Mammalian herbivores: ecosystem-level effects in two grassland national parks

I review studies on the effects of mammalian herbivore grazing on plant spe cies composition, biomass, productivity, and nitrogen cycling and the respo nses of ungulates to grazing perturbations at 2 sites in the northern Unite d States. Vegetation studies involved use of permanent and moveable exclosu res and comparison of vegetation attributes along natural grazing gradients . Animal responses to herbivore-induced vegetation changes were monitored b y 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 nit rogen (N) concentrations than plants growing outside of the colonies. Soils in prairie dog colonies had greater rates of net N-mineralization than lig htly grazed, uncolonized sites. Bison (Bison bison) grazed preferentially o n prairie dog colonies, grassland patches mowed to simulate grazing, urine patches, and recently burned patches, presumedly because of higher forage q uality in each of these patch types than on unaltered grassland. Uncolonize d grasslands used by bison and other ungulates at WCNP and areas used by wi ld horses (Equus caballus) at Bighorn Canyon National Recreation Area and t he overlapping Pryer Mountain Wild Horse Range in Montana and Wyoming showe d no consistent differences in plant species composition or in leaf N-conce ntrations compared to vegetation within exclosures. These data suggest that ungulates in these parks are managed at levels that are not presently degr ading the vegetation. Although prairie dogs have had an impact on the veget ation in local areas, their colonies provide a significant portion of the f ood bison consume at WCNP. As long as their presence does not reduce forage availability for ungulates to a potentially critical level, further contro l of their populations does not appear to be warranted.