Polar Bear Population

On May 14, 2008, the U.S. Department of the Interior announced its decision to classify polar bears as "threatened," under the Endangered Species Act.

Republican Sen. Ted Stevens of Alaska expressed his disapproval that day in a press release, saying that the classification was unnecessary:

Sen. Stevens: Scientists have observed that there are now three times as many polar bears in the Arctic than there were in the 1970s.

Steven Amstrup/USGS

Figures cited by government officials do show an increase – though not as big a jump as Stevens claims. Interior Secretary Dirk Kempthorne said at the department’s press conference that the polar bear population increased from "a low of about 12,000 in the late 1960s to approximately 25,000 today." But the size of the world’s polar bear population is subject to much debate.

Sen. Stevens’ press office told us he based his claim on numbers from the U.S. Fish and Wildlife Service’s Web site. However, we couldn’t find support for his statement on the site. Instead, we found a discussion of why it’s so hard to point to reliable numbers on the polar bear population.

Factors such as low population density, inaccessible habitats, movement of bears across international borders and budget constraints limit scientists’ abilities to accurately and precisely measure the number of polar bears. Furthermore, according to an essay by Scott L. Schliebe, polar bear project leader for the U.S. Fish and Wildlife Service, more sophisticated satellite and thermal technology exists today than was available during the 1970s, making previous population estimates less accurate than today’s approximations. 

The situation is further complicated by the fact that relatively little data, if any, reflect the entire population of polar bears as a whole. Instead, scientists tend to group them into subpopulations, known as "stocks" or "populations," based on habitat. The World Wildlife Fund estimates that today there are about 22,000 polar bears living in 20 different populations.

Some of these populations are growing; others are relatively stable. The status of certain populations is unknown. But studies by the United States Geological Survey show that some populations may, in fact, be shrinking. One USGS study from 1984-2004 showed that the number of polar bears in the West Hudson Bay stock, in Canada, decreased from 1,194 polar bears in 1987 to 935 in 2004, a 22 percent drop. USGS documented reductions in the weight of adult bears and the survival rate of newborn cubs along with this population decrease, which correlated with a loss of sea ice. A 2007 USGS report on the status of polar bears in Alaska’s Southern Beaufort Sea found a similar decrease in cub survivorship.

Yet many reports show that the total number of polar bears has increased since the late 1960s and 1970s. This growth in population is often attributed to the passage of the Marine Mammal Protection Act (MMPA) in 1972. The MMPA says that it is unlawful "for any person subject to the jurisdiction of the United States or any vessel or other conveyance subject to the jurisdiction of the United States to take any marine mammal on the high seas." The MMPA defined “take” as “to harass, hunt, capture, or kill, or attempt to harass, hunt, capture, or kill any marine mammal.”

The 1973 International Agreement on the Conservation of Polar Bears, between the United States, Russia (at that time, the Soviet Union), Norway, Denmark and Canada, applied these and other prohibitions to a much larger area. Polar bears underwent a recovery period in subsequent decades.

However, most figures regarding the number of bears in the 1970s are based on guesses. Peter Dykstra, CNN’s executive producer for Science, Technology, and Weather, wrote in a May 15 blog post that there was no consensus between the five polar bear nations regarding the population in the 1970s.

Peter Dykstra: For example, based on observer reports from Arctic villages, ships, and other sources, U.S. researchers came up with an estimate of 18,000 polar bears throughout the Arctic. The Canadian Wildlife Service set the number at 20,000. The Soviet Union submitted the low bid, estimating a worldwide population of 5,000 animals.

So why place polar bears on the list of "threatened" species if their numbers have been growing? Many scientists believe that due to climate change and resulting environmental factors, the trend is reversing. The Department of the Interior’s classification has more to do with increasing concerns for the future than with current population numbers. The Endangered Species Act of 1973 defines a "threatened" species as one "which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range." In the case of polar bears, the "foreseeable future" refers to the next 45 years.

Polar bears rely on sea ice as a platform for hunting seals, their primary source of food, and global warming and climate change are projected to cause a severe decline in the amount of sea ice. This was the reason Kempthorne cited in his May 14 press conference: "First, sea ice is vital to polar bear survival," he said. "Second, the polar bear’s sea-ice habitat has dramatically melted in recent decades. Third, computer models suggest sea ice is likely to further recede in the future." (Kempthorne also outlined several measures designed to prevent the use of the Endangered Species Act as a vehicle to set policy on climate change. The Bush administration had been reluctant to list the polar bear as "threatened," according to news reports, because it feared that listing a species due to global warming-induced habitat loss would spur attempts by environmentalists to prohibit certain activities, such as natural resource exploitation, by invoking the act.)

According to several 2007 reports by the USGS, if the predicted decline in sea ice actually takes place, the world will lose nearly two-thirds of its polar bear population by the middle of the 21st century.

-Rachel Weisel

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