Emigration and Density Dependence in Yellowstone Bison

Understanding the relative importance of density-dependent and density-independent feedback on population growth is essential for developing management strategies to conserve wildlife. Researchers examined a 99-year time series of annual counts and removals for two bison herds occupying northern and central Yellowstone National Park. Yellowstone’s aggressive management intervention recovered bison from 46 animals in 1902 to 1,500 animals in 1954. Supplemental feeding of the northern herd facilitated rapid growth (r = 0.16) from 1902 to 1952. Augmentation of the central herd with 71 animals also led to rapid growth from 1936 to 1954 (r = 0.10). Evidence of density-dependent changes in population growth rates for both herds was detected after manipulative bison management ceased in 1969 as the population increased to 3,000.

The central herd showed evidence of a constant density-dependent response from 1970 to 2000. In contrast, density dependence had a stronger effect on the growth of the northern herd from 1970 to 1981 than from 1982 to 2000. The evidence suggests that these trends reflect pulses of emigration from the central herd to the northern range beginning in 1982 in response to resource limitation that resulted from increased population density and severe snow pack. This interpretation is supported by corroborative evidence, including these findings; 1) the annual growth of the central herd was negatively correlated with snow pack but that of the northern herd was not, 2) growth rates of the central and northern herds were uncorrelated from 1970 to 1981 but significantly and negatively correlated from 1982 to 2000, and 3) the northern herd could not have sustained the high removals experienced from 1984 to 2000 without immigration. Density-related emigration from the central herd to the northern range may fuel bison emigration onto private and public lands where large-scale removals occur, exacerbating the brucellosis controversy for natural resource managers.

Updated 8/11/10