I review studies on the effects of mammalian herbivore grazing on plant species composition, biomass, productivity, and nitrogen cycling and the responses of ungulates to grazing perturbations at 2 sites in the northern United States. Vegetation studies involved use of permanent and moveable exclosures and comparison of vegetation attributes along natural grazing gradients. Animal responses to herbivore-induced vegetation changes were monitored by direct observation and by measurement of consumption at various sites. At Wind Cave National Park (WCNP), South Dakota, heavily grazed prairie dog (Cynomys ludovicianus) colonies typically had lower levels of plant biomass, were forb rather than grass dominated, and had plants with higher leaf nitrogen (N) concentrations than plants growing outside of the colonies. Soils in prairie dog colonies had greater rates of net N-mineralization than lightly grazed, uncolonized sites. Bison (Bison bison) grazed preferentially on prairie dog colonies, grassland patches mowed to simulate grazing, urine patches, and recently burned patches, presumably because of higher forage quality in each of these patch types than on unaltered grassland. Uncolonized grasslands used by bison and other ungulates at WCNP and areas used by wild horses (Equus caballus) at Bighorn Canyon National Recreation Area and the overlapping Pryor Mountain Wild Horse Range in Montana and Wyoming showed no consistent differences in plant species composition or in leaf N-concentrations compared to vegetation within exclosures. These data suggest that ungulates in these parks are managed at levels that are not presently degrading the vegetation. Although prairie dogs have had an impact on the vegetation in local areas, their colonies provide a significant portion of the food bison consume at WCNP. As long as their presence does not reduce forage availability for ungulates to a potentially critical level, further control of their populations does not appear to be warranted.
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