Destabilization of cow elk survival and population growth by chronic wasting disease

Monello, Ryan; Powers, Jenny; Hobbs, Thompson

Investigations of chronic wasting disease (CWD), a contagious prion disease of cervids, have found it can cause long-term population declines in deer (Odocoileus spp.). However, almost no data exist on the individual or population-level effects of CWD on free-ranging elk (Cervus elaphus nelsoni). We conducted a radio-telemetry study on adult female elk from 2008-2010 and determined incidence (proportion of susceptible elk infected per year), sources of mortality, annual survival, and population growth in a high-density herd from Rocky Mountain National Park (RMNP) that has been infected by CWD for over 25 years. Annual incidence of CWD was 0.08 (95% credible limits (CL) = 0.05, 0.12). Following removal of elk that tested positive for CWD via rectal biopsy at project initiation, annual survival (excluding harvest) consistently declined from 0.97 in 2008 (CL = 0.93, 0.99) to 0.85 in 2010 (CL = 0.75, 0.93). Declines in survival were due almost entirely to increases in the cause-specific mortality rate of CWD, which increased from 0.02 in 2008 (CL = 0.00, 0.05) to 0.11 in 2010 (CL = 0.04, 0.21). We used survival rates of susceptible and infected elk to develop a transition matrix, which estimated the intrinsic population growth rate (λ) of this elk herd to be 0.97 (CL = 0.90, 1.06) at the prevalence of CWD (12.9%) observed during this study. We conclude that CWD-caused mortality can exceed natural rates of mortality, destabilize cow elk survival, and cause population declines in high-density elk herds.