Polar bear consumption of terrestrial foods – new paper misses the point

A new paper by polar bear biologists (Rode et al. 2015) argues that terrestrial (land-based) foods are not important to polar bears now and will not be in the future – a conclusion I totally agree with – but they miss the point entirely regarding the importance of this issue.

Fat bears on land in the Southern Beaufort, Sue Miller photo, well-prepared for a brief summer fast.

Whatever food polar bears consume in the summer – whether they are on land or on the ice – doesn’t really matter. What matters is how many fat-rich seals they can consume between March and June each year. The fat put on in late winter/spring from gorging on baby seals carries polar bears over the summer, no matter where they spend it.

USGS polar bear biologist Karyn Rode and colleagues (press release here) have tried to frame this issue as one about future survival of polar bears in the face of declining sea ice. However, the fact that polar bears in the Chukchi Sea and Southern Davis Strait are thriving despite dramatic declines in summer sea ice (aka an extended open-water season), proves my point and disproves their premise. Bears in these regions are doing extremely well – contrary to all predictions – because they have had abundant baby seals to eat during the spring (see here and here).

Don’t forget that the sea ice models these biologists use do not predict a decline in winter ice (Dec-March) and project only slight declines in spring ice (April-June) by mid-century (Amstrup et al. 2007; Durner et al. 2009; Oakley et al. 2012; Wang et al. 2012). Polar bear biologists emphasize the generic phrase “sea ice” whenever possible (as in this paper, “Warming-induced loss of sea ice remains the primary threat faced by polar bears.”) but if you look at the predictive ice modeling papers they use, it is clear they really mean summer sea ice:

“…all GCMs project extensive winter sea ice through the end of the 21st century in most ecoregions (Durner et al. 2009).” (Amstrup et al. 2007:9)

In other words, their models show no reason to worry about the state of the critical spring feeding period between now and the middle of this century. Well-fed bears throughout the Arctic have enough fat to see them through a 4-5 month fast and even the worst-case scenario models devised suggest that most bears in productive regions like Hudson Bay [and probably, Southern Davis Strait] would survive a 6 month fast. Keep this in mind when you hear or read stories about polar bears and predicted sea ice declines: “sea ice” does not mean spring sea ice.

Here is the comment I gave to science writer Ivan Imato over at InsideScience when he asked for my opinion on the Rode et al. paper:

“I agree wholeheartedly with the authors that there is little evidence to suggest that terrestrial foods are important to polar bears now, or would be in the future. However, this squabbling over what polar bears eat, or don’t eat, in the summer hardly matters, since the critical feeding period for polar bears is March to June. Chukchi Sea and Southern Davis Strait bears, for example, are doing very well – contrary to all predictions – despite marked declines in summer sea ice because they have ample food during their critical spring feeding period when sea ice is abundant.”

[That said, I was disturbed by the fact that these authors chose to use a diminished version (“~900”) of the 2004 estimate of polar bears in Western Hudson Bay (~935, Regher et al. 2007) rather than the 2011 estimate derived from an aerial survey (~1000 bears, Stapleton et al. 2014) or the 2011 estimate derived from mark-recapture work (consistent with the ~1000 bear estimate due to differences in methods, Lunn et al. 2013). Note that both the IUCN PBSG (2014 update ) and Environment Canada (2014 assessment) use a population estimate of 1030 for Western Hudson Bay polar bears (even PBSG 2013 assessment was “1000”). Co-authors Rode and Amstrup are both PBSG members: they must know the most up-to-date estimates.  A population size of “~900” may fit their narrative of a ‘declining’ WHB population but it misrepresents the science. Knowing they would do that, why would you believe anything else they say?]

