Development and evaluation of a fish assemblage index of biotic integrity for northwestern Great Plains streams

Quantitative indicators of biological integrity are needed for streams in the Great Plains of North America, but it was not known whether the index of biotic integrity (IBI) approach would be effective in this semiarid region. Great Plains streams have a depauperate and tolerant ichthyofauna and highly variable physicochemical conditions that may mask the effects of non-point-source pollution and stream habitat degradation. We developed an IBI based on fish assemblages by screening metrics for range, responsiveness to human influence, precision, and lack of redundancy; we then tested the IBI's ability to detect anthropogenic effects by validating the index with an independent data set. The IBI was composed of 10 metrics based on species richness and composition, trophic and reproductive guilds, and age structure. These 10 metrics had many significant correlations with substrate and water chemistry variables but had fewer significant correlations with riparian condition and watershed variables. Of the watershed variables, road density had the highest number of significant correlations with final IBI metrics. The IBI was validated by demonstrating its responsiveness to aggregate measures of human influence, site-level habitat, and water chemistry, and its lack of responsiveness to factors that varied naturally, such as stream size and site elevation. The IBI was also temporally stable within and between years during repeat visits to a subset of sampled reaches. This IBI can be used as a measure of biological integrity for management of prairie streams faced with threats such as introduced species, intensive agriculture, grazing, and coalbed natural gas extraction. Although we developed this IBI based on data from Montana prairie streams only, our IBI can probably serve as a framework for other North American plains streams and our results suggest that the IBI approach may be useful in other semiarid regions of the world.