Reproduction and Survival of Yellowstone Bison

The conservation of bison from near extinction to 4,000 animals in Yellowstone National Park has led to conflict regarding the population size and the potential transmission of brucellosis to cattle. Researchers estimated survival and birth rates from 53 radiocollared adult female bison from 1995 to 2001 and used calf:adult ratios to estimate reproduction with the combined effects of pregnancy, fetal loss, and neonatal mortality from 1970 to 1997.

• Annual survival of adult females was high (0.92; 95% CI=0.87–0.95) and constant.
• Birth rates differed by brucellosis status and age. Birth rates were 0.40 calves per female (95%CI=0.15–0.65) for brucellosis-positive 3 year olds, 0.63 (95% CI=0.39–0.87) for individuals testing negative, and 0.10 (95% CI=0.00–0.24) for individuals contracting brucellosis that birth year (sero-converters). Birth rates were 0.64 (95% CI=0.52–0.76) for brucellosis-positive individuals >]4 years old, 0.81 (95% CI=0.73–0.89) for brucellosis-negative individuals, and 0.22 (95% CI=0.00–0.46) for sero-converters.
• Spring C:A ratios were negatively correlated with snow pack (b=_0.01 to_0.03, R2=0.26–0.60, P, 0.05).
• Growth rate was highly elastic to adult survival (0.51), and juvenile survival (0.36) was three times more elastic than fecundity (0.12).

Simulations suggested brucellosis eradication via vaccination would result in increased birth rates and a 29% increase in population growth (k = 1.09), possibly leading to more bison movements outside the park. These results helped park managers evaluate bison population dynamics and explore consequences of management actions and disease control programs.

Updated 8/11/10