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Population‐level influence of a recurring disease on a long‐lived wildlife host

Perez‐Heydrich, Carolina, Oli, Madan K., Brown, Mary B.
Oikos 2012 v.121 no.3 pp. 377-388
Gopherus polyphemus, at-risk population, chronic diseases, hosts, infectious diseases, models, mortality, population dynamics, respiratory tract diseases, wildlife diseases, wildlife management
Despite a heightened interest regarding the role of infectious diseases in wildlife conservation, few studies have explicitly addressed the impacts of chronic, persistent diseases on long‐term host population dynamics. Using mycoplasmal upper respiratory tract disease (URTD) within natural gopher tortoise Gopherus polyphemus populations as a model system, we investigated the influence of chronic recurring disease epizootics on host population dynamics and persistence using matrix population models and Markov chain models for temporally autocorrelated environments. By treating epizootics as a form of environmental stochasticity, we evaluated host population dynamics across varying levels of outbreak duration (ρ), outbreak recurrence (f), and disease‐induced mortality (μ). Baseline results indicated a declining growth rate (λ) for populations under unexposed or enzootic conditions (λEnzootic= 0.903, 95% CI: 0.765–1.04), and a median time to quasi‐extinction of 29 years (range: 28–30 years). Under recurring epizootics, stochastic growth rates overlapped with baseline growth rates, and ranged between 0.838–0.902. Median quasi‐extinction times under recurring epizootics also overlapped for most scenarios with those of baseline conditions, and ranged between 18–29 years, with both metrics decreasing as a function of f and μ. Overall, baseline (enzootic) conditions had a greater impact on λ than epizootic conditions, and demographic vital rates were proportionately more influential on λ than disease‐ or outbreak‐associated parameters. Lower‐level elasticities revealed that, among disease‐ and outbreak‐associated parameters, increases in μ, force of infection (φ), and f negatively influenced λ. The impact of disease on host population dynamics depended primarily on how often a population underwent an epizootic state, rather than how long the epizootic persisted within the exposed population. The modeling framework presented in this paper could be widely applied to a range of wildlife disease systems in which hosts suffer from persistent recurring diseases.