By Emily Shenk
A female polar bear stands with three yearlings in the Arctic’s Chukchi Sea. (Photograph courtesy U.S. Fish and Wildlife Service)
A recent study found that the condition of polar bears in the Arctic’s Chukchi Sea has remained stable despite sea ice loss, while the condition of another population in the Beaufort Sea has declined.
The researchers studied the overall health and reproductive rates of polar bears in the Chukchi Sea, located between Alaska and Russia, and the adjacent southern Beaufort Sea. They compared the Chukchi Sea population from 2008 to 2011 with data from the same population from decades earlier. Despite experiencing a 44-day increase in the number of reduced ice days over productive continental shelf waters during that elapsed time, the condition and reproduction rates of the Chukchi Sea polar bears remained stable.
Lead author Karyn Rode, a research wildlife biologist at the U.S. Geological Survey, said the findings were surprising given that the Chukchi Sea has experienced some of the greatest sea ice loss in the Arctic. Conversely, sea ice decline in the southern Beaufort Sea corresponded with declines in polar bear body condition and reproduction.
Along with co-author Eric Regehr of the U.S. Fish and Wildlife Service, and other collaborators, Rode examined data from polar bears captured and released between May and March in the years studied. Rode spoke with National Geographic’s Polar Bear Watch about the findings—and what they might mean for future polar bear research and conservation efforts.
What may have contributed to the healthier condition of Chukchi Sea polar bears as compared to the southern Beaufort Sea polar bears?
Body condition of polar bears of all sex and age classes in the Chukchi Sea either remained stable or improved between 1986 to 1994 and 2008 to 2011. Body size and condition were greater in the Chukchi Sea than in the southern Beaufort Sea. There are several possible explanations for these observations.
The southern Beaufort Sea is defined by a narrow, nearshore area of continental shelf that quickly leads to deep Arctic basin waters, whereas nearly the entirety of the range of polar bears in the Chukchi Sea consists of shallow continental shelf waters. As a result, even during the annual September sea ice minimum, some area of ice remains over shallow waters of the continental shelf in the Chukchi Sea. Because these waters are the preferred habitat of seals, this may allow bears there to continue to access prey during the summer, and may even concentrate the seals in a smaller area.
Biological productivity is known to be higher in the Chukchi Sea, which may translate to a greater abundance of bearded and ringed seals. A recent study conducted by Lori Quakenbush with the state of Alaska demonstrated that these seals have maintained body condition and reproduction over the past 20 to 30 years in the Chukchi Sea. Thus, the prey base for polar bears has remained intact despite changing sea ice conditions.
Declines in ice over the vast continental shelf region in the Chukchi Sea have only recently begun, in contrast with the southern Beaufort Sea where there is a longer history of ice-free days over the smaller continental shelf region. It’s possible that longer-term, cumulative effects of reduced ice over the shelf are affecting prey access for bears in the southern Beaufort Sea in a way that has not yet happened in the Chukchi Sea.
What can this study tell us about how geographic differences affect polar bears’ response to climate change? What about other ice-dependent species?
It suggests that there is ecological and environmental variation across the range of polar bears that may contribute to how and when bears respond to sea ice loss. Similarities between the lack of an apparent effect of sea ice loss on both polar bears and their primary prey species—ringed and bearded seals—in the Chukchi Sea suggest that geographic variation may affect multiple ice-dependent species, not just polar bears.
What role might diet play? Why is understanding the polar bears’ eating and fasting habits important?
How much bears were eating—as opposed to what they were eating—may explain differences in the current body condition and reproduction of polar bears in the Chukchi and southern Beaufort Seas. More bears were fasting in the spring in the southern Beaufort than in the Chukchi Sea, although diets were relatively similar for the two populations.
Loss of access to prey, resulting from a loss of the sea ice platform required for bears to effectively hunt ice seals, is the primary mechanism by which we expect polar bears to be affected by sea ice loss. Thus, determining what bears are eating and how they access those food resources is important to understanding how they will respond to continued sea ice loss.
What are the implications for population dynamics?
Body condition, reproduction, and cub survival of polar bears in the Chukchi Sea are not currently being negatively affected by sea ice loss. This suggests that overall survival is unlikely to be limited by food resources, which means that the population would likely be capable of positive growth if there were no other factors.
However, there are factors that affect survival in ways our study would not be able to detect—in particular, human-caused mortality. In the Chukchi Sea, polar bears are harvested legally by Native Alaskans for subsistence purposes, and are also harvested illegally in Russia at poorly understood levels.
A sustainable quota for subsistence harvest has been identified under an international treaty between the U.S. and Russia, but has not yet been implemented in either country. So, our results tell us something about potential population dynamics, but there are other parts of the story still missing.
Why is it important to monitor polar bears’ body condition and reproduction in relation to sea ice loss? What does this tell us about the relationship between polar bears and their habitat?
Because polar bears rely on the sea ice to access their prey, body condition and subsequently reproduction are likely to be the first indicators that sea ice loss is starting to limit their access to prey, assuming that prey populations are stable.
The western Hudson Bay population is a good example, where declines in body condition were the first indicator of a demographic response to sea ice loss. Reproduction, then survival, and ultimately population size subsequently declined.
Because polar bears inhabit vast and remote regions, body condition and reproduction are also easier to monitor accurately than population size, making them good metrics to track as sea ice changes occur.
What are the conservation ramifications of your research?
By comparing two adjacent—but very different—polar bear populations, this is one of the first studies to highlight how the response of polar bears to climate change may vary in time and space.
That is important because, short of limiting greenhouse gas concentrations, polar bear conservation will happen largely at a local and regional scale. Managers need to understand the different status of different populations in order to develop appropriate conservation measures. There is currently not a “one-size-fits-all” approach for polar bears.
For example, signs that the Chukchi Sea population thus far appears resilient to sea ice loss was critical to informing decisions of the U.S.-Russia Polar Bear Commission regarding subsistence harvest. This has allowed focus on building the systems and infrastructure in both countries to accurately monitor harvest and to regulate it in the future.
The results of this study do not negate that sea ice loss remains a significant threat to polar bears, in the Chukchi Sea or elsewhere. Indeed, research in some other parts of the Arctic has demonstrated significant negative effects of sea ice loss.
Although we have highlighted some regional differences in the response of polar bears, at the present time, all polar bears depend on sea ice for fundamental aspects of their life history. Their life on the sea ice is what makes a polar bear a polar bear. The rate and extent of sea ice loss that is projected for the Chukchi Sea, and for the Arctic as a whole, will require bears to change their behavior and potentially their ecology in ways that we have not yet seen.
We have a lot to learn about the mechanisms by which polar bears are maintaining access to food, the distribution of their prey throughout the year, and at what point sea ice loss and limited access to prey has a negative effect on polar bear populations.
We cannot simply project the results of this study into the future to say that polar bears in the Chukchi Sea or elsewhere are not threatened by sea ice loss. We can, however, use the refined understanding from this and other recent studies of how sea ice loss affects polar bears to inform local conservation efforts and provide hope that polar bears everywhere are not yet in a state of decline, but rather there is time for us to affect greenhouse gas emissions and change the trajectory of sea ice loss.