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