Polar bears, as is well known, are creatures of the sea ice, and many studies have established that declining sea ice is already impacting polar bear health and population numbers in parts of their range.
In what are probably the two best-studied of 19 polar bear sub-populations — in Canada’s western Hudson Bay, and the southern Beaufort Sea off northern Alaska — research has pointed to declines in reproductive ability and body weight, decreases in overall numbers, and stressors such as bears having to swim exceptionally long distances.
Changes in sea ice not only affect polar bears directly, but also indirectly: for example, should sea ice break up earlier in the year than usual, then ringed seals, polar bears’ favored prey, may find themselves without a sufficiently stable and long-lasting platform on which to build snow dens and nurture young pups. (Of at least equal concern with regard to ringed seals are other climate-related changes, such as a lack of snow or too much rain, which makes it hard for them to build dens that are sufficiently protective or long-lasting.)
But what might appear as a homogenous expanse of frozen water are many regional variations, and it doesn’t necessarily follow that all polar bear sub-populations will respond the same way or at the same rate. Indeed, an upcoming paper in the journal Global Change Biology has found that the bears in one area — the Chukchi-Bering Seas — have shown no decline in body size, recruitment or condition, even as those in the next door southern Beaufort Sea region have done.
In the study, Karyn Rode and Eric Regehr of the U.S. Fish & Wildlife Service and colleagues studied body size and condition of polar bears that were sedated, studied and released on sea ice in the Chukchi Sea region between 2008 and 2011 and compared the results with bears that were studied in the Beaufort during the same period, and in the Chukchi from 1986 to 1994. A small fat biopsy was conducted on the Chukchi polar bears to estimate diet.
The results were consistent with the variation among sub-populations that most scientists expect, but will nonetheless likely prove surprising to some. The more recently-studied Chukchi Sea bears “were either larger and in better condition, or similar in size and condition, to CS bears in 1986-1994,” the authors wrote.
This stands in contrast to their Beaufort near-neighbors: on average females greater than 1 year old weighed a little over 30 kg more, and similarly-aged males weighed on average 48.5 kg more, than those in the Beaufort. Additionally, the number of yearling cubs per female each spring—an indicator of reproductive success—was higher in the Chukchi.
So why should one sub-population of polar bears show signs of being so much healthier than another one close by? Rode, Regehr and colleagues noted that, although the number of ice-reduced days each year in the Chukchi increased from zero in 1986-94 to 44 in 2007-10, that was still half as many ice-reduced days as in the Beaufort. However, given that body condition remained the same in the Chukchi even as sea ice conditions deteriorated, they also looked to other explanations.
The answer, they hypothesized, is the result of a suite of environmental differences between the two regions. For example, some southern Beaufort polar bears (and all the bears in western Hudson Bay) stay on land during the summer when ice retreats, while even during the seasonal minimum ice extent, sea ice persists over the continental shelf of the Chukchi. And while the majority of southern Beaufort bears inhabit the ice year-round, in the summer months that ice now retreats far north of the continental shelf, into the deeper, less productive waters of the polar basin.
Additionally, whereas the continental shelf in the Beaufort is relatively narrow, virtually the entirety of the Chukchi is shallower than 300 meters; partly as a result, overall productivity of the marine environment is higher, and as a result the densities of many polar bear prey species are higher also.
Furthermore, the number of available prey species is greater; it may be significant, the authors note, that walruses appeared to constitute a small percentage of polar bear prey in the Chukchi Sea, whereas virtually no Beaufort Sea bears feed on them.
Finally, although warmer waters have led to sea ice loss in the Chukchi, they have also led to higher biological productivity; it is notable, the authors observe, that the condition and population size of both ringed and bearded seals are also apparently hale and hearty in the region.
None of which is to say that the Chukchi Sea bears have nothing but plain sailing ahead. The rate of sea ice decline in the region is greater than that in the Beaufort, and at some point loss of the sea ice platform from which bears hunt is likely to have negative effects on their condition and even survival.
However, the study does suggest, concluded the authors, that polar bears’ response to climate change may be “complex”, especially in the short term, with the amount of available habitat, ecosystem productivity, and the health of prey populations important considerations.
In other words: yes, polar bears are indeed creatures of the sea ice, and the continued loss of that sea ice seems certain to prove hugely detrimental to polar bears throughout their range.
But in different areas, the initial ecological responses to declining sea ice may take different forms, and it it those responses — and particularly the impact on prey species such as ringed seals — that may most strongly affect the particular contours of the paths different polar bear populations follow in the near-term as their environment changes around them.
Photograph of polar bears in the Chukchi Sea by Kieran Mulvaney