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Open-File Report 2014–1254
Evaluating and Ranking Threats to the Long-Term Persistence of Polar Bears
By Todd C. Atwood, Bruce G. Marcot, David C. Douglas, Steven C. Amstrup, Karyn D. Rode, George M. Durner, and Jeffrey F. Bromaghin
Abstract
The polar bear (Ursus maritimus) was listed as a globally threatened species under the U.S.
Endangered Species Act (ESA) in 2008, mostly due to the significant threat to their future population
viability from rapidly declining Arctic sea ice. A core mandate of the ESA is the development of a
recovery plan that identifies steps to maintain viable populations of a listed species. A substantive
evaluation of the relative influence of putative threats to population persistence is helpful to recovery
planning. Because management actions must often be taken in the face of substantial information gaps,
a formalized evaluation hypothesizing potential stressors and their relationships with population
persistence can improve identification of relevant conservation actions. To this end, we updated a
Bayesian network model previously used to forecast the future status of polar bears worldwide. We used
new information on actual and predicted sea ice loss and polar bear responses to evaluate the relative
influence of plausible threats and their mitigation through management actions on the persistence of
polar bears in four ecoregions. We found that polar bear outcomes worsened over time through the end
of the century under both stabilized and unabated greenhouse gas (GHG) emission pathways. Under the
unabated pathway (i.e., RCP 8.5), the time it took for polar bear populations in two of four ecoregions to
reach a dominant probability of greatly decreased was hastened by about 25 years. Under the stabilized
GHG emission pathway (i.e., RCP 4.5), where GHG emissions peak around the year 2040, the polar
bear population in the Archipelago Ecoregion of High Arctic Canada never reached a dominant
probability of greatly decreased, reinforcing earlier suggestions of this ecoregion’s potential to serve as
a long-term refugium. The most influential drivers of adverse polar bear outcomes were declines to
overall sea ice conditions and to the marine prey base. Improved sea ice conditions substantively
lowered the probability of a decreased or greatly decreased outcome, while an elevated marine prey base
was slightly less influential in lowering the probability of a decreased or greatly decreased outcome.
Stressors associated with in situ human activities exerted considerably less influence on population
outcomes. Reduced mortality from hunting and defense of life and property interactions resulted inmodest declines in the probability of a decreased or greatly decreased population outcome. Minimizing other stressors such as trans-Arctic shipping, oil and gas exploration, and point-source pollution had negligible effects on polar bear outcomes, but that could be attributed to uncertainties in the ecological relevance of those specific stressors. Our findings suggest adverse consequences of loss of sea ice habitat become more pronounced as the summer ice-free period lengthens beyond 4 months, which could occur in portions of the Arctic by the middle of this century under the unabated pathway. The
long-term persistence of polar bears may be achieved through ameliorating the loss of sea ice habitat, which will likely require stabilizing CO2 emissions at or below the ceiling represented by RCP 4.5. Management of other stressors may serve to slow the transition of polar bear populations to progressively worsened outcomes, and improve the prospects of persistence, pending GHG mitigation.
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First posted January 9, 2015
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Suggested citation:
Atwood, T.C., Marcot, B.G., Douglas, D.C., Amstrup, S.C., Rode, K.D., Durner, G.M., and Bromaghin, J.F., 2015,
Evaluating and ranking threats to the long-term persistence of polar bears: U.S. Geological Survey Open-File Report 2014-1254, 114 p., https://dx.doi.org/10.3133/ofr20141254.
ISSN 2331-1258 (online)
Contents
Abstract
Introduction
Materials and Methods
Results
Discussion
Conclusion
Acknowledgments
References
Appendix A. Name, Job Title, Area of Expertise, and Professional Affiliation for Each Subject Matter Expert
Appendix B. Additional Information on the Process Used to Develop and Parameterize the Model
Appendix C. Description (Including Key Assumptions) of All Nodes in the Second Generation Model
Appendix D. Normative Scenarios (Using Expected Values of Input Conditions) Run on the
Second Generation Model
Appendix E. Conditional Probability Tables for Each Summary (i.e., Intermediate) Node in the Bayesian Network Model
Appendix F. Influence Run Results for RCP 4.5 and 8.5 by Ecoregion