Singer et al. reviewed new and previously published data on ungulate numbers and soil and vegetation characteristics in and out of long-term exclosures in Yellowstone National Park (YNP), 25 years after the initiation of the Park's "natural-regulation" management model in 1968. In comparing exclosures with grazed plots throughout Yellowstone's winter range, the authors found little widespread evidence of increased levels of exotic species, greater sediment yield, soil warming or drying, changes in soil nutrients or aboveground standing crop. However, bare ground was 11-18% greater under grazing than in exclosures, resulting from a 71% reduction in litter and standing dead vegetation on grazed plots. Further, willow (Salix spp.), aspen (Populus tremuloides) and Wyoming big sagebrush (Artemisia tridentata tridentata), already uncommon prior to 1968, continued to decline under the policy of natural regulation. They conclude that the natural-regulation model for YNP was flawed in its assumptions of a single, steady state for the park, based on conditions presumed to exist in 1870 prior to establishment of the national park. The period selected as a standard (1870) was an unusual period characterized by frequent large fires and floods, common wolves, few elk, and a cooler, wetter climate.
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