Influence of Stream Location in a Drainage Network on the Index of Biotic Integrity

—The index of biotic integrity (IBI) has become a widely used tool for assessing the condition of stream fish communities and the overall biological status of streams. Because the location of a stream in a drainage network can influence the species richness offish communities and because species richness is an important component of the IBI, we examined the influence of stream spatial location on the IBI. We found that IBI scores for headwater streams in three Illinois drainage basins were significantly lower than those calculated for tributary streams of similar size connecting directly to larger streams. This difference in IBI was related to the increased species richness and to a greater number of sucker and darter species in tributaries that drain into larger, main-channel streams. Because of the influence of tributary location on the IBI, expected values for headwater tributary streams should be developed independently from those developed for main-channel tributary streams. Failure to do so can result in a substantial underestimation of the IBI of headwater tributary streams or an ovcrestimation of main-channel tributaries. The index of biotic integrity (IBI: Karr 1981) widespread application, and its proposed use as has been used widely for assessing environmental an index to set regulatory standards for stream degradation of lotic ecosystems (Ohio Environquality (Fausch et al. 1990), it is important to mental Protection Agency 1988; Hite and Berconsider factors that can influence the IBI and its trand 1989; Fausch et al. 1990; Lyons, in press). expectations independently of environmental degThe IBI is a composite index, comprising metrics radation. (generally 12) that reflect structural and functional The location of a stream in a drainage network characteristics of fish communities (Karr et al. can significantly affect the number of fish species 1986). These metrics include trophic structure in the community. Osborne and Wiley (1992) re(proportions of omnivores, insectivorous cypriported significantly fewer species of fish in lownids, and piscivores), species richness and abunorder, tributary streams in the headwaters of a dances, taxonomic richness (numbers of darter, drainage network than in similar-size streams that sunfish, sucker, and intolerant species, and somedrained into higher-order, main-channel streams times the proportion of green sunfish), and disease in the lower half of the drainage networks. Such frequency. Each metric is scored as a 1, a 3, or a tributaries of main-channel streams have been re5 depending upon whether it deviates strongly, ferred to as adventitious streams (Gorman 1986). deviates moderately, or approximates the anticiObserved differences in species richness appeared pated condition for a quality stream of a similar to be related to proximity to refugia associated size (generally assessed by stream order) in a given with higher-order streams and differential rates of region. Because of the IBFs reported sensitivity to colonization following disturbances (Osborne and broad ecological conditions (Angermeier and Karr Wiley 1992). Because species richness is an im1986; Karr et al. 1986; Fausch et al. 1990), its portant metric in the IBI and is related to other