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Combined grazing by black-tailed prairie dogs (Cynomys Idovidanus) and bison (Bison b&on) produces and maintains a series of changes in the vegetation of prairie dog colonies. However, because their grazing patterns differ in frequency and intensity through time, their individual impacts may be different. The objective of this study was to determine the individual and combined Influences of these 2 herbivores in maintaining selected vegetation characteristics of a prairie dog colony in a mixed-grass prairie at Wind Cave National Park, S.D. This was assessed by monitoring plant responses during 2 years following exclusion from grazing by 1 or both species. In spite of their different grazing patterns, prairie dogs and bison had similar and independent (i.e., additive) effects in maintaining plant community structure. For example, total above ground biomass increased 32-36s within 2 years of removal of each species, primarily as a result of increases in accumulation of graminoid biomass. Plant species diversity, equitability, and dominance concentration were similar in all treatments both years, although there were slight decreases in relative abundance of forbs and increases in relative abundance of graminoids in the second year after removal of grazers. Mean graminoid leaf nitrogen concentration (May to September) declined slightly but significantly after removal of prairie dogs (1.49 to 1.3896) in 1985, and after bison exclusion (1.64 to 1.50%) in 19ll6. We suggest that rate of vegetation change following removal of grazers depends upon weather conditions, plant species composition, and prior intensity and duration of grazing.