Survival and population growth of a free‐ranging elk population with a long history of exposure to chronic wasting disease

Abstract

ABSTRACTInvestigations of chronic wasting disease (CWD), a fatal, contagious prion disease of free‐ranging cervids, suggest the disease can cause long‐term population declines in deer (Odocoileus spp.). However, the implications of CWD for elk (Cervus elaphus nelsoni) populations are less certain. During 2008–2010, we used rectal biopsies and telemetry to observe disease transmission and survival in adult female elk from a high‐density herd in Rocky Mountain National Park (RMNP) that had been infected by CWD for over 25 years. We studied a cohort of 123 adult female elk that were determined to be free of CWD by rectal biopsy in 2008. Annual incidence of CWD was 0.08 [95% Bayesian credible interval (BCI) = 0.05, 0.12]. Annual survival probabilities of the cohort excluding harvest declined from 0.97 in 2008 (BCI = 0.93, 0.99) to 0.85 in 2010 (BCI = 0.75, 0.93). Declines in survival were attributed almost entirely to CWD; the proportion of radiocollared elk that died of CWD increased from 0.02 in 2008 (BCI = 0.00, 0.05) to 0.11 in 2010 (BCI = 0.04, 0.21). We attributed the increase to the time lag required for development of new CWD cases. We used survival rates of susceptible and infected elk to develop a projection matrix for a discrete time, female only model that estimated the intrinsic population growth rate (λ) of this elk herd to be 1.00 (BCI = 0.93, 1.05) using the prevalence of CWD (12.9%) and calf:cow ratios (24:100) observed during this study. Population declines were predicted to occur when prevalence of CWD exceeded 13% (BCI = 0, 35). However, this estimate was contingent on calf:cow ratios and harvest. Greater recruitment will offset some of the effects of CWD, whereas the inclusion of female harvest, which was excluded from this study, would likely result in lower λ values than those observed in this study. We conclude that CWD can exceed natural rates of mortality, reduce survival of adult females, and decrease population growth of elk herds. Published 2014. This article is a U.S. Government work and is in the public domain in the USA.