Sea ice looks low for this time of year but how does it compare to 2007, when summer ice habitat for polar bears hit a record-breaking low? What can the impact of 2007 ice levels on polar bears tell us about what to expect this year?
By this date in 2007 (8 August, Day 220, NSIDC Masie map below), there was almost 1 million km2 less ice than there is this year (map above). However, look which polar bear subpopulations not only survived, but thrived, through the 2007 low ice summer: Chukchi Sea, Southern Beaufort, Barents Sea, Davis Strait, Foxe Basin, Western Hudson Bay, and Southern Hudson Bay. That’s all of the subpopulations for which we have recent data.
There is more than a month left in the melt season, of course. However, while 2012 finished with a lower minimum ice extent due to a massive mid-August storm that broke up a lot of ice (Simmonds and Rudeva 2012), by the end of the first week of August (i.e, the 8th), there was more ice in 2012 than in 2007 and a bit less than this year (2012, 6.3 mkm2; 2007, 5.6 mkm2; 2015, 6.5 mkm2).
This means if less summer ice for a longer period of time impacts polar bear health and survival, conditions in 2007 should have had a noticeable impact on polar bears around the world. They didn’t. That suggests even if this September sea ice minimum is as low as 2007, it won’t have any negative impact on polar bear health or survival. The most profoundly negative documented impacts have come from thick sea ice in spring or suboptimal spring snow levels (Crockford 2015) and the evidence shows that variation in the extent of summer ice is simply irrelevant to polar bears.
In all cases, the maps below are for 2007, followed by 2015 (click to enlarge).
[2007, 278 thousand kms; 2015, 416 thousand kms – see maps below]
The latest study ended in 2011 but began in 2008, the year after the record-breaking (at the time) 2007 September low, and yet the bears were described as in excellent physical condition and reproducing well (Rode et al. 2014a). Although no population count was done, these characteristics are the signature of a population that is stable or increasing.
No wonder we are not hearing cries of anguish from the polar bear community about the low ice levels in the Chukchi Sea this year – they have already shown decreased summer sea ice not only caused no harm to polar bears but actually did them good! [because the longer open water season was advantageous for ringed seals and resulted in more fat seals for bears to eat the following spring]
[2007, 741 thousand kms; 2015, 837 thousand kms – see maps below]
The latest population study ended in 2010, although published data on research in the area is available into the spring of 2013. Oddly, 2007 – the 2nd lowest September minimum since 1979 – was the year that Southern Beaufort polar bears began to recover from the devastating losses incurred during the 2004-2006 period caused by thick spring ice conditions (Bromaghin et al. 2015). In other words, the thick spring ice caused starvation and population decline, low summer ice in 2007 did not impede recovery.
By 2012, US Fish & Wildlife researchers noted that the number of bears seen were high relative to the last decade (Polar Bear News 2014) and that body condition of the bears was normal. No massive die-off of bears or starving bears were reported by researchers working in the Southern Beaufort in the spring of 2013 (Rode et al. 2014b), the year after the lowest September minimum since 1979. Despite these extenuating circumstances, the IUCN PBSG is using the population figures generated by this study, which estimated about 907 bears (range 548-1,270) existed in 2010, a decline from the previous estimate.
More details at these previous posts:
[NSIDC defines Barents Sea oddly, so I’ve used NIS maps for 7 August (latest available) below]
The most recent Barents Sea population estimate was done in 2004 (approximately 2,650; range ~1900-3600), based on an aerial survey (Aars et al. 2009). From the data presented online by Norwegian biologists Jon Aars and Magnus Andersen from spring studies, the proportion of females with yearlings and cubs of the year were about average in 2008 (compared to 1993-2013).
Davis Strait & Baffin Bay together
[2007, 70 thousand kms; 2015, 227 thousand kms – see maps below]
Both of these areas are defined as “seasonal” habitats for polar bears, similar to Hudson Bay, because the bears spend variable amounts of time on land when the ice melts in summer and thus live off their stored fat. The latest study, by Lily Peacock and colleagues (2013), for Davis Strait generated mark-recapture data for 2005-2007, in the fall. They estimated the number of bears in Davis Strait at about 2,158, a substantial increase over the estimate of about 1,400 bears in 1993. Bears in the southern portion of Davis Strait in particular were in good condition despite reduced sea ice due to the abundance of harp seals available to them in the spring. This suggests no immediate negative impact from the low sea ice levels in 2007.
In Baffin Bay, 2,074 bears (range, 1544-2604) were thought to live in Baffin Bay in 1997, at which time the population was stable or slightly increasing. A reduction in population size was suspected due to over-hunting so the IUCN PBSG reduced the 1997 estimate to ~1546 bears (690-2402) based on that assumption. A study supposedly completed in 2013 and promised at the end of 2014 has still not been published or released in preliminary fashion. I’ve come across no hints regarding the results of this study. Therefore, there is no evidence one way or another regarding a possible impact of the low sea ice levels of 2007 on the polar bears of Baffin Bay.
Foxe Basin, Western Hudson Bay, & Southern Hudson Bay together
[2007, 93 thousand km2; 2015, 230 thousand km2 – see maps below]
Foxe Basin The most recent population data comes from an aerial survey of Foxe Basin just after the low 2007 sea ice summer, in 2009/2010, which generated an estimate of ~2,580 bears, slightly above the early 1990s estimate of ~2,300 (Stapleton et al. 2012). While the two methods (aerial survey and mark-recapture) are not directly comparable, the aerial survey does suggest the population either did not decline as a result of the 2007 ice levels or had recovered by 2009/2010.
