Fish and amphibian tolerance values and an assemblage tolerance index for streams and rivers in the western USA

Aquatic species' tolerances to overall human disturbance are key components of biological assessments of aquatic ecosystems. These tolerance classifications enable development of metrics for use in multimetric indexes, such as the index of biotic integrity (IBI). Usually, species are classified as being tolerant, moderately tolerant, or intolerant (sensitive) to human disturbance. Traditionally, for fish-based IBIs, these assignments are based on a combination of professional judgment and information from state fish books. We used fish and amphibian species data in conjunction with chemical, physical, and landscape indicators of human disturbance collected at 1,001 stream and river sites in 12 western states (USA) sampled by the Environmental Monitoring and Assessment Program in 2000-2004. Using principal components analyses, we created synthetic disturbance variables for water nutrients, site-scale physical habitat, catchment-scale land use, and overall human disturbance. We calculated species' tolerance values for the four synthetic disturbance variables as weighted (by relative abundances) averages plus SDs. For each site, we used the tolerance values (based on the overall synthetic disturbance variables) and relative abundances of species to calculate an assemblage tolerance index (ATI) score. We discuss (1) bow the tolerance values could be used in establishing species tolerance classifications appropriate for regional species pools and (2) potential application of the ATI to the IBI and to bioassessments in general.