns? We collected quantitative samples of benthic invertebrates along with a suite of abiotic (i.e., water temperature, canopy cover, gradient, substrate grain size, bankfull width, distance from headwaters, elevation, latitude, and maximum current speed) and biotic (i.e., salmonid density and benthic and sestonic chlorophyll-a concentration) variables from 29 sites across 16 small streams on the Olympic Peninsula in Washington State, USA. We created a mixed-effects model to analyze taxonomic diversity and abundance metrics in relation to environmental variables. We analyzed benthic invertebrate assemblages (taxa, functional habit groups, and functional feeding groups) composition via ordination (non-metric multidimensional scaling) and then plotted vectors for the most important group of non-parametrically correlated environmental variables explaining the variation in the assemblages. We found that invertebrate diversity increased significantly with sestonic and benthic algal biomass and gradient and decreased significantly with bankfull width and the interaction between sestonic and benthic algal biomass, and abundance increased significantly with sestonic algal biomass and bankfull width (p < 0.05). Additionally, the invertebrate assemblage was best explained by bankfull width, gradient, salmonid density, and sestonic algal biomass, which accounted for 27.8%, 21.9%, and 23.5% of the variation in taxonomic, functional habit group, and functional feeding group composition, respectively. These assemblage analyses indicate that biotic factors, often overlooked in aquatic surveys, may be vital ecological drivers and processes underlying invertebrate assemblage patterns in aquatic ecosystems. In addition, our findings show how environmental changes resulting from natural disturbance, forest harvest, restoration, or climate change may result in changes in benthic invertebrate taxonomic and functional assemblage structure.