Reproduction and Survival of Yellowstone Bison

Abstract

The conservation of bison (Bison bison) from near extinction to >4,000 animals in Yellowstone National Park has led to conflict regarding overabundance and potential transmission of brucellosis (Brucella abortus) to cattle. We estimated survival and birth rates from 53 radiocollared adult female bison during 1995–2001, and we used calf:adult (C:A) ratios to estimate reproduction with the combined effects of pregnancy, fetal loss, and neonatal mortality during 1970–1997. 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 (β = −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 3 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 (γ = 1.09), possibly leading to more bison movements outside the park. Our results will help park managers evaluate bison population dynamics and explore consequences of management actions and disease control programs.

State‐space modeling to support management of brucellosis in the Yellowstone bison population

“…The posterior distribution of the realized growth rate reflecting the influence of Brucellosis (median k ¼ 1.07; BCI ¼ 1.03, 1.11; Fig. 8B) was centered on the previous point estimate (k ¼ 1.07) for Yellowstone bison obtained by Fuller et al (2007b). We estimated that brucellosis reduced the growth rate of the Yellowstone bison population by 5% points (Fig.…”

“…However, 95% confidence limits on previous estimates of adult survival (Fuller et al (2007b) The posterior distribution of the growth rate we would expect for the Yellowstone bison population in the absence of infection with brucellosis (median ¼ 1.11; BCI ¼ 1.07, 1.16; Fig. 8A) strongly overlapped the point estimate of the brucellosis-free growth rate of bison populations obtained by Fuller et al 2007b (k ¼ 1.09), Vanvuren and Bray 1986 (k ¼ 1.10), but did not include the estimate of Fuller et al (2007b), k ¼ 1.05. The posterior distribution of the realized growth rate reflecting the influence of Brucellosis (median k ¼ 1.07; BCI ¼ 1.03, 1.11; Fig.…”

“…There is a rich, multi-decadal database that can be used to support estimation of parameters and to understand the effect of brucellosis on the population's growth rate and its demography. Published research studies provide prior information on the influence of brucellosis on vital rates (Fuller et al 2007b). The ability to evaluate alternative models of disease transmission is enhanced by periodic reductions in animal numbers following removals by park management.…”

“…We estimated separate, time-invariant fecundities for seronegative animals ( f n ), animals that have seroconverted and are actively infectious ( f c ), and recovered animals that are seropositive ( f p ); see Fig. 2A and Fuller et al 2007b). These terms include neonatal survival as well as the number of offspring born per female.…”

“…We used informative prior distributions for probability of vertical transmission and recrudescence derived from mark-recapture studies (section Infectiousness) and for fecundity parameters reported in Fuller et al (2007b); see Table 3. The model of Fuller et al (2007a) for bison fecundity includes the Palmer Drought f g s ði;tÞ n ði;tÀ1Þ þ P i2 2;3;5;6;7 f g s ði;tÞ n ði;tÀ1Þ…”