A new paper on Baffin Bay polar bears reports data on body condition and litter sizes collected as part of a major study of the region completed in 2013 compared to sea ice declines since the 1990s; based on a computer model, the authors predict that in 37 years time (if sea ice declines continuously), the incidence of twin litters could “largely disappear.” However, no decline in population numbers was predicted and a critical caveat acknowledges that factors other than changes in sea ice could have affected the body condition and litter size data the authors analyzed, which means the conclusions are scientifically inconclusive.
The last (2013) polar bear population survey of Baffin Bay (SWG 2016) generated an estimate of almost 3,000 (2,826; range 2,059-3,593), which means that regardless of some slight changes in body condition and litter size over the last two decades (which may or may not have been caused by loss of sea ice), there are currently a lot of bears in Baffin Bay.
A summary of the Baffin Bay paper on NASA’s Earth Observatory “image of the day” website (see map above) for 4 February 2020 [“Polar bears struggle as sea ice declines”, which seems to be serving as a press release] quoted lead author Kristin Laidre (U. Washington), as saying [my bold]:
“Climate-induced changes in the Arctic are affecting polar bears,” said Laidre, who was the main author of the study. “They are an icon of climate change, but they’re also an early indicator of climate change because they are so dependent on sea ice…
Polar bears are a harbinger for the future,” said Laidre. “The changes we document here are going to affect everyone around the globe.”
Writers at the UK’s Metro widely-read but free newspaper interpreted this as “Climate change is killing polar bears and it’s an ominous ‘harbinger of the future’, Nasa warns” (4 February 2020). Oh dear.
According to the paper (Laidre et al. 2020 in press), polar bears arrived on land in the summer an average of 20 days earlier in the 2000s than they did in the 1990s (4 August vs. 24 August) but departed from land in the fall only 7 days later, an insignificant difference (8 November vs. 1 November). The NASA folks redrew the Laidre paper graph showing sea ice decline and timing of bears off and on the land (in fall and summer, respectively):
In total, Baffin Bay bears are now spending about three months onshore (about 90 days).
To put this in perspective, polar bears in Western and Southern Hudson Bay spend almost five months on shore over the summer (about 150 days) yet even this length of time has not had a marked impact on polar bear health or population size: some decline has been claimed for both but as I have stated repeatedly (Crockford 2017, 2019a,b), according to the Committee on the Status of Endangered Wildlife in Canada (COSEWIC 2018, released in June 2019) neither is statistically significant.
Regarding body condition, Laidre and colleagues found fewer Baffin Bay bears were in ‘very good’ condition in the 2000s than in the 1990s (13% vs. 39%) but more bears in the 2000s were in ‘good’ condition than in the 1990s (66% vs. 44%); the percentage of bears in ‘poor’ condition were about the same in both periods (17% in 1990s vs. 21% in 2000s), see their Table 2.
This overall decline in body condition was said to be correlated to sea ice cover and to have impacted mean litter size for cubs of the year (but not litters of yearling cubs, see their Table 4): in other words, the size of litters for cubs that had survived their first year (and have the best chance of surviving to adulthood) did not change significantly between the 1990s and the 2000s.
Oddly, the population size of Kane Basin polar bears to the north of Baffin Bay has been found to have increased significantly between the 1990s and 2000s (SWG 2016), as was the size of the Davis Strait population to the south of Baffin Bay (Crockford 2019a; Peacock et al. 2013; Rode et al. 2012). However, the apparent 36% increase in Baffin Bay population size between 1997 (about 2,074) and 2013 (about 2,826) was dismissed as untrustworthy due to methodological differences between the two estimates (Crockford 2019a, b; SWG 2016; York et al. 2016), although the 2013 population size estimate of 2,826 is not disputed. In other words, although we cannot tell for sure if the population size increased since 1997, there are currently a lot of bears in Baffin Bay.
The author’s caveat (2nd last sentence of their paper) is critical:
“We note, however, that the functional and temporal relationships between declines in body condition and recruitment, and declines in subpopulation size, are poorly understood and that the trend of the BB subpopulation is currently unknown (SWG 2016).”
This warning is not included in the abstract of the paper or in the online summary provided by NASA or by fellow polar bear specialist Andrew Derocher (20 January 2020). But it is a really important limitation of the study: sea ice may not be the only factor (or even the main one) affecting body condition and litter size. For example, changes in size of prey population sizes or their distribution, or even snow depth in spring could have had a crucial impact.
In other words, because no other factor besides sea ice cover was considered in this study, no definitive conclusions can be drawn from the results of the study. Correlation of body condition and litter size declines with sea ice decline does not prove that sea ice decline is the cause of those declines. It’s just been made to look that way, which was apparently acceptable to the journal that published the paper.
COSEWIC. 2018. COSEWIC assessment and status report on the Polar Bear Ursus maritimus in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. Online version here: https://www.canada.ca/en/environment-climate-change/services/species-risk-public-registry/cosewic-assessments-status-reports/polar-bear-2018.html
Crockford, S.J. 2017. Testing the hypothesis that routine sea ice coverage of 3-5 mkm2 results in a greater than 30% decline in population size of polar bears (Ursus maritimus). PeerJ Preprints 19 January 2017. Doi: 10.7287/peerj.preprints.2737v1 Open access. https://peerj.com/preprints/2737/
Crockford, S.J. 2019a. State of the Polar Bear Report 2018. Global Warming Policy Foundation Report 32, London.
Laidre, K.L, Atkinson, S., Regehr, E.V., Stern, H.L, Born, E.W., Wiig, Ø., Lunn, N.J. and Dyck, M. 2020 in press. Interrelated ecological impacts of climate change on an apex predator. Ecological Applications https://doi.org/10.1002/eap.2071 [open access]
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
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
SWG [Scientific Working Group to the Canada-Greenland Joint Commission on Polar Bear]. 2016. Re-Assessment of the Baffin Bay and Kane Basin Polar Bear Subpopulations: Final Report to the Canada-Greenland Joint Commission on Polar Bear. +636 pp. http://www.gov.nu.ca/documents-publications/349
York, J., Dowsley, M., Cornwell, A., Kuc, M. and Taylor, M. 2016. Demographic and traditional knowledge perspectives on the current status of Canadian polar bear subpopulations. Ecology and Evolution 6(9):2897-2924. DOI: 10.1002/ece3.2030