‘Species-threatening’ population declines vs. polar bear declines

In 2008, polar bears in the United States were declared ‘threatened’ under the Endangered Species Act (USFWS 2008). The IUCN (to which the Polar Bear Specialist Group (PBSG) belongs) lists the polar bear as “vulnerable” (IUCN Red List 2012). In Canada (where 60% of the world’s polar bears reside), the polar bear is listed as a ‘species of special concern’ (COSEWIC 2008:iii).

As Jonathan Adler pointed out in an excellent article that appeared on the heels of the American ESA listing decision (Adler 2008:112), “Insofar as the listing is based upon climate models, ice-melt projections, and assumptions about the effects of habitat loss on the bear’s prospects for survival in the wild, its scientific basis is quite speculative.” These are also, as I understand it, unprecedented criteria for ESA listing – no other species has been listed as endangered or threatened based on such speculation of future conditions.

So what do actual polar bear population declines look like?

The Western Hudson Bay, Canada (WHB) subpopulation is the only one that has recently declined by a statistically-significant amount. [see previous post here on the status numbers] The WHB polar bear subpopulation makes up 3-5% of the global total for the species, currently estimated at 20,000-25,000 animals.

In this post, I’ll compare the documented WHB population decline to the declines seen in a few truly endangered animals, just to put the much-bandied about numbers into some perspective.

Western Hudson Bay polar bear numbers
Polar bears in WHB declined from an estimated 1194 (in 1987) to 935 (in 2004), a drop of 22%. However, as the most recent PBSG report points out (Obbard et al. 2010:58), the population had been stable before 1998 (Stirling et al. 1999:302, see below), which means this decline really occurred between 1998 and 2004 (over 7 years).  This suggests that the decline could be a temporary response to short-term changes (such as discussed in this previous post).

Figure 1. Population estimate graph for 1981 to 1997 that appears in Stirling et al. 1999.

Figure 1. Population estimate graph for 1981 to 1997 that appears as Fig. 6 in Stirling et al. 1999. Note the stable level of the population from 1986 to 1997.

Zac Unger, in the latest issue of Canadian Geographic magazine, calls the  decline in WHB numbers “startling” and states that the trend lines from the study that documented the decline suggest that “by 2011, the population would fall to as low as 676.” [this projection to 2011, by the way, appears to originate from a statement made in Atkinson et al. 2012 on page 32, although the number given there is “around 650”].

It is now 2012. Has there been a 43% decline in polar bear numbers in WHB since 1998 – in other words, are there now only 676 (or “around 650”) bears? There have been no additional surveys of the WHB subpopulation using the “mark-recapture method” since 2004 (see previous posts here, and here,) so we don’t really know.

However, in 2011 the government of Nunavat commissioned an aerial count of WHB polar bears – a method not used before in this region – and the study generated an estimate of 1,013 (Atkinson et al. 2012  Stapleton, Atkinson et al. 2012), not appreciably different from the 2004 count of 935. Unfortunately, an aerial survey is not comparable to the previous mark-recapture studies (the methods are just too different), so we cannot really say for sure if any change has occurred.

The 2011 Nunavat study does, however, suggest to me the possibility that the decline documented in 2004 was temporary and that a slight rebound in numbers – rather than a continued decline – may have occurred. Temporary declines followed by population rebounds happen relatively frequently in mammals, including polar bears and ringed seals, often in response to extreme winter conditions (see previous posts here, and here on this topic. More on this in a subsequent post).

Population declines of endangered species
Here I offer some perspective on the issue of what constitutes a truly “startling” population decline. Below are some actual population numbers that have been documented for marine mammals classified as “endangered” – these are actual declines, not “projections” or “predictions.”

Steller sea lion (Eumetopias jubatus), western stock (Aleutians to northern Gulf of Alaska, declared endangered by the ESA in 1993): declined to approximately 18,000 individuals in 2000, from about 140,000 in the 1950s, an 87% reduction (NMFS 2008).

