IUCN Polar Bear Specialist Group says its global population estimate was “a qualified guess”

Last week (May 22), I received an unsolicited email from Dr. Dag Vongraven, the current chairman of the IUCN Polar Bear Specialist Group (PBSG).

The email from Vongraven began this way:

“Dr. Crockford

Below you’ll find a footnote that will accompany a total polar bear population size range in the circumpolar polar bear action plan that we are currently drafting together with the Parties to the 1973 Agreement. This might keep you blogging for a day or two.” [my bold]

It appears the PBSG have come to the realization that public outrage (or just confusion) is brewing over their global population estimates and some damage control is perhaps called for. Their solution — bury a statement of clarification within their next official missive (which I have commented upon here).

Instead of issuing a press release to clarify matters to the public immediately, Vongraven decided he would let me take care of informing the public that this global estimate may not be what it seems.

OK, I’ll oblige (I am traveling in Russia on business and finding it very hard to do even short posts – more on that later). The footnote Vongraven sent is below, with some comments from me. You can decide for yourself if the PBSG have been straight-forward about the nature of their global population estimates and transparent about the purpose for issuing it.

Here is the statement that the PBSG proposes to insert as a footnote in their forthcoming Circumpolar Polar Bear Action Plan draft:

“As part of past status reports, the PBSG has traditionally estimated a range for the total number of polar bears in the circumpolar Arctic. Since 2005, this range has been 20-25,000. It is important to realize that this range never has been an estimate of total abundance in a scientific sense, but simply a qualified guess given to satisfy public demand. It is also important to note that even though we have scientifically valid estimates for a majority of the subpopulations, some are dated. Furthermore, there are no abundance estimates for the Arctic Basin, East Greenland, and the Russian subpopulations. Consequently, there is either no, or only rudimentary, knowledge to support guesses about the possible abundance of polar bears in approximately half the areas they occupy. Thus, the range given for total global population should be viewed with great caution as it cannot be used to assess population trend over the long term.” [my bold]

So, the global estimates were “…simply a qualified guess given to satisfy public demand” and according to this statement, were never meant to be considered scientific estimates, despite what they were called, the scientific group that issued them, and how they were used (see footnote below).

All this glosses over what I think is a critical point: none of these ‘global population estimates’ (from 2001 onward) came anywhere close to being estimates of the actual world population size of polar bears (regardless of how scientifically inaccurate they might have been) — rather, they were estimates of only the subpopulations that Arctic biologists have tried to count.

For example, the PBSG’s  most recent global estimate (range 13,071-24,238) ignores five very large subpopulation regions which between them potentially contain 1/3 as many additional bears as the official estimate includes (see map below). The PBSG effectively gives them each an estimate of zero.

Figure 1. Based on previous PBSG estimates and other research, there may be ~6,000-9,000 (perhaps less but maybe more) bears living in the regions marked in black that are not included in the most recent PBSG “global population estimate” – their population estimates are “zero.” CS, Chukchi Sea; LS, Laptev Sea; KS, Kara Sea; EG, East Greenland; AB, Arctic Basin.

Based on previous PBSG estimates and other research reports, it appears there are probably at least another 6,000 or so bears living in these regions and perhaps as many as 9,000 (or more) that are not included in any PBSG “global population estimate”: Chukchi Sea ~2,000-3,000; East Greenland, ~ 2,000-3,000; the two Russian regions together (Laptev Sea and Kara Sea), another ~2,000-3,000 or so, plus 200 or so in the central Arctic Basin. These are guesses, to be sure, but they at least give a potential size

In other words, rather than assigning a “simple, qualified guess” for these subpopulations that have not been formally counted as well as those that have been counted (generating a total figure that is indeed a “global population estimate,” however inaccurate), the PBSG have been passing off their estimate of counted populations as a true global population estimate, with caveats seldom included.

However, at the last tally (2013), 26% of the world’s polar bear subpopulations are not accounted for by the PBSG and geographically, those subpopulations occupy close to half of the world’s polar bear habitat. See previous posts here and here.

In their proposed footnote, the PBSG seem to object to the fact that these global population estimates — which they provided — have been used to suggest that polar bear numbers have increased since the 1960s (although we cannot say by precisely what amount). In addition, the estimates have been used to suggest that polar bear numbers worldwide have been relatively stable over the last 30 years, not declining.

