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Peter Molnar

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Péter Molnár, Ph.D. Candidate

Modelling life history strategies and the population dynamics of polar bears in a changing environment

Polar bears ( Ursus maritimus ) live throughout the ice-covered waters of the circumpolar Arctic, and depend on the sea ice for their survival. They feed almost exclusively on ringed seals ( Phoca hispida ) and bearded seals ( Erignathus barbatus ) 1, 2 , and rely on the sea ice to access these prey species. Furthermore, sea ice is used as a platform for traveling, mating, and to raise their cubs. A warming climate that alters sea ice distribution or even reduces its extent is expected to have profound effects on polar bears 3, 4 .

Adult female polar bear caught in the Beaufort Sea, spring 2005.

Current research has provided strong evidence that major changes in sea distribution and abundance have been occurring in the past decades. The total amount of sea ice cover in the Arctic has declined by 14% since 1978 5 , the perennial sea ice cover is declining at a rate of about 9% per decade 6 , and the multiyear ice cover has become significantly thinner in some regions of the polar basin during the 1990s 7 . Furthermore, some studies have suggested that Hudson Bay, which is the southernmost area that is inhabited by polar bears, might be completely ice-free by the middle of this century 8 .

Adult male polar bear.

It is expected that climatic warming would affect polar bears first at the southern limits of their distribution, and in fact, some possible effects of climatic warming on polar bears have already been detected. Hudson Bay becomes ice-free every year for approximately four months between July and November. This forces polar bears ashore, where they must survive on the fat reserves they have acquired during the rest of the year while preying on seals on the sea ice. Due to increased spring temperatures sea ice break-up in western Hudson Bay is now occurring 2˝-3 weeks earlier than in the early 1980s 9 . This effectively has prolonged the fasting period for polar bears and has resulted in declining body conditions and reduced reproductive parameters 10 . Further changes in the life histories of polar bears in western Hudson Bay have been acknowledged, but could not decisively be linked to climate change yet. This includes a later age of first reproduction and a later weaning age for cubs 11, 10 . Other potentially detrimental effects of climate change on polar bears have been suggested, such as a reduced accessibility of terrestrial denning sites, increased energetic demands for movement in a more fragmented and more dynamic sea ice habitat and a resulting greater difficulty to find mates and prey 3, 4 .

Due to the complexities of the system, it has been impossible to make precise predictions about the timeframe and manner in which climatic warming will affect polar bears. However, polar bears have been studied intensively, and long-term ecological research done over the past twenty years has resulted in one of the most complete, long-term data sets for any species within the Arctic marine ecosystem. This puts me into a unique position, as it permits the construction, parameterization and validation of realistic mathematical models of the population dynamics and ecology of polar bears and their prey. This modelling approach will help to effectively deal with the complexities of the system. Explicitly describing and incorporating the underlying mechanisms that govern the population dynamics will ultimately not only allow us to better understand historical patterns, and to identify the underlying driving mechanisms, but we can also use the models to make projections into the future and to analyze for possible impacts from climate change. Such a mechanistic approach has successfully been used before to address various ecological questions for example in insects, fish, sea turtles, birds, deer, wolves, elephants, whales and sea lions; however, very little work has been done on bears.

Polar bear habitat north of Tuktoyaktuk, NWT, Canada.

My project involves developing life history models that link the changes in the life history of bears (e.g., reduced litter sizes, later weaning age of cubs, later age of first reproduction) to climatic conditions. As the sea ice now breaks up earlier than it used to, bears have less time to feed on seals and a longer time to fast, and thus, it is expected that they become food-stressed. This could obviously result in reduced life history parameters. My aim, however, is to quantify this relationship based on individual energetics and predator-prey dynamics. This will put me, for example, in the position to address the question "What would happen if the ice broke up even another week earlier?".

Another objective of these models is the investigation of the respective roles of evolutionary and plastic responses of polar bears to different environmental circumstances. This will improve our understanding as of to what extent and how fast the bears can adapt to a changing environment.

