Papers on polar bear populations
Posted by Ari Jokimäki on March 15, 2010
This is a list of papers on polar bear populations. Emphasis is on climate change effect on the populations. The list is not complete, and will most likely be updated in the future in order to make it more thorough and more representative.
UPDATE (June 9, 2012): Stirling & Derocher (2012), Stirling & Parkinson (2006), Stirling et al. (1999), and Stirling & Derocher (1993) added.
UPDATE (October 15, 2010): Regehr et al. (2006) added. Thanks to Barry for pointing it out, see the comment section below.
Effects of Climate Warming on Polar Bears: A Review of the Evidence – Stirling & Derocher (2012) “Climate warming is causing unidirectional changes to annual patterns of sea ice distribution, structure, and freeze-up. We summarize evidence that documents how loss of sea ice, the primary habitat of polar bears (Ursus maritimus), negatively affects their long-term survival. To maintain viable subpopulations, polar bears depend upon sea ice as a platform from which to hunt seals for long enough each year to accumulate sufficient energy (fat) to survive periods when seals are unavailable. Less time to access to prey, because of progressively earlier breakup in spring, when newly-weaned ringed seal (Pusa hispida) young are available, results in longer periods of fasting, lower body condition, decreased access to denning areas, fewer and smaller cubs, lower survival of cubs as well as bears of other age classes and, finally, subpopulation decline toward eventual extirpation. The chronology of climate-driven changes will vary between subpopulations, with quantifiable negative effects being documented first in the more southerly subpopulations, such as those in Hudson Bay or the southern Beaufort Sea. As the bears’ body condition declines, more seek alternate food resources so the frequency of conflicts between bears and humans increases. In the most northerly areas, thick multiyear ice, through which little light penetrates to stimulate biological growth on the underside, will be replaced by annual ice which facilitates greater productivity and may create habitat more favorable to polar bears over continental shelf areas in the short term. If the climate continues to warm and eliminate sea ice as predicted, polar bears will largely disappear from the southern portions of their range by mid-century. They may persist in the northern Canadian Arctic Islands and northern Greenland for the foreseeable future, but their long-term viability, with a much reduced global population size in a remnant of their former range, is uncertain.” Ian Stirling, Andrew E. Derocher, Global Change Biology, DOI: 10.1111/j.1365-2486.2012.02753.x.
Predicting 21st-century polar bear habitat distribution from global climate models – Durner et al. (2009) “We used location data from satellite-collared polar bears and environmental data (e.g., bathymetry, distance to coastlines, and sea ice) collected from 1985 to 1995 to build resource selection functions (RSFs). … We applied the RSFs to monthly maps of 21st-century sea ice concentration projected by 10 general circulation models (GCMs) used in the Intergovernmental Panel of Climate Change Fourth Assessment Report, under the A1B greenhouse gas forcing scenario. … Mean loss of optimal polar bear habitat was greatest during summer; from an observed 1.0 million km2 in 1985–1995 (baseline) to a projected multi-model mean of 0.32 million km2 in 2090–2099 (−68% change). Projected winter losses of polar bear habitat were less: from 1.7 million km2 in 1985–1995 to 1.4 million km2 in 2090–2099 (−17% change). … Although a reduction in the total amount of optimal habitat will likely reduce polar bear populations, exact relationships between habitat losses and population demographics remain unknown.” [Full text]
Rebuttal of “Polar Bear Population Forecasts: A Public-Policy Forecasting Audit” – Amstrup et al. (2009) “In summary, we show that the AGS audit offers no valid criticism of the USGS conclusion that global warming poses a serious threat to the future welfare of polar bears and that it only serves to distract from reasoned public-policy debate.” [Full text]
Effects of climate change on polar bears – Wiig et al. (2008) A review article. “In this article, we review the effects on polar bears of global warming that have already been observed, and try to evaluate what may happen to the polar bears in the future. Many researchers have predicted a wide range of impacts of climate change on polar bear demography and conditions. A predicted major reduction in sea ice habitat will reduce the availability of ice associated seals, the main prey of polar bears, and a loss and fragmentation of polar bear habitat will ultimately lead to large future reductions in most subpopulations. It is likely that polar bears will be lost from many areas where they are common today and also that the total population will change into a few more distinctly isolated populations.”
