Graphing polar bear population estimates over time

I’ve already commented on the 2013 update of polar bear population status released by the IUCN Polar Bear Specialist Group (PBSG).

However, I thought it might be interesting to graph the changes in global population estimates over time (from 1981-2013) — not just the actual estimates from PBSG status tables (with their min/max error ranges) but those totals plus the so-called “inaccurate” estimates that the PBSG have dropped from their accounts in recent (2005-2013) assessments: Chukchi Sea, East Greenland, Queen Elizabeth Islands (now known as the “Arctic Basin”), and Laptev Sea.

In 2001, those “inaccurate” estimates contributed 5,000-5,400 bears to the global total, but now they’re gone — no bears from those regions contribute to the official totals listed on recent PBSG status tables.

Adding those dropped estimates back into the global totals makes it possible to generate a graph in which the global estimates are truly comparable over time.

To see how the dropped estimates influenced the perception of population change over time, I’ve also graphed the estimates given by the PBSG in their status tables. I’ve combined the two into one image (Fig. 1, click to enlarge) to make comparison easy.

Figure 1. Upper graph uses totals reported in PBSG status tables, with min/max; Lower graph uses the same figures, but adds back in the so-called "inaccurate" estimates dropped 2005-2013. The 1960 figure * is a ballpark estimate

Figure 1. Upper graph uses totals reported in PBSG status tables, with min/max; Lower graph uses the same figures, but adds back in the so-called “inaccurate” estimates dropped 2005-2013. The 1960 figure * is a ballpark estimate

What is apparent is that the global population of polar bears has not declined over the last 30 years, as the PBSG status tables indicate.

Although it’s true that the official estimates for 2005 and 2009 publicized by the PBSG (“20,000-25,000”) also indicated no recent change, the numbers listed on their status tables don’t tell the same story. What’s the point of insisting on supposedly more accurate “scientific” estimates if you don’t use them to generate your official total?

Note that the most recent status table released last week, for 2013, did not include a new total — if you wanted to know if the global total had changed (and if so, by how much), you had to add the figures up yourself. Nowhere on the PBSG website does it now list an official population estimate. What’s that about?

Finally, note that I’ve included a ballpark estimate for 1960 that reflects international concern that many years of unregulated hunting had seriously depleted populations, as occurred for numerous other species (indicated as a broad range rather than a single figure). And while it is true that no one truly knows how many polar bears there were in 1960, it is also true that by that time, over-hunting had occurred on a large scale for decades, and that there were fewer bears then than there are now (see another discussion of that topic here).

NOTES ON METHODS
Where the PBSG status table for any given year (list below) gives a total range of estimates (maximum and minimum), as in 2001, I’ve used the PBSG totals. Where the status tables do not give a total for the maximum, minimum and average numbers (i.e., 2005, 2009, 2013), I’ve used those listed in the first column of each table (Fig. 2 below, and upper panel of Fig. 1). For example, the min/max and average estimates (rounded to nearest 10) from the status tables for 2005-2013 are:

2005 15,130-27,760 (average 21,420)
2009 14,220-25,160 (average 19,610)
2013 13,070-24,240 (average 18,349)

Fig. 2 Graph using the figures listed in PBSG status tables only.

Fig. 2 Graph using the figures listed in PBSG status tables only.

While I acknowledge that earlier estimates (from the 80s and 90s) used different methods than later ones, which may have made them less accurate, they were the “best available science” at the time. The purpose of this exercise is to compare the PBSG published numbers from these documents:

1981 8th Meeting – Oslo, Norway (Anonymous 1985: Table 1)
1993 11th Meeting – Copenhagen, Denmark (Wiig et al. 1995: Table 1)
1997 12th Meeting – Oslo, Norway (Derocher et al. 1998: Table 1)
2001 13th Meeting – Nuuk, Greenland (Lunn et al. 2002: Table 1)
2005 14th Meeting – Seattle, Washington (Aars et al. 2006: Table 1)
2009 15th Meeting – Copenhagen, Denmark (Obbard et al. 2010: Table 1)
2013 Online status update, posted February 14, 2014

As I’ve pointed out previously, adjustments made to the PBSG status tables that dropped to zero estimates that had formerly been included in the global totals suggested that by 2009, the totals must actually have increased by 2650 to 5700 bears since 2001.

So to make the population estimates comparable over time, we really need to add back in the ballpark estimates for the Queen Elizabeth Islands (which includes any bears residing there and in the Arctic Basin, last estimated at 200 bears), East Greenland (2,000 bears) and the Chukchi Sea (2,000 bears)1, which were dropped in the 2005 and 2009 PBSG assessments, as well as the estimate for the Laptev Sea (800-1200 bears, dropped in 2013, discussed here).

[I did not include adjustments for Barents Sea changes in this graph (as I did previously), to keep the comparison simple and because some people might not agree that the Barents Sea adjustment qualifies as the same kind of change as the dropped estimates]

Figure 3. Graph using figures from the PBSG status tables plus "inaccurate" estimates dropped between 2005 & 2013.

Figure 3. Graph using figures from the PBSG status tables plus “inaccurate” estimates dropped between 2005 & 2013.

Footnote:
1. Although the estimate for this region is indeed less “scientifically” accurate than those from regions that have been surveyed, it is very clear that large number of bears reside in the Chukchi Sea. For example, Rode and colleagues (Rode et al. 2014) used an estimate of 2,000 for the Chukchi Sea region (“based on extrapolation of den surveys”) to argue that the Chukchi and the Southern Beaufort population of bears were of “similar size.” PBSG biologists apparently like to have it both ways: ignore the Chukchi estimate when it doesn’t suit them (for global estimates) but include it when they need it for statistical work.

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/

Amstrup, S.C. and Wiig, O. (eds.) 1991. Polar Bears: Proceedings of the 10th meeting of the Polar Bear Specialists Group IUCN/SSC, Oct. 25-29, 1988, Sochi, USSR. Gland, Switzerland and Cambridge UK, IUCN. http://pbsg.npolar.no/en/meetings/

Anonymous. 1985. Polar Bears: Proceedings of the 8th meeting of the Polar Bear Specialists Group IUCN/SSC, 15-19 January, 1981, Oslo, Norway. Gland, Switzerland and Cambridge UK, IUCN. http://pbsg.npolar.no/en/meetings/

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. http://pbsg.npolar.no/en/meetings/

Lunn, N.J., Schliebe, S., and Born, E.W. (eds.). 2002. Polar Bears: Proceedings of the 13th working meeting of the IUCN/SSC Polar Bear Specialists Group, 23-28 June , 2001, Nuuk, Greenland. Gland, Switzerland and Cambridge UK, IUCN. http://pbsg.npolar.no/en/meetings/

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/

Rode, K.D., Regehr, E.V., Douglas, D., Durner, G., Derocher, A.E., Thiemann, G.W., and Budge, S. 2014 [now in print]. Variation in the response of an Arctic top predator experiencing habitat loss: feeding and reproductive ecology of two polar bear populations. Global Change Biology 20(1):76-88. http://onlinelibrary.wiley.com/doi/10.1111/gcb.12339/abstract 

Wiig, Ø., Born, E.W., and Garner, G.W. (eds.) 1995. Polar Bears: Proceedings of the 11th working meeting of the IUCN/SSC Polar Bear Specialists Group, 25-27 January, 1993, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN. http://pbsg.npolar.no/en/meetings/

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