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Populationâlevel influence of a recurring disease on a longâlived wildlife host
- PerezâHeydrich, 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 longâterm 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 (Ï), outbreak recurrence (f), and diseaseâinduced mortality (Î¼). Baseline results indicated a declining growth rate (Î») for populations under unexposed or enzootic conditions (Î»Enzootic= 0.903, 95% CI: 0.765â1.04), and a median time to quasiâextinction of 29 years (range: 28â30 years). Under recurring epizootics, stochastic growth rates overlapped with baseline growth rates, and ranged between 0.838â0.902. Median quasiâextinction times under recurring epizootics also overlapped for most scenarios with those of baseline conditions, and ranged between 18â29 years, with both metrics decreasing as a function of f and Î¼. Overall, baseline (enzootic) conditions had a greater impact on Î» than epizootic conditions, and demographic vital rates were proportionately more influential on Î» than diseaseâ or outbreakâassociated parameters. Lowerâlevel elasticities revealed that, among diseaseâ and outbreakâassociated parameters, increases in Î¼, force of infection (Ï), and f negatively influenced Î». 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.