Effects of Earlier Sea Ice Breakup on Survival and Population Size of Polar Bears in Western Hudson Bay – Regehr et al. (2007) “We used a flexible extension of Cormack–Jolly–Seber capture–recapture models to estimate population size and survival for polar bears (Ursus maritimus), one of the most ice-dependent of Arctic marine mammals. We analyzed data for polar bears captured from 1984 to 2004 along the western coast of Hudson Bay and in the community of Churchill, Manitoba, Canada. The Western Hudson Bay polar bear population declined from 1,194 (95% CI = 1,020–1,368) in 1987 to 935 (95% CI = 794–1,076) in 2004. … Survival of juvenile, subadult, and senescent-adult polar bears was correlated with spring sea ice breakup date, which was variable among years and occurred approximately 3 weeks earlier in 2004 than in 1984. We propose that this correlation provides evidence for a causal association between earlier sea ice breakup (due to climatic warming) and decreased polar bear survival.”
Polar Bear Population Status in the Southern Beaufort Sea – Regehr et al. (2006) An U.S. Geological Survey report. “Polar bears depend entirely on sea ice for survival. In recent years, a warming climate has caused major changes in the Arctic sea ice environment, leading to concerns regarding the status of polar bear populations. Here we present findings from long-term studies of polar bears in the southern Beaufort Sea (SBS) region of the U.S. and Canada, which are relevant to these concerns. We applied open population capture-recapture models to data collected from 2001 to 2006, and estimated there were 1,526 (95% CI = 1,211; 1,841) polar bears in the SBS region in 2006. The number of polar bears in this region was previously estimated to be approximately 1,800. Because precision of earlier estimates was low, our current estimate of population size and the earlier ones cannot be statistically differentiated. For the 2001–06 period, the best fitting capture-recapture model provided estimates of total apparent survival of 0.43 for cubs of the year (COYs), and 0.92 for all polar bears older than COYs. Because the survival rates for older polar bears included multiple sex and age strata, they could not be compared to previous estimates. Survival rates for COYs, however, were significantly lower than estimates derived in earlier studies (P = 0.03). The lower survival of COYs was corroborated by a comparison of the number of COYs per adult female for periods before (1967–89) and after (1990–2006) the winter of 1989–90, when warming temperatures and altered atmospheric circulation caused an abrupt change in sea ice conditions in the Arctic basin. In the latter period, there were significantly more COYs per adult female in the spring (P = 0.02), and significantly fewer COYs per adult female in the autumn (P < 0.001). Apparently, cub production was higher in the latter period, but fewer cubs survived beyond the first 6 months of life. Parallel with declining survival, skull measurements suggested that COYs captured from 1990 to 2006 were smaller than those captured before 1990. Similarly, both skull measurements and body weights suggested that adult males captured from 1990 to 2006 were smaller than those captured before 1990. The smaller stature of males was especially notable because it corresponded with a higher mean age of adult males. Male polar bears continue to grow into their teens, and if adequately nourished, the older males captured in the latter period should have been larger than those captured earlier. In western Hudson Bay, Canada, a significant decline in population size was preceded by observed declines in cub survival and physical stature. The evidence of declining recruitment and body size reported here, therefore, suggests vigilance regarding the future of polar bears in the SBS region." [Full text]
Possible Effects of Climate Warming on Selected Populations of Polar Bears (Ursus maritimus) in the Canadian Arctic – Stirling & Parkinson (2006) “Polar bears depend on sea ice for survival. Climate warming in the Arctic has caused significant declines in total cover and thickness of sea ice in the polar basin and progressively earlier breakup in some areas. Inuit hunters in the areas of four polar bear populations in the eastern Canadian Arctic (including Western Hudson Bay) have reported seeing more bears near settlements during the open-water period in recent years. In a fifth ecologically similar population, no changes have yet been reported by Inuit hunters. These observations, interpreted as evidence of increasing population size, have resulted in increases in hunting quotas. However, long-term data on the population size and body condition of polar bears in Western Hudson Bay, as well as population and harvest data from Baffin Bay, make it clear that those two populations at least are more likely to be declining, not increasing. While the ecological details vary in the regions