Western Hudson Bay The internal government report on the recent (2005-2011) WHB mark-recapture work contains a population estimate only. It has no figures on changes (if any) on number of cubs, size of litters, or condition of bears over time for 1984-2011 (previous study period ended in 2004). The authors (Lunn et al. 2013:18) calculated a new estimate for the population at 2004 (i.e., the previous count), using the same method they used for their new count in 2011, which generated an estimate of 742 (630-872) for 2004, vs. the 806 (653-984) estimated for 2011. This indicates there has been no decline in population numbers since the last estimate was calculated in 2004 and suggests no negative influence from the low sea ice levels of 2007. The IUCN Polar Bear Specialist Group considers the WHB subpopulation to be stable (as of 24 January 2015), with an estimate of approximately 1030 bears. Lunn et al (2013: 15) also found no significant trend in either breakup or freeze-up dates over the period 2001-2010 (using a definition of 50% ice cover), which includes 2007.
Southern Hudson Bay The current population estimate is 951 bears (662-1366) based on an aerial survey done in 2012 (Obbard et al. 2013), no change since 2005 (900-1000 bears). This suggests no lingering negative consequences from low ice conditions in 2007, if indeed there was any impact at all.
Aars, J. 2013. Variation in detection probability of polar bear maternity dens. Polar Biology 36:1089-1096. http://link.springer.com/article/10.1007%2Fs00300-013-1331-7
Aars, J., Marques, T.A., Buckland, S.T., Andersen, M., Belikov, S., Boltunov, A., and Wiig, Ø. 2009. Estimating the Barents Sea polar bear subpopulation. Marine Mammal Science 25:35-52. http://onlinelibrary.wiley.com/doi/10.1111/j.1748-7692.2008.00228.x/abstract
Bromaghin, J.F., McDonald, T.L., Stirling, I., Derocher, A.E., Richardson, E.S., Rehehr, E.V., Douglas, D.C., Durner, G.M., Atwood, T. and Amstrup, S.C. 2015. Polar bear population dynamics in the southern Beaufort Sea during a period of sea ice decline. Ecological Applications 25(3):634-651. http://www.esajournals.org/doi/abs/10.1890/14-1129.1 Open Access.
Crockford, S.J. 2015. “The Arctic Fallacy: sea ice stability and the polar bear.” GWPF Briefing 16. The Global Warming Policy Foundation, London. Pdf here.
Derocher 2005. Population ecology of polar bears at Svalbard, Norway. Population Ecology 47:267-275. http://www.springerlink.com.ezproxy.library.uvic.ca/content/765147518rp35613/fulltext.pdf
Lunn, N.J., Regehr, E.V., Servanty, S., Converse, S., Richardson, E. and Stirling, I. 2013. Demography and population assessment of polar bears in Western Hudson Bay, Canada. Environment Canada Research Report. 26 November 2013. PDF HERE
Mauritzen, M., Derocher, A.E. and Wiig, Ø. 2001. Space-use strategies of female polar bears in a dynamic sea ice habitat. Canadian Journal of Zoology 79:1704-1713. http://www.nrcresearchpress.com/doi/abs/10.1139/z01-126#.Ul1dxhDkSUk
Obbard, M.E., K.R. Middel, S. Stapleton, I. Thibault, V. Brodeur, and C. Jutra. 2013. Southern Hudson Bay polar bear aerial survey, 2011 and 2012. Final report. Unpublished report. Wildlife Research and Monitoring Section, Ontario Ministry of Natural Resources, Peterborough, ON. 43p. Pdf here.
Peacock, E., Taylor, M.K., Laake, J., and Stirling, I. 2013. Population ecology of polar bears in Davis Strait, Canada and Greenland. Journal of Wildlife Management 77:463–476. http://onlinelibrary.wiley.com/doi/10.1002/jwmg.489/abstract?deniedAccessCustomisedMessage=&userIsAuthenticated=false
Polar Bear News 2013-14. 2013. Polar bear newsletter of the US Fish & Wildlife Service, Anchorage, Alaska. Pdf here.
Rode, K.D., Peacock, E., Taylor, M., Stirling, I., Born, E.W., Laidre, K.L., and Wiig, Ø. 2012. A tale of two polar bear populations: ice habitat, harvest, and body condition. Population Ecology 54:3-18. http://link.springer.com/article/10.1007/s10144-011-0299-9
Rode, K.D., Regehr, E.V., Douglas, D., Durner, G., Derocher, A.E., Thiemann, G.W., and Budge, S. 2014a. 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 [updated February 9 2014]
Rode, K. D., Pagano, A.M., Bromaghin, J.F., Atwood, T.C., Durner, G.M. and Simac K.S. 2014b. Effects of capturing and collaring on polar bears: Findings from long-term research on the southern Beaufort population. Wildlife Research. 41 (4):311-322. doi 10.1071/WR13225 [paywalled] http://www.publish.csiro.au/?paper=WR13225 Supplementary Info: 10.1071/WR13225_AC Pdf here.
Simmonds, I. and Rudeva, I. 2012. The great Arctic cyclone of August 2012, Geophysical Research Letters 39, L23709, doi:10.1029/2012GL054259. http://www.agu.org/pubs/crossref/2012/2012GL054259.shtml
Stapleton, S., Peacock, E., and Garshelis, D. 2012. Foxe Basin polar bear aerial survey. Nunavut Wildlife Research Trust, Government of Nunavut, Igloolik. Pdf here.