Regional Trends of Biodiversity Indices in the Temperate Mesic United States: Testing for Influences of Anthropogenic Land Use on Stream Fish while Controlling for Natural Landscape Variables

The biodiversity of stream fishes is critically threatened globally, and a major factor leading to the loss of biodiversity is anthropogenic land use in stream catchments, which act as stressors to stream fishes. Declines in the biodiversity of stream fish are often identified by a loss of species or fewer individuals comprising assemblages, but biological degradation can also occur with increases in non-native species and/or the spread of fish tolerant to anthropogenic land use, suggesting the importance of accounting for the distinctness of assemblages along with richness and diversity to best characterize the response of stream fish assemblages to anthropogenic landscape stressors. We summarized stream fish assemblages from 10,522 locations through multiple biodiversity indices and then quantified index responsiveness to natural landscape variables and anthropogenic land use in stream network catchments across five freshwater ecoregions in the temperate mesic portion of the United States. Indices included species richness, Shannon’s diversity, Pielou’s evenness, beta diversity, taxonomic diversity, and taxonomic distinctness. First, we tested for correlations among indices across freshwater ecoregions and found that while species richness and Shannon’s diversity were always highly correlated, taxonomic distinctiveness was not highly correlated with other biodiversity indices measured except taxonomic diversity. Then, we used multiple linear regression to predict biodiversity indices in each of the five freshwater ecoregions to identify significant landscape variables from natural landscape and anthropogenic land uses. Most indices were consistently predicted by catchment area, and many were predicted by elevation, except for beta diversity, emphasizing the importance of these natural landscape variables on biodiversity. In contrast, taxonomic distinctness was often predicted by the amount of urban land use in the catchment, but the direction of the relationship varied. The proportion of agriculture land use in the network catchment was a more consistent predictor of species richness, beta diversity, and Shannon’s diversity. Our analyses show that taxonomic distinctness in freshwater fishes characterize a unique element of biodiversity in relationships with anthropogenic land uses in a streams network catchment. Taxonomic distinctness may also be an effective metric for the bioassessment of stream fishes along with richness and diversity indices to help preserve biodiversity in regard to current and future anthropogenic land uses.