Incorporating community assembly mechanisms in biotic indices improves index performance

Macroinvertebrate biotic indices are widely used in the assessment of freshwater systems. One criticism of these indices is that they typically neglect assembly processes such as dispersal and species interactions. These assembly processes can influence community composition, leading to communities that are unrepresentative of local ecological conditions. Combining multiple water quality, macroinvertebrate community, and dispersal-related trait datasets, we built two stressor-specific (total nitrogen, total phosphorus) biotic indices for the south-central United States. From there, we quantified dispersal capacity, interspecific interactions, and niche breadth for each taxon present. These assembly mechanisms were then incorporated into each index, downweighing taxa that were high dispersers, high interactors, and/or exhibited a wide niche breadth. Initial and corrected indices were then tested and compared based on their continuous predictive performance and discriminative ability between nutrient levels. Our results show that indices with corrections for assembly mechanisms outperformed indices without corrections. These assembly mechanism corrections were then applied to the Salt Belt Index (SBI), a biotic index developed to measure salt intrusion in streams. Corrections also improved SBI performance, and specific corrections were identified that improved all three indices. Given their ubiquity in freshwater assessment, biotic indices should incorporate additional assembly mechanisms to more accurately reflect local conditions and improve index performance.