Population demography of the Yellowstone National Park bison herds

Abstract

The bison population of Yellowstone National Park (YNP) represents an outstanding conservation success story, as their numbers have increased from just 46 in 1902 to >4,000 today. Bison are keystone herbivores in YNP, and are central to several management controversies due to their infection with brucellosis. I integrated 100 years of historical count data with 7 years of recent vital rate data, and parameterized a matrix model using these vital rates to validate population growth estimates and to project future management scenarios involving disease control through vaccination and/or culling programs. From count data, I determined that historical population growth rates were heavily influenced by supplemental feeding, resulting in high population growth rates (ë = 1.17). I found evidence for density dependence in two subpopulations, the northern and central herds. The herds displayed divergent population dynamics over 1970-1981 and 1982-2000, as density dependence weakened in the northern herd and increased in the central herd, and herd growth rates were negatively correlated after 1982 (R2=0.40, P<0.01) I postulate the severe winter of 1982 catalyzed emigration from the central herd to the northern herd. Survival and birth rates did not differ between the herds 1995-2001, and calf-adult ratios did not differ between the herds or the periods. I found that snow pack was negatively associated with the central herd population growth rates, and with spring calf ratios and recruitment (R2=0.26-0.60, P<0.05). Birth rates were not variable with density-dependent or density-independent factors, but did vary according to serological status for brucellosis and with age structure, as primiparous bison had lower birth rates than adults. I did not detect reproductive or survival senescence. Adult survival rates were high and static (0.92). Integrating these vital rates into a matrix model resulted in a population growth rate estimate of ë=1.07, which closely corroborated an estimate of ë from count data during 1990-2000 (ë=1.05). Simulating the effects of brucellosis eradication through vaccination programs resulted in ë = 1.09, roughly a 29% increase. I concluded brucellosis eradication could further increase bison population growth rates, exacerbating conflicts outside YNP.