Those are pretty broadly defined and loosely bounded “trends” but if the PBSG objects to even those uses of their “global population estimate,” they perhaps have only themselves to blame.

If their estimate of the counted portion of the global population was only a guess, why have they steadfastly refused to guess the population size of the uncounted portion of the population? Why have the PBSG refused to issue an estimate of the entire global population, even if it’s only their best guess? Curious, that.

Footnote. Quotes below show how the PGSG global population estimates have been described and used in the scientific literature by its members in their peer reviewed papers — look for any mention that this number does not represent an estimate of the total number of polar bears in the world:

1) “Polar bears have successfully occupied virtually all available sea ice habitats throughout the circumpolar Arctic and the global population was last estimated at 21,500-25,000 individuals (IUCN/SSC Polar Bear Specialist Group, 2002).” [Derocher, Lunn and Stirling 2004:163]

2) “There are currently estimated to be ~24,600 polar bears (Aars et al. 2006) distributed over a maximum of 1.5 X 10⁷ km² of northern hemisphere sea ice (mean 1979-2006 winter estimate; National Snow and Ice Data Center, Boulder, Colorado, USA; Cavalieri et al. 2006, Meier et al. 2006).” [Durner et al. including Amstrup, Stirling, Wiig, and Derocher 2009:26]

3) “Polar bears still inhabit the majority of their historic range in 19 subpopulations (Fig. 1) with a total population estimated at 20,000-25,000 (Obbard et al., 2010), although data with which to reliably ascertain the sizes of several subpopulations are either nonexistent or dated.” [Stirling and Derocher 2012:2694]

Only the last (#3) attempted a caveat but still failed to make clear that estimates for four very large subpopulation regions (Chukchi Sea, East Greenland, Kara Sea and Arctic Basin) were not included in that estimate.

And the PBSG itself, how has it described and used these figures? The two most recent examples below from the proceedings of their last two meetings [the most recent status update (2013) offered no global figure at all – you could add up the numbers given, which I have done – but the PBSG itself no longer cites a ‘global’ figure on their website.

2005 (Aars et al. 2006)

“The total number of polar bears worldwide is estimated to be 20,000-25,000.” [pg. 33]

…

“The world’s polar bears are distributed in 19 subpopulations over vast and sometimes relatively inaccessible areas of the Arctic. Thus, while the status of some subpopulations in Canada and the Barents Sea is well documented, that of several others remains less known [sic]. Thus, it is not possible to give an accurate estimate of the total number of polar bears in the world, although the range is thought to be 20-25,000.” [pg. 61, press release] [my bold]

2009 (Obbard et al. 2010)

“The total number of polar bears worldwide is estimated to be 20,000-25,000.” [pg. 31]

…

“The total number of polar bears is still thought to be between 20,000 and 25,000. However, the mixed quality of information on the different subpopulations means there is much room for error in establishing that range. That potential for error is cause for concern, given the ongoing and projected changes in habitat and other potential stressors.” [pg. 86, press release] [my bold]

References
Aars, J., Lunn, N. J. and Derocher, A.E. (eds.) 2006. Polar Bears: Proceedings of the 14th Working Meeting of the IUCN/SSC Polar Bear Specialist Group, 20-24 June 2005, Seattle, Washington, USA. Occasional Paper of the IUCN Species Survival Commission 32. IUCN, Gland (Switzerland) and Cambridge (UK). http://pbsg.npolar.no/en/meetings/

Derocher, A.E., Lunn, N.J. and I. Stirling. 2004. Polar bears in a warming climate. Integrative and Comparative Biology 44:163-176. Open access http://icb.oxfordjournals.org/content/44/2/163.abstract

Durner, G.M., Douglas, D.C., Nielson, R.M., Amstrup, S.C., McDonald, T.L. and 12 others. 2009. Predicting 21st-century polar bear habitat distribution from global climate models. Ecological Monographs 79:25-58. http://www.esajournals.org/doi/abs/10.1890/07-2089.1

Obbard, M.E., Theimann, G.W., Peacock, E. and DeBryn, T.D. (eds.) 2010. Polar Bears: Proceedings of the 15th meeting of the Polar Bear Specialists Group IUCN/SSC, 29 June-3 July, 2009, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN. http://pbsg.npolar.no/en/meetings/

Stirling, I. and Derocher, A.E. 2012. Effects of climate warming on polar bears: a review of the evidence. Global Change Biology 18:2694-2706. doi:10.1111/j.1365-2486.2012.02753.x