Reduced Ice and Polar Bears in Beaufort and Chukchi Seas

by Hilary Bruegl

Polar bears in the Arctic rely on sea ice as a means of locating and hunting for seals, their primary food source. Because populations of polar bears can be quite variable, their responses to climate change also depend on reproductive and hunting strategies employed by each population, especially when faced with declining sea ice. The Chukchi Sea (CS) population of polar bears was found to have greater body size and overall condition in a period of four years between 2008–2011 as compared to previous CS population data from 1986–1994 as well as compared to the 2008–2011 Beaufort Sea (SB) population of polar bears (Rode et al. 2014). The SB population of polar bears has been exposed to declining sea ice conditions for longer periods of time than the CS population, allowing for compounding effects over generations, which may account for some differences in population response in 2008–2011.

Karyn D. Rode and colleagues evaluated the behavioral and physical responses of two arctic polar bear populations as they relate to increase in global temperatures and subsequent reduction in sea ice conditions, including the increasing distance from the edge of the continental shelf to the nearest ice pack. Bears were immobilized with a tranquilizer dart by helicopter and measured. Condition was estimated by measuring skull width, body mass, and energy density, which relates body mass and body length. Recruitment was measured as yearling litter size, relative number of females with cubs, and number of cubs per female. To determine diet, a 6–mm fat biopsy was taken from the hindquarters and analyzed for fatty acid signature. Additionally, blood samples were taken in order to determine whether or not the bears were fasting and for how long.

The most relevant variable for this study is the change in climate over the two periods of data collection. The authors found a significant increase in the number of reduced ice days on the CS continental shelf in 2008–2011 as compared to 1986–1994. There were 44 days of reduced summer ice as compared to 0 in previous years. The number of reduced ice days in 2007–2010 was nearly doubled in the SB. Both locations exhibited similar mean minimum distance from the continental shelf to the September ice pack. This distance increased 445km in 2008–2011 as compared to 1986–1994.

Using these ice quality data, the authors found an increase in body condition in CS bears correlating with a decrease in ice quality across the two data-recording periods. In addition, CS bears were found to be significantly larger than bears in the SB population. A correlation was found between an increase the number of reduced ice days and overall reduction in body condition in both locations during 2008–2011. Measurements were made in September each year.

Overall, the CS polar bears appear to be in better health and overall fitness than the SB bears. The CS polar bear population had a higher number of females with cubs relative to the SB population, suggesting that better condition plays a role in overall recruitment. Because number of days of ice reduction per year has been steadily increasing for a number of years in the SB continental shelf, it is reasonable to expect lesser condition and fewer cubs produced relative to the CS population, which only recently began to see reduction in sea ice. By comparing these two populations we can see the effects of long-term climate change on an Arctic polar bear population. While the CS population may still be in excellent condition, it is possible that with further warming and ice reduction, their condition and recruitment will fall similarly to that of the SB population. This study proposes further monitoring of recently affected populations to study their response to climate change in the future.

Rode, K. D., Regehr, E. V., Douglas, D. C., Durner, G., Derocher, A. E., Thiemann, G. W., Budge, S. M. 2014. 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, 76–88. Full paper: http://bit.ly/1vWQyMo

2 thoughts on “Reduced Ice and Polar Bears in Beaufort and Chukchi Seas

  1. This is solid overview of Rode et al. The contrast between the Chukchi Sea and Southern Beaufort Sea polar bear populations nicely highlights the variation in responses we are seeing across the Arctic. The Chukchi Sea is ecologically and oceanographically different from the Beaufort Sea and the main difference is the extent of the continental shelf. While polar bears are widely distributed across the Arctic, the highest densities occur over the continental shelves where marine productivity is higher and thus, ringed seal densities are higher. We are seeing similar differences between adjacent populations in other areas. In Hudson Bay, the western population is showing the effects of sea ice loss sooner than the southern one. In this area, sea ice break-up patterns are the ecological issue associated with the difference. These 2 populations overlap to a degree on the sea ice so they are exploiting the same resources. With 19 relatively discrete populations (sometimes called subpopulations using the IUCN terminology) we are seeing 19 different scenarios play out with some populations showing dramatic change and others showing little. As in most issues in ecology, the patterns are complex and the data noisy. At the end of it all, however, the polar bear issue is a simple one – it is a habitat loss issue. Reasonable scientists don’t expect adaptation by polar bears to sea ice loss any more than we would for a highly specialized bird losing its habitat in the Amazon rainforest. While polar bears are able to do some amazing things, they only exist where sea ice persists for enough of the year. The experiment of climate change (albeit slower) was done for polar bears at the end of the last glaciation: we only have polar bear fossils in the Baltic Sea. The main prey of polar bears, ringed seals, still exist there and some sea ice forms each winter but it was not enough for the bear to persist. Similarly, we expect extirpation of polar bear populations over much of the Arctic by mid-century based on sea ice projections. Most ecologists would not expect a highly specialized species adapted to a highly specialized habitat to adapt to a dramatically different habitat in the span of decades. Besides, in the Arctic, we already have the terrestrial bear niche filled by the brown bear (Ursus arctos).

    Like

  2. Thanks for featuring our paper. What’s often overlooked in many (often political) discussions about polar bears is that we shouldn’t expect every subpopulation in every region to be identically affected by climate change. Some subpopulations, like the Southern Beaufort, are more vulnerable than others, like the Chukchi Sea. There are a host of ecological factors (e.g., relative area of continental shelf, prey diversity) that will influence the manner and timing of a population’s response to climate change, even of we don’t yet understand exactly how. But ecological nuances aside, we know enough about polar bears (and all species for that matter) to know that ultimately, habitat loss = species loss. If polar bears are doing relatively well in a particular region (see the Chukchi Sea), it doesn’t change that equation.

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s