Signs that Davis Strait polar bears are at carrying capacity

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 a Nunatsiaq|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 km2 but 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).

Peacock et al. note that the density of bears in Davis Strait comes out to 5.1 bears/1,000 km2 of sea ice habitat, which is “greater than polar bear densities in other seasonal-ice subpopulations, which are approximately 3.5 bears/1,000 km2 (Taylor and Lee 1995).” Bears in Hudson Bay, for example, also live in a ‘seasonal-ice’ habitat.

Peacock et al. characterize the Davis Strait subpopulation as having “low recruitment rates [low birth rates], average adult survival rates, and high population density” and conclude – here’s the kicker – thatlow reproductive rates may reflect negative effects of greater densities or worsening ice conditions.” [my bold]

In other words, this subpopulation is showing changes expected in populations affected by declines in sea ice or one that has reached its carry capacity (i.e. “negative effects of greater densities”) but that it is not possible to distinguish between the two.

This is actually not the first time this conclusion has been reached. As discussed previously here, Andrew Derocher (2005) stated in regards to his study of Svalbard area polar bears in the Barents Sea (1988-2002): “given that the population may be showing density-dependent responses, it is not possible to differentiate the climatic effects from population effects.” [my bold]

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. 

References
Derocher 2005. Population ecology of polar bears at Svalbard, Norway. Population Ecology 47:267-275. http://link.springer.com/article/10.1007/s10144-005-0231-2

Derocher, A., Garner, G.W., Lunn, N.J., and Wiig, Ø. (eds.) 1998. Polar Bears: Proceedings of the 12th meeting of the Polar Bear Specialists Group IUCN/SSC, 3-7 February, 1997, Oslo, Norway. Gland, Switzerland and Cambridge UK, IUCN.

Peacock, E., Taylor, M.K., Laake, J., and Stirling, I. 2013. Population ecology of polar bears in Davis Strait, Canada and Greenland. Journal of Wildlife Management 77:463–476. http://onlinelibrary.wiley.com/doi/10.1002/jwmg.489/abstract?deniedAccessCustomisedMessage=&userIsAuthenticated=false

Taylor, M., and Lee, J. 1995. Distribution and abundance of Canadian polar bear populations: a management perspective. Arctic 48:147-154. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/1236/1261

Vongraven, D. and Peacock E. 2011. Development of a pan-Arctic monitoring plan for polar bears: background paper. Circumpolar Biodiversity Monitoring Programme, CAFF Monitoring Series Report No. 1, CAFF International Secretariat, Akureyri, Iceland. Available at http://www.caff.is/publications/

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