All this squabbling over whether or not consumption of terrestrial foods will save polar bears from prophesied declines in summer sea ice is a red herring: it doesn’t matter what, if anything, polar bears eat in summer — what matters is how much they eat in the spring

For more on the terrestrial foods topic, see my detailed discussion in this previous post, and this recent (March 30) ScienceNews report on yet another, largely anecdotal ‘polar bears resort to bird eggs because of declining sea ice’ story (see photo below, based on a new paper by Prop and colleagues), which was also covered March 31 at the DailyMail (“Polar bears are forced to raid seabird nests as Arctic sea ice melts – eating more than 200 eggs in two hours,with lots of hand-wringing and sea ice hype but little mention of the fact that there are many more bears now than there were in the early 1970s around Svalbard or that the variable, cyclical, AMO (not global warming) has had the largest impact on sea ice conditions in the Barents Sea).

“Young polar bears raid the nesting grounds of barnacle geese, searching for eggs and newly hatched goslings in the Nordenskiöldkysten region of Spitsbergen, Norway.” Jouke Prop photo.

Young polar bears raid the nesting grounds of barnacle geese, searching for eggs and newly hatched goslings in the Nordenskiöldkysten region of Spitsbergen, Norway.” Jouke Prop photo.

Amstrup, S.C., Marcot, B.G. and Douglas, D.C. 2007. Forecasting the range-wide status of polar bears at selected times in the 21st century. Administrative Report, US Geological Survey. Reston, Virginia. http://www.usgs.gov/newsroom/special/polar_bears/

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. http://www.esajournals.org/doi/abs/10.1890/07-2089.1

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

Prop, J., Aars, J., and 14 colleagues. 2015. Climate change and the increasing impact of polar bears on bird populations. Frontiers in Ecology and Evolution. Vol. 3 Article 33. 25 March 2015 doi: 10.3389/fevo.2015.00033 http://journal.frontiersin.org/article/10.3389/fevo.2015.00033/full Open Access, Pdf here.

Regehr, E.V., Lunn, N.J., Amstrup, S.C., and Stirling, I. 2007. Survival and population size of polar bears in western Hudson Bay in relation to earlier sea ice breakup. Journal of Wildlife Management 71: 2673-2683. http://onlinelibrary.wiley.com/doi/10.2193/2006-180/abstract

Rode, K.D., Robbins, C.T., Nelson, L. and Amstrup, S.C. 2015. Can polar bears use terrestrial foods to offset lost ice-based hunting opportunities? Frontiers in Ecology and the Environment 13(3): 138–145, doi:10.1890/140202
http://www.esajournals.org/doi/abs/10.1890/140202 [paywalled]

“Increased land use by polar bears (Ursus maritimus) due to climate-change-induced reduction of their sea-ice habitat illustrates the impact of climate change on species distributions and the difficulty of conserving a large, highly specialized carnivore in the face of this global threat. Some authors have suggested that terrestrial food consumption by polar bears will help them withstand sea-ice loss as they are forced to spend increasing amounts of time on land. Here, we evaluate the nutritional needs of polar bears as well as the physiological and environmental constraints that shape their use of terrestrial ecosystems. Only small numbers of polar bears have been documented consuming terrestrial foods even in modest quantities. Over much of the polar bear’s range, limited terrestrial food availability supports only low densities of much smaller, resident brown bears (Ursus arctos), which use low-quality resources more efficiently and may compete with polar bears in these areas. Where consumption of terrestrial foods has been documented, polar bear body condition and survival rates have declined even as land use has increased. Thus far, observed consumption of terrestrial food by polar bears has been insufficient to offset lost ice-based hunting opportunities but can have ecological consequences for other species. Warming-induced loss of sea ice remains the primary threat faced by polar bears.”

Stapleton S., Atkinson, S., Hedman, D., and Garshelis, D. 2014. Revisiting Western Hudson Bay: using aerial surveys to update polar bear abundance in a sentinel population. Biological Conservation 170:38-47. http://www.sciencedirect.com/science/article/pii/S0006320713004618#

Wang M., Overland, J.E., Stabeno, P. 2012. Future climate of the Bering and Chukchi Seas projected by global climate models. Deep-Sea Research Part II: Topical Studies in Oceanography 65–70: 46–57. http://www.sciencedirect.com/science/article/pii/S0967064512000367

Comments are closed.