In a review article published in the April 2015 issue of Frontiers in Ecology and the Environment, titled "Can Polar Bears Use Terrestrial Foods to Offset Lost Ice-Based Hunting Opportunities?" the authors report that polar bears forced on shore due to sea-ice loss may be eating terrestrial foods, including berries, birds, and eggs, but any nutritional gains are limited to a few individuals and likely cannot compensate for lost opportunities to consume their traditional, lipid-rich prey—ice seals. They conclude that "Warming-induced loss of sea ice remains the primary threat faced by polar bears."

The second study, "Evaluating and Ranking Threats to the Long-Term Persistence of Polar Bears," reports results from a recently updated USGS forecasting model indicating that the state of the worldwide polar bear population will likely worsen over time through the end of this century, mainly because of continuing sea-ice loss.

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The modeling effort examined the prognosis for polar bear populations in four ecoregions (see map) using sea-ice projections from the Intergovernmental Panel on Climate Change for two greenhouse gas emission scenarios: one scenario in which climate warming is stabilized by century's end because of reduced greenhouse gas emissions, and the other in which the unabated rise of greenhouse gas emissions leads to increased warming by century's end. Under both scenarios, the outcome for the worldwide polar bear population will very likely worsen over time through the end of the century.

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“Substantial sea ice loss and expected declines in the availability of marine prey that polar bears eat are the most important specific reasons for the increasingly worse outlook for polar bear populations,” said Todd Atwood, research wildlife biologist with the USGS and lead author of the modeling study. “We found that other environmental stressors such as trans-Arctic shipping, oil and gas exploration, disease and contaminants, sustainable harvest, and defense-of-life takes, had only negligible effects on polar bear populations—compared with the much larger effects of sea-ice loss and associated declines in their ability to access fat-rich marine mammal prey.”

Few foods are as energetically dense as marine prey. Studies suggest that polar bears consume the highest lipid diet of any species, which provides all essential nutrients and is ideal for maximizing fat deposition and minimizing energetic requirements. Potential foods found in the terrestrial environment are dominated by high-protein, low-fat animals and vegetation. Although most bear species consume at least some vegetation, bears that consume high dietary proportions of vegetation are among the smallest. For example, Arctic grizzly bears whose diet is primarily vegetation are the smallest of their species and occur at the lowest densities of any populations. Thus, it would be difficult for polar bears, the largest of all bears, to ingest and digest sufficient volumes of terrestrial foods required to support their large body size.

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"The evidence thus far suggests that increased consumption of terrestrial foods by polar bears is unlikely to offset declines in body condition and survival resulting from sea-ice loss," said Karyn Rode, a USGS research wildlife biologist who is the lead author of the Frontiers in Ecology and the Environment paper and a coauthor of the modeling study.

The modeling study was conducted by USGS scientists together with collaborators from the U.S. Forest Service and Polar Bears International. See also, the USGS Fact Sheet “Changing Arctic Ecosystems: Updated forecast: Reducing carbon dioxide (CO₂) emissions required to improve polar bear outlook.” The review article was written by researchers at the USGS, Washington State University, and Polar Bears International. Both studies are part of the USGS Changing Arctic Ecosystems Initiative. For further information, read about USGS Polar Bear Research.