Historical distribution and changes in populations of sagebrush steppe species.
Dobkin, David*,1, Sauder, Joel1, 1 High Desert Ecological Research Institute, Bend, OR
ABSTRACT- Sagebrush-dominated landscapes comprise one of the most extensive habitat types in the United States. These so-called "rangelands" constitute one of our most imperiled and neglected ecosystems due to the profound, ecologically-transformative influence of livestock grazing followed by alteration of natural fire regimes and consequent invasion by exotic plant species. Although relatively few species of mammals and birds are endemic to sagebrush shrubsteppe landscapes of the Intermountain West, we identified 24 species of small mammals and 37 species of birds for which these landscapes comprise a significant portion of their total geographic range. We developed preliminary assessments of changes in distributions and population status for mammals by extracting survey and population data from the scientific literature, putting this information into a GIS, comparing these distributions to distribution maps of Hall(1981), and seeking patterns in species responses to habitat conditions. For birds, we used Breeding Bird Survey data and filtered it further to a refined, maximally reliable sample of 349 routes. Avian abundances across three time periods were calculated and used in ArcGIS natural-neighbor analyses to identify spatial patterns of decline and increase among time periods. Nearly all of the mammals appear to have diminished geographic ranges, and many are apparently uncommon to rare in appropriate habitats. Across the 11 western states, southeast Oregon and much of Nevada constitute a region of distinctly higher species richness for this suite of mammals. For most of the bird species examined, BBS data produced statistically unreliable trend estimates at the ecoregional level, reflecting the lack of adequate sampling for these species across the region. There was little difference in spatial patterns of avian species richness for 1968-1983 compared with 1984-2001 either for upland or riparian bird species, but comparisons of Jaccard Indexes between time periods indicated that species composition of riparian bird communities varied substantially.
Key words: population change, shrubsteppe birds, geographic distribution, shrubsteppe mammals
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