Estimating observed-to-expected taxonomic richness in streams and lakes to support existence valuation of biological health
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The U.S. EPA is striving to quantify the existence value of biological health in the Nation’s streams and lakes through stated preference (SP) surveys. Through focus groups, we concluded that the public understood observed-to-expected taxonomic richness (O/E) better than other candidate indicators of biological health. To quantify how value varies with proximity to resource, we need spatial interpolations of O/E for all perennial streams and lakes within the conterminous US. To make these interpolations, we used random forest to model O/E with data from the National Aquatic Resource Surveys and StreamCat/LakeCat datasets for the Eastern, Plains, and Western ecoregions. Benthic macroinvertebrates (BMI) and plankton were used for streams and lakes, respectively. The models explained 25-30% of the variation in BMI O/E scores (RMSE: 0.25-0.27) and 13-36% in plankton O/E scores (RMSE: 0.22-0.25). Maps of model residuals showed no visual spatial biases, however, there were noticeable differences in predicted values at ecoregion boundaries. To improve model performances, we are exploring modeling O and E separately, using different modeling algorithms, and including new landscape variables. We will also discuss challenges and solutions for combining model interpolations from streams and lakes.