A Structured Approach for Developing Indices of Biotic Integrity: Three Examples from Streams and Rivers in the Western USA

Abstract In the late 1990s the Environmental Monitoring and Assessment Program of the U.S. Environmental Protection Agency developed a structured set of tests to evaluate and facilitate selection of metrics for indices of biotic integrity (IBIs). These IBIs were designed to be applicable across multistate regions as part of a national assessment of all U.S. waters. Here, we present additional steps in, and refinements to, that IBI development process. We used fish and amphibian assemblage data from 932 stream and river sites in 12 western U.S. states to develop IBIs for Mountains, Xeric, and Plains ecoregions. We divided 237 candidate metrics into nine metric classes representing different attributes of assemblage structure and function. For each ecoregion we sequentially eliminated metrics by testing metric range, signal‐to‐noise ratios, responsiveness to disturbance, and redundancy to select the best metric in each class. The IBIs for the Mountains and Plains each had seven metrics and the Xeric IBI had five. In the Mountains, half of the estimated stream length that could be assessed had IBI scores greater than 62 (out of 100). In the Xeric and Plains, half the stream length had scores no greater than 50 and no greater than 37, respectively. An estimated 16% of Xeric stream length had scores greater than 62 (the median for the Mountains), while 5% of Plains stream length had scores that exceeded 62. This IBI development process is less subjective and more streamlined and has more clearly defined criteria for metric selection and scoring than those used in the past, while maintaining a strong ecological foundation.