In this report, certain other metrics were used, when appropriate, as descriptors of 

 the benthic community response to habitat or water quality but were not incorporated into 

 the bioassessment metric battery, either because they have not yet been tested for 

 reliability in streams of western Montana, or because results of such testing did not show 

 them to be robust at distinguishing impairment, or because they did not meet other 

 requirements for inclusion in the metric battery. These metrics and their use in predicting 

 the causes of impairment or in describing its effects on the biotic community are 

 described below. 



• The modified biotic index. This metric is an adaptation of the Hilsenhoff Biotic 

 Index (HBI, Hilsenhoff 1987), which was originally designed to indicate organic 

 enrichment of waters. Values of this metric are lowest in least impacted 

 conditions. Taxa tolerant to saprobic conditions are also generally tolerant of 

 warm water, fine sediment and heavy filamentous algae growth (BoUman, 

 impublished data). Loss of canopy cover is often a contributor to higher biotic 

 index values. The taxa values used in this report are modified to reflect habitat 

 and water quality conditions in Montana (Bukantis 1998). Ordination studies of 

 the benthic fauna of Montana's foothill prairie streams showed that there is a 

 correlation between modified biotic index values and water temperature, substrate 

 embeddedness, and fine sediment (Bollman 1998). In a study of reference 

 streams, the average value of the modified biotic index in least -impaired streams 

 of western Montana was 2.5 (Wisseman 1992). 



• Taxa richness. This metric is a simple count of the number of unique taxa present 

 in a sample. Average taxa richness in samples from reference streams in western 

 Montana was 28 (Wisseman 1992). Taxa richness is an expression of biodiversity, 

 and generally decreases with degraded habitat or diminished water quality. 

 However, taxa richness may show a paradoxical increase when mild nutrient 

 enrichment occurs in previously oUgotrophic waters, so this metric must be 

 interpreted with caution. 



• Percent shredders. Shredding organisms consume large particles of detritus such 

 as leaves, needles and wood. Foothill and prairie streams with healthy riparian 

 vegetation and sufficient instream structure to retain detritus will have large 

 numbers of shredders Often, this feeding group dominates the fauna of headwater 

 streams. The abundance of shredders generally increases in the fall, when leaf and 

 blade input to streams maximizes. In another study, average shredder contribution 

 in western Montana reference streams was 8% (Wisseman 1 992). 



• Percent predators. Aquatic invertebrate predators depend on a reliable source of 

 invertebrate prey, and their abundance provides a measure of the trophic 

 complexity supported by a site. Less-disturbed sites have more plentiful habitat 

 niches to support diverse prey species, which in turn support abundant predator 

 species. 



• Number of "dinger" taxa. So-called "dinger" taxa have physical adaptations that 

 allow them to cling to smooth substrates in rapidly flowing water. Aquatic 

 invertebrate "dingers" are sensitive to fine sediments that fill interstices between 

 substrate particles and eliminate habitat complexity. Animals that occupy the 

 hyporheic zones are included in this group of taxa. Expected "dinger" taxa 



