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Extinction Countdown

Extinction Countdown

News and research about endangered species from around the world

New Polar Bear Counting Method Creates Confusion

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A few weeks ago, the director of wildlife for Nunavut, Canada, made an unexpected declaration, claiming that the number of polar bears (Ursus maritimus) in the western Hudson Bay region is increasing, even though scientists say the population is declining. Western Hudson Bay is one of 19 distinct polar bear subpopulations, and previous research has suggested that the animals in that region could die out in 25 to 30 years as climate change eliminates the sea ice that they rely on for hunting and breeding. Media outlets such as Forbes and the Web sites of various climate change skeptics quickly picked up on the announcement and published headlines such as "Polar Bears Hate Al Gore."

Too bad they all got their facts wrong.

The misinterpretations stem from a new aerial study (pdf) of the polar bear population in western Hudson Bay, conducted under the auspices of the Nunavut Department of Environment. The survey, conducted in August 2011, estimated the total population of polar bears in the region to be 1,013 animals, derived from a statistically plausible range of between 717 and 1,430 bears. The actual number of bears observed was 701.

Before this aerial survey, the most recent estimate of polar bears in that region was 935 animals in 2004, down from 1,194 in 1987. The new number, according to the wildlife director, shows that polar bear populations are increasing.

But here's the thing: all previous counts of the polar bear population in western Hudson Bay used a different methodology called capture–recapture, in which animals are quite literally captured, studied, marked and released. Later recaptures can then provide a statistical estimate of population size. The differing methodology between the two studies—plus the fact that the aerial surveys were conducted over a slightly different and larger geographical area than the capture–recapture studies—means that the two counts are apples and oranges: They can't be compared against each other in the hopes of determining a trend.

And in fact, the aerial study does not compare itself against the earlier capture–recapture surveys, nor does it claim that populations are increasing. Instead, it reads, "This aerial survey-based estimate is not significantly different from the 2004 mark–recapture estimate." It does point out that the 2004 capture–recapture study predicted the population would fall to 650 by this time, but says it is too early to make any real comparisons. Another capture–recapture survey was apparently also conducted in 2011 and estimates will be released later this year, at which time the report says the two studies can be formally compared.

The aerial survey did come up with one interesting discovery: The western Hudson Bay population has a very low level of young bears, with yearlings (young between one and two years old) representing just 3 percent of the observed animals. According to Steven Amstrup, chief scientist for Polar Bears International, the number of yearlings in a population should be closer to 15 percent. "This is classic population dynamics," Amstrup says. "If you have declining habitat quality, the first thing you're going to see impacted is the survival of young." He points out adult polar bears have energy stores that the young lack, so adults are more likely to survive tough times than are their offspring.

In addition to the low number of cubs, the report says the average observed litter size was the lowest ever recorded in all three Hudson Bay polar bear subpopulations—western and southern Hudson Bay and Foxe Basin—suggesting that reproductive output in the region "was poor in 2011."

Another interesting element about the report is its reason for existence in the first place. The aerial survey was a response to concerns from the Inuit population that they are seeing more polar bears entering their communities, giving rise to a belief that there are more bears in the region rather than fewer. The report calls this a "disparity" between science and "traditional ecological knowledge." It says the Inuit have doubts about the accuracy of the previous capture–recapture surveys and have called for new research to reassess the population.

"Many people in the ground haven't really believed the estimate derived by the scientists because they are seeing more bears," Amstrup says. He also says the fact that bears appear where they did not previously roam indicates that the animals' food system is broken. "They're going into new territories looking for something to eat."

The lowering of hunting quotas has also caused some ill will in the region. Last year Nunavut raised the quota in western Hudson Bay, where hunting has had both traditional and economic importance, from eight bears to 21, still well below the 56 allowed in 2006. Quotas were lowered based on the projections from the earlier population studies, so any studies that show an increasing population could be used to bring quotas back up.

Ultimately, though, Amstrup says the biggest concern is not whether polar bear populations are increasing or decreasing today, but whether or not they will be able to survive into the next century. "If we stay on the path we're on with greenhouse gases, we're going to lose them all," he says.

Polar bears are listed as vulnerable to extinction by the International Union for Conservation of Nature. The species is listed as threatened under the U.S. Endangered Species Act. Canada lists it as a species of "Special Concern." Canada is home to two thirds of the world's polar bears.

Photo: A polar bear in Nunavut by Rich Durant, via Flickr. Used under Creative Commons license

The views expressed are those of the author and are not necessarily those of Scientific American.

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