occupied by the five different populations discussed in this paper, analysis of passive-microwave satellite imagery beginning in the late 1970s indicates that the sea ice is breaking up at progressively earlier dates, so that bears must fast for longer periods during the open-water season. Thus, at least part of the explanation for the appearance of more bears near coastal communities and hunting camps is likely that they are searching for alternative food sources in years when their stored body fat depots may be depleted before freeze-up, when they can return to the sea ice to hunt seals again. We hypothesize that, if the climate continues to warm as projected by the Intergovernmental Panel on Climate Change (IPCC), then polar bears in all five populations discussed in this paper will be increasingly food-stressed, and their numbers are likely to decline eventually, probably significantly so. As these populations decline, problem interactions between bears and humans will likely continue, and possibly increase, as the bears seek alternative food sources. Taken together, the data reported in this paper suggest that a precautionary approach be taken to the harvesting of polar bears and that the potential effects of climate warming be incorporated into planning for the management and conservation of this species throughout the Arctic.” Ian Stirling and Claire L. Parkinson, Arctic, Vol. 59, No. 3 (Sep., 2006), pp. 261-275 [Full text]
Observations of mortality associated with extended open-water swimming by polar bears in the Alaskan Beaufort Sea – Monnett & Gleason (2006) “We speculate that mortalities due to offshore swimming during late-ice (or mild ice) years may be an important and unaccounted source of natural mortality given energetic demands placed on individual bears engaged in long-distance swimming. We further suggest that drowning-related deaths of polar bears may increase in the future if the observed trend of regression of pack ice and/or longer open water periods continues.” [Full text]
The influence of climate variability on polar bear (Ursus maritimus) and ringed seal (Pusa hispida) population dynamics – Rosing-Asvid (2006) “Unusually high polar bear (Ursus maritimus Phipps, 1774) predation on ringed seal (Pusa hispida (Schreber, 1775)) pups and increased survival of polar bear cubs during mild springs is documented in published articles. Strong predation on newborn ringed seal pups in early spring, however, is likely to lower the overall energy intake of polar bears if ringed seal pups are their main food, because the energetic value of ringed seal pups increases 7–8 times during the 6 week lactation period. So although hunting success in early spring increases cub survival during the period after den emergence,when they are most vulnerable, it is likely to increase the number of starving bears later in the season.” [Full text]
Population ecology of polar bears at Svalbard, Norway – Derocher (2005) “The population ecology of polar bears at Svalbard, Norway, was examined from 1988 to 2002 using live-captured animals. … However, the variation in reproduction and body mass in the population show a relationship between large-scale climatic variation and the upper trophic level in an Arctic marine ecosystem. Similar change in other polar bear populations has been attributed to climate change, and further research is needed to establish linkages between climate and the population ecology of polar bears.” [Full text]
Polar Bears in a Warming Climate – Derocher et al. (2004) “Polar bears (Ursus maritimus) live throughout the ice-covered waters of the circumpolar Arctic, particularly in near shore annual ice over the continental shelf where biological productivity is highest. However, to a large degree under scenarios predicted by climate change models, these preferred sea ice habitats will be substantially altered. … In the short term, climatic warming may improve bear and seal habitats in higher latitudes over continental shelves if currently thick multiyear ice is replaced by annual ice with more leads, making it more suitable for seals. … The effects of climate change are likely to show large geographic, temporal and even individual differences and be highly variable, making it difficult to develop adequate monitoring and research programs. All ursids show behavioural plasticity but given the rapid pace of ecological change in the Arctic, the long generation time, and the highly specialised nature of polar bears, it is unlikely that polar bears will survive as a species if the sea ice disappears completely as has been predicted by some.” [Full text]
