LB 2-12 - Using joint species distribution models to establish biological benchmarks for streams and rivers of the contiguous United States

Using reference sites to establish appropriate biological assessment benchmarks is challenging because of the ubiquity of anthropogenic impacts. We circumvent the need for reference sites using joint species distribution models to predict the benthic macroinvertebrate assemblage expected (E) to occur if nutrients, salinity, physical habitat, and climate were minimally influenced by human activities. We then compare the observed (O) assemblage to E and assess biological condition for each site. We used random forest models to relate natural and anthropogenic factors to observed nutrient and salinity concentrations and then predicted minimally disturbed values for each site by setting the anthropogenic effects to zero. We obtained minimally disturbed values for physical habitat and climate from published sources. Depending on the ecoregion, random forest models explained 22 to 79% of the variation in the chemical gradients and, in general, hindcasted values for all gradients were lower than present-day observations. The species distribution models fitted using present-day conditions accurately explained occurrences (AUC > 0.70) and assemblage level characteristics. We then substituted minimally disturbed values for each gradient as predictor variables in the fitted models and generated a distribution of plausible observed-to-expected ratios (O:E) by comparing O to 1250 random samples from the posterior predictive distribution of E. We measured biological condition as the probability that the observed assemblage is different from the assemblage expected to occur if the site was in its minimally disturbed state. We found that the assemblages observed at 57% of sites across the contiguous United States had a probability > 0.5 of being different from the expected assemblage. This presentation demonstrates an alternative method for establishing biological benchmarks without relying on reference sites and we discuss its advantages, disadvantages, and complexities. The views expressed in this presentation are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency.