impairment than with natural environmental gradients Each of the six metrics developed 

 and tested for western Montana ecoregions is described below. 



1. Ephemeroptera (mayfly) taxa richness. The number of mayfly taxa declines as 

 water quality diminishes Impairments to water quality which have been 

 demonstrated to adversely affect the ability of mayflies to flourish include elevated 

 water temperatures, heavy metal contamination, increased turbidity, low or high pH, 

 elevated specific conductance and toxic chemicals. Few mayfly species are able to 

 tolerate certain disturbances to instream habitat, such as excessive sediment 

 deposition 



2. Plecoptera (stonefly) taxa richness. Stoneflies are particularly susceptible to 

 impairments that affect a stream on a larger or reach-level scale, such as loss of 

 riparian canopy, streambank instability, and alteration of morphological features such 

 as pool fi-equency and fijnction, riffle development and sinuosity. Just as all benthic 

 organisms, they are also susceptible to smaller scale habitat loss, such as by sediment 

 deposition, loss of interstitial spaces between substrate particles, or unstable substrate. 



3. Trichoptera (caddisfly) taxa richness. Caddisfly taxa richness has been shown to 

 decline when sediment deposition affects their habitat. In addition, the presence of 

 certain case-building caddisflies can indicate good retention of woody debris and lack 

 of scouring flow conditions 



4. Number of sensitive taxa. Sensitive taxa are generally the first to disappear as 

 anthropogenic disturbances increase The list of sensitive taxa used here includes 

 organisms sensitive to a wide range of disturbances, including warmer water 

 temperatures, organic or nutrient pollution, toxic pollution, sediment deposition, 

 substrate instability and others. Unimpaired streams of western Montana typically 

 support at least four sensitive taxa (Bollman 1998). 



5. Percent filter feeders. Filter-feeding organisms are a diverse group, they capture 

 small particles of organic matter, or organically enriched sediment material, ffom the 

 water column by means of a variety of adaptations, such as silken nets or hairy 

 appendages. In forested montane streams, filterers are expected to occur in 

 insignificant numbers Their abundance increases when canopy cover is lost and 

 when water temperatures increase and the accompanying growth of filamentous algae 

 occurs. Some fihering organisms, specifically the Arctopsychid caddisflies 

 (Arctopsyche spp. and Parapsyche spp.) build silken nets with large mesh sizes that 

 capture small organisms such as chironomids and eariy-instar mayflies. Hence, they 

 are considered predators and in this study their abundance does not contribute to the 

 percent filter feeders metric. 



6. Percent tolerant taxa. Tolerant taxa are ubiquitous in stream sites, but when 

 disturbance increases, their abundance increases proportionately. The list of taxa used 

 here includes organisms tolerant of a wide range of disturbances, including warmer 

 water temperatures, organic or nutrient pollution, toxic pollution, sediment 

 deposition, substrate instability and others. 



Scoring criteria for each of the six metrics are presented in Table 2. Metrics differ in 

 their possible value ranges and in the direction the values move as biological conditions 

 change. For example, Ephemeroptera richness values may range fi-om zero to ten taxa or 



