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Multitaxon distribution models reveal severe alteration in the regional biodiversity of freshwater invertebrates

Knowledge of current and historical conditions is needed to guide conservation and restoration policies, but such information is lacking for most taxa. This problem is especially severe for small, inconspicuous taxa, such as the thousands of invertebrate species that inhabit stream and other freshwater ecosystems. We describe a novel application of River Invertebrate Prediction and Classification System (RIVPACS)-type multitaxon distribution models that, when applied to the biological survey data routinely collected in support of water-quality monitoring programs, can quantify the regional biodiversity status of hundreds of taxa. We used models developed for 2 stream data sets (North Carolina and the Mid-Atlantic Highlands, USA) to illustrate the potential of this approach. The models were calibrated with data collected from sites considered to be in reference condition and predict how probabilities of capture for each taxon in a data set vary across natural environmental gradients. When applied to survey data from multiple sites, predicted probabilities of capture can be summed across sites to estimate the taxon-specific frequencies of collection (Fe) expected under reference conditions. Comparison of observed frequencies (Fo) with Fe provides a quantitative measure of how individual taxon frequencies of collection have shifted relative to estimated reference conditions. In these 2 data sets, Fe was statistically different from Fo for >70% of taxa, implying wholesale changes have occurred in the stream invertebrate biodiversity of both regions. Fo was <Fe for most taxa, including those predicted to have been the historically most common taxa, indicating significant biodiversity loss. This type of multitaxon, regional-scale assessment of biodiversity status complements the site-specific, community-level assessments typically used by water resource managers to assess the biological integrity of individual water bodies. Together, the 2 approaches can provide a more robust, multiscale understanding of the effects landscape and waterway alteration have had on the diversity of freshwater biota.