Population demography of the Yellowstone National Park bison herds
Fuller, Julie Ann
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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.