Gray whale (Eschrichtius robustus): declined to approximately 200-300 individuals by 1938 after decades of over-hunting, recovered to 21,113 by 1987/88. Pre-decline numbers unknown. (Clapham et al. 1999:47).

Sea otter (Enhydra lutris): declined to less than 2,000 individuals by 1911 after many decades of over-hunting, recovered to approximately 100,000 by the mid-1980s. Since then, the population in SW Alaska (northern Gulf of Alaska and the Aleutians) had a second overall decline of 55-67%, with some local declines of over 90% (Sea Otter Recover Team 2007:6; USFWS 2005).

Northern elephant seal (Mirounga angustirostris): declined to approximately 20-100 individuals by 1900 after decades of over-hunting, recovered to approximately 175,000 by 2000. Pre-decline numbers unknown (Weber et al 2000:1287).

Guadalupe fur seal (Arctocephalus townsendi): declined to approximately 60 individuals by 1928 after decades of over-hunting, recovered to 12,176 by 2003. Pre-decline numbers unknown (Rick et al. 2009:488).

In summary, the only statistically significant decline documented among polar bear subpopulations occurred in Western Hudson Bay over the seven years between 1998 and 2004, a 22% drop in numbers that left approximately 935 individuals remaining. This decline was not enough to change the global estimate for the species, which remained at 20,000-25,000 individuals, and it is not known if this decline has continued into the present. In contrast, several truly endangered marine mammal species are notable for their populations having dropped by 87% or more over several decades, in some cases leaving only a few hundred individuals or less remaining over the entire range of the species or subspecies.

References
Adler, J.H. 2008. An animal to save the world: climate change and the polar bear. The New Atlantis Summer:111-115. http://www.thenewatlantis.com/publications/an-animal-to-save-the-world

Atkinson, S., Hedman, D., Garshelis, D. and Stapleton S. 2012. Western Hudson Bay polar bear aerial survey 2011 final report. May 14. Prepared for the Government of Nunavut. env.gov.nu.ca/sites/default/files/wh_final_report_may_2012.pdf

[UPDATE Sept. 12 2013: The correct order of authors for the above report is:
Stapleton S., Atkinson, S., Hedman, D., and Garshelis, D. 2012. Western Hudson Bay polar bear aerial survey. May 14. Final project report prepared for the Government of Nunavut and project collaborators. 56 pp.

Clapham, P. J., Young, S. B. and Brownell Jr., R. T. 1999. Baleen whales: conservation issues and the status of the most endangered populations. Mammal Review 29:35-60.

Rick, T. C., DeLong, R. L., Erlandson, J. M., Braje, T. J., Jones, T. L., Kennett, D. J., Wake, T. A., and Walker, P. L. 2009. A trans-Holocene archaeological record of Guadalupe fur seals (Arctocephalus townsendi) on the California coast. Marine Mammal Science 25:487-502.

Sea Otter Recovery Team 2007. Recovery strategy for the sea otter (Enhydra lutris) in Canada. Species at Risk Act Recovery Strategy Series. Fisheries and Oceans Canada, Vancouver.

Weber, D. S., Stewart, B. S., Garza, J. C., and Lehman, N. 2000. An empirical genetic assessment of the severity of the northern elephant seal population bottleneck. Current Biology 10:1287-1290.

IUCN Red List 2012. http://www.iucnredlist.org/ search on “polar bear” Dec. 15, 2012

USFWS 2005. Endangered and threatened wildlife and plants; determination of Threatened Status for the Southwest Alaska distinct population segment of the northern sea otter (Enhydra lutris kenyoni); final rule. Federal Register 70:46366.

USFWS 2008. Endangered and threatened wildlife and plants; determination of Threatened Status for the polar bear (Ursus maritimus) throughout its range; final rule. Federal Register 73:28211-28303.

Unger, Z. 2012. The truth about polar bears. Canadian Geographic December:28-42. http://www.canadiangeographic.ca/magazine/dec12/polar_bears.asp

Comments are closed.