Effects of Grazing on Vegetation

Grazing by large native ungulates and semiaridity are the two main forces that have had a large infuence in shaping the current-day structure of the shortgrass steppe ecosystem (Milchunas et al., 1988). With the uplift of the Rocky Mountain chain during the Miocene (approximately one million years ago), forests of the Great Plains were gradually replaced by grasslands (Axelrod, 1985). Large grazing and browsing animals inhabited the Great Plains during the middle to late Pleistocene, as did grasses of the genera Stipa, Agropyron, Oryzopsis, and Elymus (Axelrod, 1985; Stebbins, 1981). Bison occurred both east and west of the Rockies during the Wisconsin glacial period in the latter part of the Pleistocene (Wilson, 1978). During the early Holocene, approximately 10,000 years ago, bison and grasses of the genera Bouteloua, Buchloë, Andropogon or Schizachyrium, and Sorghastrum concomitantly increased throughout the Great Plains (Stebbins, 1981), but bison did not proliferate west of the continental divide (Mack and Thompson, 1982; Van Vuren, 1987). The natural shift in fauna from horses, pronghorn, and camels to bison and wild sheep from Eurasia is thought to have favored the spread of shortgrasses such as Bouteloua and Buchloë (Stebbins, 1981). Furthermore, grassland flora east and west of the Rocky Mountains probably had separate origins (Leopold and Denton, 1987). The shortgrass steppe is unique from other North American semiarid ecosystems in having bison play an important role. Bison did not proliferate west of the Rocky Mountains as they did on the Great Plains to the east. This is due in part to a lack of coincidence in timing of bison lactation and the phenological development of C3 grasses in the more Mediterranean–like climate west of the Rockies, in contrast to the mix of C3 and C4 grasses and pattern of spring–summer precipitation on the Great Plains (Mack and Thompson, 1982). Other explanations for the low numbers of bison west of the Rocky Mountains include physiographic barriers restricting immigration (Kingston, 1932), low p rotein content of forage (Daubenmire, 1985; Johnson, 1951), heavy snowfall as a cause of mortality (Daubenmire, 1985), and low aboveground primary production coupled with disjunct suitable habitat (Van Vuren, 1987). Bison a lso did not prosper in the southwestern United States, nor did a large herbivore fauna develop in South America (Stebbins, 1981).