Polar Bear Distribution and Abundance on the Southwestern Hudson Bay Coast During Open Water Season, in Relation to Population Trends and Annual Ice Patterns – Stirling et al. (2004) “We concluded that 1) the coastal survey data reliably indicated the population trends in Manitoba and Ontario; 2) little exchange occurred between the Western Hudson Bay (Manitoba) and Southern Hudson Bay (Ontario) populations; 3) between 1971 and 2001, there was a statistically significant trend toward earlier breakup of sea ice off the Manitoba coast, but not off the Ontario coast; 4) the onset of ice absence along the coast had no significant relationship to the number of bears present in each sub-sampling area within either the Manitoba or the Ontario population, but did significantly influence the distribution of bears on the coastline of each province independently of the other; 5) timing of the surveys can influence the results; and 6) adult male and female bears both showed a high degree of fidelity to specific areas during summer, independent of the pattern of ice breakup.” [Full text]
Polar Bears and Seals in the Eastern Beaufort Sea and Amundsen Gulf: A Synthesis of Population Trends and Ecological Relationships over Three Decades – Stirling (2002) “The changes in the sea ice environment, and their consequent effects on polar bears, are demonstrable in parallel fluctuations in the mean ages of polar bears killed each year by Inuit hunters. In 1989, the decadal-scale pattern in fluctuations of ice conditions in the eastern Beaufort Sea changed in response to oceanographic and climatic factors, and this change has resulted in greater amounts of open water in recent years. In addition, climatic warming will be a major environmental factor if greenhouse gas emissions continue to increase. It is unknown whether the ecosystem will return to the pattern of decadal-scale change exhibited in previous decades, or how polar bears and seals will respond to ecological changes in the future, but research on these topics is a high priority.” [Full text]
Long-Term Trends in the Population Ecology of Polar Bears in Western Hudson Bay in Relation to Climatic Change – Stirling et al. (1999) “From 1981 through 1998, the condition of adult male and female polar bears has declined significantly in western Hudson Bay, as have natality and the proportion of yearling cubs caught during the open water period that were independent at the time of capture. Over this same period, the breakup of the sea ice on western Hudson Bay has been occurring earlier. There was a significant positive relationship between the time of breakup and the condition of adult females (i.e., the earlier the breakup, the poorer the condition of the bears). The trend toward earlier breakup was also correlated with rising spring air temperatures over the study area from 1950 to 1990. We suggest that the proximate cause of the decline in physical and reproductive parameters of polar bears in western Hudson Bay over the last 19 years has been a trend toward earlier breakup, which has caused the bears to come ashore in progressively poorer condition. The ultimate factor responsible for the earlier breakup in western Hudson Bay appears to be a long-term warming trend in April-June atmospheric temperatures.” Ian Stirling, Nicholas J. Lunn and John Iacozza, Arctic, Vol. 52, No. 3 (Sep., 1999), pp. 294-306.
Possible Impacts of Climatic Warming on Polar Bears – Stirling & Derocher (1993) “If climatic warming occurs, the first impacts on polar bears (Ursus maritimus) will be felt at the southern limits of their distribution, such as in James and Hudson bays, where the whole population is already forced to fast for approximately four months when the sea ice melts during the summer. Prolonging the ice-free period will increase nutritional stress on this population until they are no longer able to store enough fat to survive the ice-free period. Early signs of impact will include declining body condition, lowered reproductive rates, reduced survival of cubs, and an increase in polar bear-human interactions. Although most of these changes are currently detectable in the polar bears of western Hudson Bay, it cannot yet be determined if climatic change is involved. In the High Arctic, a decrease in ice cover may stimulate an initial increase in biological productivity. Eventually however, it is likely that seal populations will decline wherever the quality and availability of breeding habitat are reduced. Rain during the late winter may cause polar bear maternity dens to collapse, causing the death of occupants. Human-bear problems will increase as the open water period becomes longer and bears fasting and relying on their fat reserves become food stressed. If populations of polar bears decline, harvest quotas for native people will be reduced and eventually eliminated. Tourism based on viewing polar bears would become extirpated from at least the southern part of their range. If climatic warming occurs, the polar bear is an ideal species through which to monitor the cumulative effects in arctic marine ecosystems because of its position at the top of the arctic marine food chain.” Ian Stirling and Andrew E. Derocher, Arctic, Vol. 46, No. 3 (Sep., 1993), pp. 240-245. [Full text]