If a tree falls in the forest: terrestrial habitat loss predicts caddisfly (Insecta: Trichoptera) assemblages and functional feeding group biomass throughout rivers of the North-central United States

Context: Understanding the specific natural and anthropogenic drivers of aquatic insect assemblages and feeding ecology is critical to managing aquatic ecosystems. Utilizing the counts of benthic specimens found in most studies is suboptimal due to sampling difficulties, lack of taxonomic resolution, and the tacit assumption that specimens of all sizes are ecologically equivalent. These problems may be overcome by measuring the biomass of winged adults instead. Objectives: This study quantified the importance of natural and anthropogenic variables affecting adult caddisflies in the north-central United States in habitats ranging from designated wilderness to intense agriculture. Methods: Nearly 650 streams were sampled throughout a 1.2 million km² area. The relative ability of 52 variables to predict (1) caddisfly species assemblages and (2) functional feeding group (FFG) biomass was assessed. Results: The percentage of intact habitat, particularly at the whole-watershed scale, was most effective at predicting both metrics. The biomass of all FFGs except filtering collectors, and that of 85% of species, decreased as intact habitat decreased. Assemblages of least disturbed streams, conversely, were primarily affected by gradient and width, and generally followed patterns of river continuity. In all streams, increasing stream temperatures associated with decreasing intact habitat led to decreasing species richness; whereas increasing temperatures associated with decreasing gradient or increasing width in least disturbed streams led to increasing species richness. Conclusions: Natural variables are important predictors of caddisfly assemblages in undisturbed streams; however, habitat loss overwhelms such variables throughout much of the north-central US, leading to declines in nearly all species.