Biogeographic Considerations for Fish-Based Indices of Stream Health in Regions with High Species Richness and Endemism: A Perspective from the Southeastern US

Agencies monitoring aquatic ecosystems desire to accurately measure the similarity of species assemblages to undisturbed states to assess ecological “health”. Over the past century, numerous fish-based indices have been developed to estimate the abstract property of stream health. The Index of Biotic Integrity (IBI) is the predominant technique used by state and federal agencies in the United States and has been widely used by these agencies following its inception over 40 years ago. However, biogeographic patterns can often confound the application of traditional IBI frameworks in highly speciose regions. While nearly all regional IBI versions are developed to suit particular geographic regions, few measure contemporary biogeographic changes (such as loss or maintenance of endemic species) even though these properties are critical components of “healthy” ecosystems in highly diverse regions. Quantitative analyses from recent decades have shown that the proportion of endemic fishes may decline, but that other taxonomically and ecologically similar widespread (albeit native) species may replace them. Herein, we summarize challenges using traditional trait-based fish IBIs in the southeastern United States—a region with unparalleled fish species richness and endemism within the temperate world. In our experience, complex biogeographic patterns and scant biological information for many fish species in this region often hinder the ability of traditional fish-based biotic integrity indices to measure stream health. Tailoring indices and metrics to better suit biogeographic patterns and incorporating traits such as species identity and endemism may help to further refine stream health indices in highly diverse regions.