Patterns in alpha, beta, and gamma diversity in lotic fish and macroinvertebrates across the conterminous U.S.
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Understanding taxonomic richness patterns across varying geographic extents offers useful biological conservation insights because richness is driven by multiple proximal and distal factors. Most research on aquatic ecosystem richness patterns has been limited to sites (alpha richness) or small areas (watersheds; beta richness) and focused on one taxonomic group. We investigated macroinvertebrate and fish richness at regional and continental extents based on data collected at 3,550 sites during USEPA’s National Rivers and Streams Assessments (NRSA). To assess how geographic area influenced beta richness, we used various-grain hydrologic units and ecoregions. At the ecoregion extent (Omernik Level III), fish beta richness ranged from 8-235 species (median=71), whereas macroinvertebrate beta richness ranged from 105-366 genera (median=201). Richness was strongly related to sampling effort (number of sites sampled), thus, richness was converted to residual richness and related to ecoregion landscape data using regression and random forest analyses. Ecoregion %agricultural and %developed were the most important variables for fish residual richness. Whereas ecoregion %forest was the most important variable for macroinvertebrate residual richness. These findings suggest that biodiversity conservation approaches for one assemblage are unlikely to suffice for the other.