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An Introduction to the Index of Biotic Integrity

Original IBI Metrics (Karr 1981)
Species Richness and Composition Metrics Indicator Species Metrics Trophic Function Metrics Reproductive Function Metrics Abundance and Condition Metrics

The Index of Biotic Integrity (IBI), the original multi-metric index, was first developed by Dr. James Karr for use in small warmwater streams (ie., too warm to support salmonids) in central Illinois and Indiana (Karr, J. R. 1981. Assessment of biotic integrity using fish communities. Fisheries (Bethesda) 6: 21-27. (7 pp., 193KB, About PDF)). The original version had 12 metrics that reflected fish species richness and composition, number and abundance of indicator species, trophic organization and function, reproductive behavior, fish abundance, and condition of individual fish (see Table).

Each metric received a score of five points if it had a value similar to that expected for a fish community characteristic of a system with little human influence, a score of one point if it had a value similar to that expected for a fish community that departs significantly from the reference condition, and a score of three points if it had an intermediate value. Sites with high biotic integrity had relatively high numbers of total species, sucker (Catostomidae) species, darter (Crystallaria, Ammocrypta, Etheostoma, and Percina; Percidae), sunfish (Centrarchidae excluding Micropterus) species, and intolerant species; high relative abundance of top carnivores and insectivorous cyprinid species; high overall fish abundance; and low relative abundance of the tolerant green sunfish (Lepomis cyanellus), omnivores, hybrids, and fish with diseases or deformities. Expectations for species richness metrics increased with increasing stream order, and were derived from an empirical relationship between stream size and maximum number of species present, termed the maximum species richness (MSR) line (Fausch et al. 1984). The total IBI score was the sum of the 12 metric scores and ranged from 60 (best) to 12 (worst) (some authors have reduced the lowest score to zero).

The original version of the IBI quickly became popular, and was used by many investigators to assess warmwater streams throughout the central United States. Karr and colleagues explored the sampling properties and effectiveness of the original version in several different regions and different types of streams (Fausch et al. 1984; Karr et al. 1986).

As the IBI became more widely used, different versions were developed for different regions and different ecosystems. These new versions had a multimetric structure, but differed from the original version in the number, identity, and scoring of metrics. New versions developed for streams and rivers in the central United States generally retained most of the metrics used in the original IBI, modifying only those few that proved insensitive to environmental degradation in a particular geographic area or type of stream.

Currently, almost all states use some version of the IBI, whether for fish, benthic macroinvertebrates or algae. The figure to the right shows the status as of 1995.

Figure 1

Figure - Use of the IBI in state and tribal programs in 1995.

Learn more about Metrics and the Index of Biotic Integrity

(The text above was adapted from Simon Exit EPA Disclaimer and Lyons "Application of the Index of Biotic Integrity to Evaluate Water Resource Integrity in Freshwater Ecosystems," Chapter 16, in Davis and Simon Exit EPA Disclaimer. 1995. Biological Assessment and Criteria - Tools for Water Resource Planning and Decision Making.)

Biological Indicators | Aquatic Biodiversity | Statistical Primer

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