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AGU Fall Meeting 2016: Polar bear numbers set to drop by one-third

by Liz Kalaugher

It’s highly likely that the world’s polar bear population, currently 26,000 strong, will decrease by a third within the next 35-40 years. That’s according to Kristin Laidre of the University of Washington, US, speaking at a press conference at the AGU Fall Meeting in San Francisco.

This finding supports the listing of polar bears as vulnerable by the International Union for Conservation of Nature (IUCN) Red List, which requires projection of species population numbers after three generations. Laidre and colleagues found that the average age of a female polar bear with dependent cubs is 11.5 years so 35 years corresponds to roughly three generations.

Polar bears use ice for traveling, hunting, mating and, in some cases, for their maternity dens. From 1979 to 2015, Arctic sea ice shrank at an average rate of 53,100 square km per year.

“When we look forward several decades, climate models predict such profound loss of Arctic sea ice that there’s little doubt this will negatively affect polar bears throughout much of their range, because of their critical dependence on sea ice,” said Laidre in a press release.

There are 19 sub-populations of polar bears around the Arctic Circle, living in 4 eco-regions. Some of these populations have never been studied whilst the number of animals in others have only been counted one or two times. Two or three populations have been studied on an annual basis, Laidre explained.

Some regions of the Arctic, such as the Chukchi Sea, are highly ecologically productive. Even though sea ice cover here has decreased, polar bears in the area are in better condition than they were 20 years ago; there could be a time lag before the full effects of the ice loss kick in. In other areas, like western Hudson Bay, polar bear survival and reproduction have declined as sea ice availability has dropped.

Laidre, Eric Regehr of the US Fish and Wildlife Service, and colleagues made their predictions by analysing sea ice extent from satellite observations and projecting this forward by 35 years. They evaluated three different relationships of polar bear numbers to sea ice – a decline proportional to the sea ice loss, and two changes based on previous data – before taking a median value across all three scenarios.

This revealed that the probability of a more than 30% reduction in polar bear numbers in the next 35-40 years is 0.71. The probability of a decrease in headcount of more than half was just 0.07.

Laidre and colleagues reported their findings in Biology Letters and The Cryosphere.

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