BIRD SPECIES ASSEMBLAGES AS INDICATORS OF BIOLOGICAL INTEGRITY IN GREAT-BASIN RANGELAND

The study evaluates the potential for bird species assemblages to serv e as indicators of biological integrity of rangelands in the Great Bas in in much the same way that fish and invertebrate assemblages have be en used as indicators in aquatic environments. Our approach was to ide ntify metrics of the bird community using relatively simple sampling m ethods that reflect the degree of rangeland degradation and are consis tent over a variety of vegetation types and geographic areas. We condu cted the study in three range types (i.e., potential natural plant com munity types) in each of two widely separated areas of the Great Basin : south-eastern Idaho (sagebrush steppe range types) and west-central Utah (salt-desert shrub range types). Sites were selected in each rang e type to represent three levels of grazing impact, and in Idaho inclu ded sites modified for crested wheatgrass production. Birds were sampl ed by point counts on 9 100-m radius plots at 250-m spacing on each of 20 sites in each area during the breeding season. In sagebrush-steppe , 964 individuals in 8 species of passerine birds were used in analyse s. Five metrics were significantly related to impact class, both when analyzed within range type and when analyzed with all range types comb ined. Species richness, relative abundance of shrub obligate species, and relative abundance of Brewer's sparrow were generally lower for th e higher impact classes; whereas the reverse was true for dominance by a single species and for relative abundance of homed larks. In contra st, total number of individuals did not differ significantly as a func tion of impact class. In salt-desert shrub, a total of 843 birds in 4 species were included in analyses, 98% of which were homed larks. None of the metrics identified above was significantly related to impact c lass. Two metrics for breeding birds in sagebrush steppe (species rich ness and dominance) showed little overlap between values for the extre mes of impact class, and thus they have potential as indicators of bio logical integrity. However, the sensitivity of these metrics appears t o be greatest at the high impact end of the spectrum, which suggests t hey may have limited utility in distinguishing between sites having li ght and moderate impact.