Thunder on the Yellowstone revisited: an assessment of management of native ungulates by natural regulation, 1968-1993

Natural regulation of native ungulates was initiated in 1968 in Yellowstone National Park (YNP) based on the premise that ungulates would reach an equ ilibrium with their plant resources. The natural-regulation management mode l stated: density dependence wilt regulate ungulates (i.e., a dynamic equil ibrium will result between ungulates and their food supply, within some bou nds of vegetation and soil effects); and no retrogression of soil and veget ation will occur from elk (Cervus elaphus) grazing during this process. The historical record indicated that elk were abundant in the system and elk w ere primarily food limited before settlement by European man (i.e., wolves [Canis lupus] and Native Americans were only an adjunct to the density depe ndent population regulation of ungulates). Density dependence was demonstra ted in elk, but not in bison (Bison bison). No widespread evidence of overg razing was observed through 1993 in study sites within vegetation communiti es that comprised about 97% of the winter range. No evidence of increased e xotics, increased sediment yield, warming or drying of the soil, changes in soil nutrients, or differences in aboveground standing-crop biomass of pla nts was found between grazed and ungrazed plots. Ungulate herbivory apparen tly stimulated aboveground production of grasses, enhanced nitrogen and mac ronutrients in grasses, increased nutrient cycling, and enhanced measures o f fitness in 6 common plants. However, exposed soil surface (bare ground an d pebbles combined) was 11-18% greater on grazed than ungrazed plots, appar ently due to a 71% decline in dead and standing litter on grazed plots. Per cent live-plant basal cover, however, did not differ on grazed versus ungra zed plots, and there was no difference in soil microclimate or sediment yie ld. Differences in the abundance of 12% of the herbaceous species were foun d in grazed versus ungrazed sites (16 of 128 species); 10 were declines and 6 were increases. Willow (Salix spp.) and aspen (Populus tremuloides) decl ines predated the new management policy, but their slow declines also conti nued after 1968. Three uncommon plant species (aspen, willow, and Wyoming b ig sagebrush [Artemisia tridentata tridentata]) and 1 herbivore(moose [Alce s alces]) declined under natural-regulation management. Two uncommon specie s of woody browse (aspen, Wyoming big sagebrush) were overutilized by ungul ates (consumption of >2/3 current annual growth occurred). We conclude 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 p rior to establishment of the national park. The period selected as a standa rd (1870) was an unusual period characterized by frequent large fires and f loods, common wolves, few elk, and a cooler, wetter climate. We also conclu de there is a high level of uncertainty surrounding what elk densities were in pre-Columbian times (element 3 of the management model) and what effect s wolves and Native Americans had in regulating the elk population.