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Hierarchical models provide insight into wildlife and disease management

Date

2014

Authors

Geremia, Chris, author
Hobbs, N. Thompson, advisor
White, P. J., committee member
Miller, Michael W., committee member
Hoeting, Jennifer A., committee member
Antolin, Michael F., committee member

Journal Title

Journal ISSN

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Abstract

Wildlife diseases can alter host populations with cascading effects throughout ecosystems and human economies that rely on those wildlife. Pathological effects can be the ultimate cause of wildlife population decline through depressing host reproduction and survival. Otherwise, less virulent pathogens can harm host populations indirectly, through management actions imposed on wildlife populations harboring diseases that harm people or their livelihoods. Hierarchical Bayesian methods provide a framework for factoring highly dimensional problems into lower dimensional ones. These techniques decompose a problem into data, the underlying process, and parameters, and identify uncertainty associated with each component. Appropriately quantifying uncertainty fosters clearer understanding of wildlife and disease management problems. Bison (Bos bison) migrating from Yellowstone National Park into the state of Montana during winter and spring concern ranchers on lands surrounding the park because bison can transmit brucellosis (Brucella abortus) to cattle. Migrations have been constrained with bison being lethally removed or moved back into the park. I, and several coauthors (we) developed a state-space model to support decisions on bison management aimed at mitigating conflict with landowners outside the park. The model integrated recent GPS observations with 22 years (1990-2012) of aerial counts to forecast monthly distributions and identify factors driving migration. Wintering areas were located along decreasing elevation gradients and bison accumulated in wintering areas prior to moving to progressively lower elevation areas. Bison movements were affected by time since the onset of snow pack, snow pack magnitude, standing crop, and herd size. Migration pathways were increasingly used over time, suggesting experience or learning influenced movements. To support adaptive management of Yellowstone bison, we forecast future movements to evaluate alternatives. Our approach of developing models capable of making explicit probabilistic forecasts of large herbivore movements and seasonal distributions is applicable to managing the migratory movements of large herbivores worldwide. These forecasts allow managers to develop and refine strategies in advance, and promote sound decision-making that reduces conflict as migratory animals come into contact with people. Chronic wasting disease (CWD) is a fatal, neurodegenerative prion disease that affects members of the deer family (Cervidae). There is worldwide concern that the disease may harm ecosystems and human economies by causing demise of deer populations. Little is known about effects of the disease on population dynamics. We studied a mule deer population where CWD has been present for at least four decades. We developed a disease model to estimate the effect of CWD on population growth rate and extent that the epidemic is increasing. Our model integrated capture-mark-recapture histories of adult female mule deer during a four year study with long-term population monitoring data on abundance, composition, and CWD prevalence. Our model was capable of deciphering probabilities of infection and correct identification of infected individuals from disease tests. We provide compelling evidence that prion epidemics can affect mule deer populations both locally and at coarse spatial scales. Chances of population decline were greatest at the wintering subpopulation scale, but differences in infection rate among subpopulations caused CWD to have virtually no effect on growth in some wintering subpopulations. At larger scales, deer populations showed some natural resistance against CWD by localizing areas of higher infection. Overall, disease effects were subtle and the protracted time-scale of the epidemic is likely much longer than the thirty year time span of our research. As a result, we could not identify the inevitable fate of deer populations with CWD. Our findings do suggest, in the nearer-term (e.g., decades), mule deer populations persisting at lower levels after disease establishment.

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Subject

brucellosis
bison
hierarchical Bayes
mule deer
Yellowstone
CWD

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