Sagebrush Steppe Habitats and Their Associated Bird Species in South Dakota, North Dakota, and Wyoming: Life on the Edge of the Sagebrush Ecosystem

Sagebrush lands in the western Dakotas are at the eastern edge of the distribution of sagebrush in the United States, forming a transition zone from the true shrubsteppe of Wyoming and Montana to the mixed-grass prairie that dominates most of the Dakotas. Because this transition zone is different from the two major ecosystems that lie to the east and west of it, the characteristics of the vegetation are unique and the avian associations consist of species from both the grassland and the true shrubsteppe. Various activities, including grazing, conversion to tillage agriculture, introduction of exotic species, herbicide application, and roadbuilding have significantly altered and/or reduced sagebrush throughout its range. The purpose of this study was to characterize the vegetation and avian associations found in the transitional sagebrush habitats of the western Dakotas. Data from this ecotone were then compared with data from the interior of the sagebrush ecosystem (central Wyoming) and interior of the grassland ecosystem (western, central, and eastern South Dakota). In 2001-2002, 100 m long, fixed-width belt transects were surveyed for avian species on the same sites where four 50 m vegetation transects were performed. Eight-hundred ninety-six birds representing 33 species were detected on 200 transects in South Dakota, 249 individuals representing 21 species were detected on 95 transects in North Dakota, and 125 individuals representing seven species were detected on 27 study sites in Wyoming. Avian abundance, species richness, and Shannon diversity indices were higher in South Dakota and Wyoming than in North Dakota. Differences in local vegetation characteristics between the states probably accounted for the differences in avian community measures, particularly the abundance and percent cover of shrubs and sagebrush and mean percent cover of grasses in the understory. Associations of local vegetation with the six most abundant bird species were different in some cases from the relationships found in the true shrubsteppe and grassland ecosystems. Landscape variables, summarized from buffers drawn around survey sites at diameters of 200, 400, and 800 m, were unrelated to most bird species except the Brewer’s Sparrow (a sagebrush obligate species) which was present more often where there were more shrubs and sagebrush in the buffers and Horned Larks which were absent when there were more hayfields in the buffer. Fifty-percent habitat occupancy models were developed for the six most abundant bird species, and all of the models accurately predicted the occurrence and non-occurrence of these bird species on the sites used to test the models. Results of this study provide justification for the conservation of sagebrush habitats in the western Dakotas. To aid future conservation, I recommend that agencies conduct periodic monitoring to determine vegetation and landuse trends in sagebrush habitats, because if these habitat types are reduced greatly in quality and/or extent sagebrush obligate bird species including the Sage Grouse, Brewer’s Sparrow, Sage Thrasher, and Sage Sparrow will be eliminated from the Dakotas.