Did polar bear numbers in E. Beaufort fluctuate each decade due to thick ice years?

Now that we have a plausible explanation (previous post here) for why shorefast ice in the Eastern Beaufort got too thick for ringed seals every ten years or so, it’s time to talk about the effect that this recurring sea ice phenomenon might have had on polar bear population numbers.

We know from the reports of polar bear biologists that without fat young seals to eat in the spring, some bears in those thick-ice springs came close to starving and many mothers lost all or most of their cubs (Amstrup et al. 2006; Stirling 2002; Stirling and Lunn 1997; Stirling et al. 1980; Stirling et al. 2008). This presumably had some impact on population numbers – the question is: how bad was it?

None of the reports on the effects of the thick ice have given us any indication of how many polar bears might have died or lost their cubs. However, Ian Stirling and colleagues (Stirling et al. 2011) recently published a paper on the Northern Beaufort subpopulation that looked, at first glance, to have done just that.

You have to keep in mind that the geographic area in question – the Eastern Beaufort – is not an official polar bear subpopulation region – at least, not any more. As Fig. 1 below shows, the Eastern Beaufort was once its own, strictly Canadian region (or at least, a strictly Canadian research region) see previous post here), but management is now shared between two subpopulations and managed by two governments (Canada and the USA). About half of the bears of the “Eastern Beaufort” reside in the ‘Northern Beaufort’ subpopulation and the other half live in the ‘Southern Beaufort’ subpopulation.

Figure 1. Re-jigging of polar bear subpopulations now splits what used to be an entirely a Canadian research segment, called the “Eastern Beaufort” (map on the left, from Stirling and Lunn 1997), into management regions called “Southern Beaufort” (shared with the USA) and “Northern Beaufort,” with the Canada-USA border at 141 W (map on the right, from Stirling et al. 2011, Fig. 1). Labels added. Most of the polar bears sampled for the Stirling et al. paper were captured along the west and south coasts of Banks Island, although a few were captured in M’Clure Strait and in Amundsen Gulf.

Despite the changing boundaries, ringed seals and polar bears in the Eastern Beaufort have been the focus of research since the early 1970s. In part, this is because the region has been targeted for oil exploration and studies on both species have been part of the associated ecological impact assessments (Stirling et al. 1993).

Getting back to the point, did Stirling et al. 2011 find fluctuations in polar bear numbers in the Northern Beaufort that might reflect the periodic bouts of thick spring ice in the Eastern Beaufort? Unfortunately, no — the data lack necessary precision. You’ll see why, I think, from the summary below. 

First, have a look at Fig. 2 below, modified slightly from the 2011 paper by Stirling and colleagues (I’ve added some labels) – it’s a summary of mark-recapture work around Banks Island, from 1972 to 2006:

Figure 2. “Model-averaged estimates” of polar bear population numbers in the N. Beaufort, from 1972 to 2006 (Stirling et al 2011:871, Fig. 4), labels for years and estimates with confidence intervals added (from their Table 9). Note the huge confidence intervals for the data points (the lines extending up and down from the triangles, e.g. 1122 ± 584 for 1987). Lack of precision in the data preclude any conclusions being drawn from them.

The enormous confidence intervals shown in the Fig. 2 graph were a red-flag for me – I’m no statistics expert, but those are some crazy-large error bars!

So I asked a colleague of mine, eminent ecologist Dr. Daniel Botkin (see his website here), for a second opinion on this graph. He said (and said I could quote him):

“The confidence intervals are so large that nothing can be concluded.”

In other words, it is not possible to presume (based on the data presented) – as Stirling et al. have indeed done – that the population was stable over the period in question. The population might have been stable or it might have fluctuated – it’s simply not possible to tell. To say otherwise is giving the data more credence than is warranted.

It is not even possible to say, with any confidence, how many bears there were in 2006 – as Stirling et al. concede.

Their estimate for 2006 (767 ± 416, down from 1345 ± 480 in 2005), they say, is probably low. They state:

“The low estimate of population size in 2006 (Table 9) [Fig. 2, above], which reduced the mean for the 2000s, should be viewed with caution. …the empirical observations suggest there was a major change in the distribution of bears in 2006. We obtained a smaller capture sample than in previous years, despite searching over a similar total number of kilometers of sea ice habitat in search of polar bears. Intensive studies in the adjacent SB [Southern Beaufort] region also indicated that changes in the distribution or availability may have reduced the local abundance of polar bears there in 2006 (Regehr et al. 2010, indicating the effect prevailed over the entire Beaufort Sea. If, as our observations suggest, the population estimate for 2006 is biased low, the estimates of ~1200-1300 in 2004 and 2005 may more accurately reflect the current number of polar bears in NB [Northern Beaufort].”[my bold]

In other words, maybe “~1200-1300” is a more accurate estimate for the number of bears present in 2006, but maybe it’s not. Was the 1989 estimate “biased low” for the same reason — if so, why not a “caution” for that one as well?

Sadly, it’s disappointing to have to say so, but it appears that we are still in the dark about how much polar bear populations were affected by the recurring episodes of thick spring ice in the Eastern Beaufort. There may have been fluctuations in the numbers but it’s just not possible to say, given the available data.

References
Amstrup, S.C., Stirling, I., Smith, T.S., Perham, C. and Thiemann, B.W. 2006. Recent observations of intraspecific predation and cannibalism among polar bears in the Southern Beaufort Sea. Polar Biology 29:997–1002. Pdf here.

Stirling, I. 2002. Polar bears and seals in the eastern Beaufort Sea and Amundsen Gulf: a synthesis of population trends and ecological relationships over three decades. Arctic 55 (Suppl. 1):59-76. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/issue/view/42

Stirling, I. and Lunn, N.J. 1997. Environmental fluctuations in arctic marine ecosystems as reflected by variability in reproduction of polar bears and ringed seals. In Ecology of Arctic Environments, Woodin, S.J. and Marquiss, M. (eds), pg. 167-181. Blackwell Science, UK.

Stirling, I., Andriashek, D., and Calvert, W. 1993. Habitat preferences of polar bears in the western Canadian Arctic in late winter and spring. Polar Record 29:13-24. http://tinyurl.com/kowzw2g

Stirling, I., McDonald, T.L., Richardson, E.S., Regehr, E.V., and Amstrup, S.C. 2011. Polar bear population status in the northern Beaufort Sea, Canada, 1971-2006. Ecological Applications 21:859-876. http://www.esajournals.org/doi/abs/10.1890/10-0849.1

Stirling, I., Richardson, E., Thiemann, G.W. and Derocher, A.E. 2008. Unusual predation attempts of polar bears on ringed seals in the southern Beaufort Sea: possible significance of changing spring ice conditions. Arctic 61:14-22. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/3/3

Stirling, I., Schweinsburg, R.E., Kolenasky, G.B., Juniper, I., Robertson, R.J., and Luttich, S. 1980. Proceedings of the 7th meeting of the Polar Bear Specialists Group IUCN/SSC, 30 January-1 February, 1979, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN., pg. 45-53.http://pbsg.npolar.no/en/meetings/  and pdf of except here.