Polar Bear Population Dynamics

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Information on the status and trends of polar bear populations are needed to inform management of polar bears under US laws and international agreements. The USGS maintains a long-term research program focused on the population dynamics of the southern Beaufort Sea polar bear population.  In addition, the USGS collaborates with the US Fish and Wildlife Service in population studies in the Chukchi Sea.

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Polar bear population status in the southern Beaufort Sea

In the southern Beaufort Sea of the United States and Canada, prior investigations have linked declines in summer sea ice to reduced physical condition, growth, and survival of polar bears.  In 2015, we published a study that used mark–recapture models to investigate the population dynamics of polar bears in the southern Beaufort Sea from 2001 to 2010, years during which the spatial and temporal extent of summer sea ice generally declined. This study found that low survival, particularly of cubs, from 2004 through 2006 led to a 25–50% decline in population size in less than 10 years.  Currently, we are developing spatial capture-recapture models that incorporate resource selection into the abundance modeling process. This new approach should provide a biologically-relevant mechanism for bear density to vary within the study area, thereby reducing uncertainty associated with abundance estimates.  Further we are conducting a variety of research to better understand the factors affecting survival and reproduction.

Forecasting the influence of environmental and anthropogenic stressors on polar bears

US and international conservation planning efforts, including management of polar bears as a threatened species under the Endangered Species Act and the circumpolar action plan for polar bears, require assessments that evaluate the temporal and spatial pattern over which sea ice loss and other factors may affect polar bear populations. We have developed Bayesian network models to forecast the circumpolar status of polar bears, evaluate the potential benefit of mitigated greenhouse gas emissions on sea ice habitat, and the influence of environmental and anthropogenic stressors on population persistence. Consistently, sea ice conditions are the most influential determinant of long-term population outcomes. Adverse consequences of loss of sea ice habitat become more pronounced as the summer ice‐free period lengthens beyond four months, which could occur in most of the Arctic basin after mid‐century if greenhouse gas emissions are not promptly reduced. Until further sea ice loss is stopped, management of other stressors may serve to slow the transition of populations to progressively worsened outcomes, and improve the prospects for their long‐term persistence.