A demographic analysis of the Yellowstone National Park bison (Bison bison) herds from 1902 to 2000

Julie A. Fuller and Robert A. Garrott

The Yellowstone National Park (YNP) bison population is increasingly the subject of public and political attention. Bison are considered by many to be "charismatic megafauna", and are popular tourist attractions to YNP. However, bison harbor the disease brucellosis (Brucella abortus), which is of concern to livestock-producing states around YNP. In recent years, bison have been leaving YNP boundaries in winter more frequently than they did historically (NPS 2000), with some animals spending part of winter on U.S. Forest Service grazing allotments or on private lands. This behavior worries cattle producers, and has led to considerable debate about why bison are leaving the park now more than in the past. There is conjecture that bison leave YNP more frequently due to the ease of travel on groomed and packed of trails maintained for snowmachines during winter time. Alternately, bison may be leaving YNP simply to extend their range as a response to their increasing population.

The objective of this project is to thoroughly analyze the population demography of the YNP bison herds using more than 100 years of count and removal information (1902-present) and 34 years of herd composition surveys (1970-present); one of the largest, long-term data sets for a large herbivore population. The YNP bison population will provide an excellent natural experiment in effects of density on a population: they have been subject to periods of encouraged growth, unrestrained growth, and they have been managed around set population levels. Bison experience negligible predation, thus are regulated through density-dependent factors, weather effects, and human removals. An examination of bison population trends and demographic structure in relation to density-dependent and density-independent factors may help clarify the extent to which bison leave the park due to density-dependent pressures or due to the presence of low-energy travel routes, such as the groomed roads for snow machines in the winter time.

Funding:  National Park Service