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New Study: Polar Bear Population Growing Despite Declining Sea Ice

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Susan Crockford, Polar Bear Science

Exciting news about polar bears in eastern Canada: the peer-reviewed paper on the Davis Strait subpopulation study has finally been published (Peacock et al. 2013). It concludes that despite sea ice having declined since the 1970s, polar bear numbers in Davis Strait have not only increased to a greater density (bears per 1,000 km2) than other seasonal-ice subpopulations (like Western Hudson Bay), but it may now have reached its ‘carrying capacity.’

This is great news. But where is the shouting from the roof-tops? This peer-reviewed paper (with its juicy details of method and analysis results), considered by some to be the only legitimate format for communicating science, was published February 19, 2013. No press release was issued that I could find and consequently, there was no news coverage. Funny, that.

There was a bit of shouting back in 2007 when the study ended and the preliminary population count was released – polar bear biologist Mitch Taylor is quoted in the Telegraph (March 9 2007) as saying:

“There aren’t just a few more bears. There are a hell of a lot more bears.”

There was also a CBC news item in January 2007 and aNunatsiaq|Online report in October 2009 when the official government report was completed. But these were all based on preliminary information and focused on the population increase only.

This new paper (Peacock et al. 2013) reveals that the story in Davis Strait is about more than simple population growth. Small wonder no one is drawing attention to it.

Davis Strait is most southerly subpopulation of polar bears (Fig. 1), because some bears move down as far as southern Newfoundland (470N) when sea ice is at its maximum in the spring. Davis Strait, in total area, is almost as large as the three Hudson Bay subpopulations together – Western Hudson Bay, Southern Hudson Bay and Foxe Basin – according to area data given by Vongraven and Peacock (2011). However, a lot of that area is land and not all of the water is ice-covered, even in late spring. The actual “suitable ice habitat in spring” (determined by Taylor and Lee 1995) averages only 420,100 km2, which is about 16% of the total area.

[Note that Davis Strait is one of the subpopulations heavily impacted by whalers between the 1890s and the 1930s (see previous post here), so bears in this region have probably been recovering since then (discussed previously here).]

Figure 1. The Davis Strait subpopulation region runs from just below the Arctic Circle at the north end to at least 470N in the south (map on the left from Vongraven and Peacock 2011: Figure 6). In total area, it covers 2.62 million km2 but much of that area is land or open water, even in late spring. The actual “suitable ice habitat in spring” (determined by Taylor and Lee 1995) is only 420,100 km2, which is about 16% of the total area. This seems a reasonable average: the map on the right shows the sea ice extent at the end of March 2010 (NSIDC).

Figure 1. The Davis Strait subpopulation region runs from just below the Arctic Circle at the north end to at least 470N in the south (map on the left from Vongraven and Peacock 2011: Figure 6). In total area, it covers 2.62 million km2but the actual “suitable ice habitat in spring” (Taylor and Lee 1995) is only 420,100 km2, which is about 16% of the total area. This seems a reasonable enough average: the map on the right shows the sea ice extent at the end of March 2010 (NSIDC).

The new study, by Lily Peacock, Mitch Taylor and two other colleagues, compared data from mark-recapture studies done in 1974-1979 to those undertaken in 2005- 2007. They state that in Davis Strait, “the overall amount of sea ice declined and breakup has become progressively earlier” since the 1970s.

However, in spite of this decline in sea ice, they estimated the number of bears at about 2,158, a substantial increase over the estimate of about 1,400 bears in 1993 (Derocher et al. 1998:27 – see previous discussion here). […]

So, now we have at least two reports in the peer-reviewed literature that state flat out that the presumed negative effects of declining sea ice on a population’s size are indistinguishable from a population that is as large as it can get.

Hard to believe, isn’t it? Rather than being proven victims of Arctic sea ice in a “death spiral” due to global warming, when they finally present the data, biologists have to admit that they cannot actually tell the difference between a polar bear population that is so large that it can no longer increase and one that is suffering a population decline because of reduced sea ice. 

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