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Polar Bears in the Chukchi Sea Doing Very Well Despite Large Sea Ice Losses

Reference
Rode, K.D., Regehr, E.V., Douglas, D., Durner, G., Derocher, A.E., Thiemann, G.W. and Budge, S. 2013 (accepted). Variation in the response of an Arctic top predator experiencing habitat loss: feeding and reproductive ecology of two polar bear populations. Global Change Biology DOI: 10.1111/gcb.12339.
The latest assessment report of the IUCN Polar Bear Specialist Group (Obbard et al. 2010) declared that the Chukchi Sea subpopulation (which includes polar bears that live in the northern Bering Sea, with management shared between the USA and Russia) was "declining," based primarily on anticipated effects from "one of the highest rates of sea ice loss in the Arctic." By 2009, very little work had been done on Chukchi polar bears and no population survey had been undertaken: the official population estimate for the Chukchi region was zero. In contrast, polar bears in the neighbour subpopulation to the east, the Southern Beaufort (shared between the USA and Canada), have been studied since the early 1970s (e.g. Stirling 2002; Stirling et al. 2003, 2006, 2008; Stirling and Lunn 1997).

Both the Chukchi Sea and the Southern Beaufort Sea subpopulations have been classified as "divergent" ice ecoregions by researchers attempting to predict how polar bear habitat might fare over the next 25 to 95 years based on computer-projected sea ice declines (Amstrup 2011; Amstrup et al. 2008, 2010; Durner et al. 2009). Durner and colleagues, for example, said that "within the Divergent ecoregion, rates of decline are projected to be greatest in the Southern Beaufort, Chukchi, and Barents Sea subpopulations." They also determined that "optimal polar bear habitat" in the Chukchi declined by 8% per decade between 1979 and 2006, while similar habitat in the Southern Beaufort declined by 4.8% per decade.

Anticipated effects of summer sea ice decline on polar bears are reduced body size and condition (i.e., relative fatness), reduced juvenile survival, reduced litter size and population decline. According to one study, declines in body size and condition, as well as the number of first-year cubs and yearlings per female, have been documented in the Southern Beaufort, changes which appeared to coincide with declines in summer sea ice (Rode et al. 2010).

Karyn Rode and colleagues captured, measured and released polar bears on the sea ice between mid-March and early May, 2008-2011; others did similar work in 1986-1994. They compared data collected on body condition, litter size and juvenile survival ("reproductive indices") in the Chukchi Sea between the two periods (1986-1994 and 2008-2011). They also compared body condition and reproduction in the Chukchi and neighboring Southern Beaufort for the period 2008-2011. They evaluated these metrics in relation to sea ice loss and prey availability; for Chukchi bears in 2008-2011 only, they also determined diet composition from analysis of fat samples ("fatty acid signatures") and fasting behavior from analysis of blood samples ("levels of blood urea nitrogen and creatinine").

The authors found that "in 2008-2011, CS [Chukchi] bears were either larger and in better condition, or similar in size and condition, to CS bears in 1986-1994." They also found "no difference in the number of yearlings per female, yearling litter size, or the annual percentage of females with yearlings between periods in the CS. ...Overall, CS bears in 2008-2011 were larger and in better condition than SB [Southern Beaufort] bears during the same period." The number of yearlings per female in spring was higher in the CS than in the SB.

These results were unexpected: because the Southern Beaufort and Chukchi Seas are considered similar ice ecoregions, polar bears were predicted to respond similarly to summer sea ice loss.

Most surprisingly, Chukchi bears were also larger and heavier than virtually all other subpopulations studied and "spring COY litter sizes are among the highest reported for 18 of 19 polar bear populations. ...spring litter sizes of CS yearlings from the study were also higher than other populations." The authors pointed out that "the larger body mass of adult females in the CS corresponded not only with larger litter sizes, but also with heavier yearlings (Fig.5) which have a greater chance of survival."

Rode et al. stated in their discussion that "body size, condition, and reproductive indices of CS polar bears did not decline over time between 1986-1994 and 2008-2011 despite a 44-day increase in the number of reduced-ice days. Furthermore, CS bears were larger, in better condition, and appeared to have higher recruitment compared to the adjacent SB population during 2008-2011. These differences were biologically significant."

The authors concluded that in the Chukchi subpopulation "body condition was maintained or improved when sea ice declined" and that "continued high biological productivity in the Chukchi and northern Bering seas may be allowing polar bears and their prey to prosper despite habitat loss." Regarding Southern Beaufort bears, they stated: "Our evaluation of nutritional ecology for polar bears is consistent with lower prey availability in the SB compared to the CS."

Two conclusions can be drawn from this study: 1) declines in summer sea ice extent can markedly benefit polar bear survival; 2) extent of sea ice loss in summer is not the paramount determiner of polar bear health and population status, at least over the short term.

Additional References
Amstrup, S.C. 2011. Polar bears and climate change: certainties, uncertainties, and hope in a warming world. Pgs. 11-20 in R.T. Watson, T.J. Cade, M. Fuller, G. Hunt, and E. Potapov (eds.), Gyrfalcons and Ptarmigan in a Changing World, Volume 1. The Peregrine Fund, Boise, Idaho.

Amstrup, S.C., DeWeaver, E.T., Douglas, D.C., Marcot, B.G., Durner, G.M., Bitz, C.M. and Bailey, D.A. 2010. Greenhouse gas mitigation can reduce sea-ice loss and increase polar bear persistence. Nature 468: 955-958.

Amstrup, S.C., Marcot, B.G., Douglas, D.C. 2008. A Bayesian network modeling approach to forecasting the 21st century worldwide status of polar bears. Pgs. 213-268 in Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications, E.T. DeWeaver, C.M. Bitz, and L.B. Tremblay (eds.). Geophysical Monograph 180. American Geophysical Union, Washington, D.C.

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.

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 Specialist Group IUCN/SSC, 29 June-3 July, 2009, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN.

Rode, K.D., Amstrup, S.C., and Regehr, E.V. 2010. Reduced body size and cub recruitment in polar bears associated with sea ice decline. Ecological Applications 20: 768-782.

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.

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.

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.

Archived 9 October 2013