Furthermore, I am working on models that are aimed at evaluating the effects of sex-selective harvest on the population dynamics. This is a particularly important topic because some subpopulations appear to be declining 12 . In Canada, most polar bear harvesting is managed by a quota system which is based on population size estimates, and the maximum sustainable yield from a simulation model 13, 14 . Management recommendations are aimed toward encouraging hunters to be more selective for larger adult males in preference to adult females to increase the sustainable yield 14, 15 . While previous modelling studies have suggested that such a strategy might only be optimal under special circumstances 16 , the consequences of current management policies on polar bear conservation have not been evaluated yet. In fact, the population sex ratio has become strongly skewed towards females in all of Canada's polar bear populations 15 , and some concerns were phrased that eventually this could lead to a population decline. In addition, changing sea ice dynamics might further complicate the mating of polar bears. For instance, increasingly fragmented sea ice might make it more difficult for bears to find mates. A modelling approach will improve our understanding of how sex-selective harvest and climatic warming might affect the mating dynamics of polar bears. This will ultimately help us to determine appropriate management strategies.

I am originally from Munich, Germany, where I studied Mathematics at the Ludwig-Maximilians-University Munich, specializing in probability theory. I completed my M.Sc. degree in 2003 with distinction, and started my doctoral studies at the University of Alberta in September of the same year. I am an interdisciplinary student in Mathematical and Statistical Biology, co-supervised by Dr. Mark Lewis (Mathematical & Statistical Sciences / Biological Sciences) and Dr. Andrew E. Derocher (Biological Sciences), and also a member of the Centre for Mathematical Biology at the University of Alberta.

For more information contact:

Péter Molnár
CAB 550 Centre for Mathematical Biology

Dept. of Mathematical and Statistical Sciences
University of Alberta
Edmonton, Alberta
Canada, T6G 2G1
Phone: (780) 492-4756

Andrew E. Derocher
CW405 Biological Sciences Center
University of Alberta
Edmonton, Alberta
Canada, T6G 2E9
Phone: (780) 492-5570
Fax: (780) 492-9234

Cited literature:

1. Stirling I., Archibald W.R., 1977. Aspects of predation of seals by polar bears. J. Fish. Res. Board Can. 34: 1126-1129.

2. Smith T.G., 1980. Polar bear predation of ringed and bearded seals in the land-fast sea ice habitat. Can. J. Zool. 58: 2201-2209.

3. Stirling I. and Derocher A.E., 1993. Possible impacts of climatic warming on polar bears. Arctic 46: 240-245.

4. Derocher A.E., Lunn N.J. and Stirling, I., 2004. Polar bears in a warming climate. Integrative and Comparative Biology 44: 163-176.

5. Vinnikov, K.V., Robock, A., Stouffer, R.J., Walsh, J.E., Parkinson, C.L., Cavalieri, D.J., Mitchell, J.F.B., Garrett, D. and Zakharov, V.F., 1999. Global warming and northern hemisphere ice extent. Science 286: 1934-1937.

6. Comiso, J.C., 2002. A rapidly declining perennial sea ice cover in the Arctic.Geophys. Res. Lett. 29: 1956.

7. Rothrock, D.A., Yu, Y. and Maykut, G.A., 1999. Thinning of the Arctic sea-ice cover. Geophys. Res. Lett. 26: 3469-3472.

8. Gough, W.A. and Wolfe, E., 2001. Climate change scenarios for Hudson Bay, Canada, from general circulation models. Arctic 54: 142-148.

9. Gough, W.A., Cornwell, A.R. and Tsuji, L.J.S., 2004. Trends in seasonal sea ice duration in southwestern Hudson Bay. Arctic 57: 298-304.

10. Stirling, I., Lunn, N.J., and Iacozza, J., 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climate change. Arctic 52: 294-306.

11. Stirling, I., and Lunn, N.J., 1997. Environmental fluctuations in arctic marine ecosystems as reflected by variability in reproduction of polar bears and ringed seals. In: Ecology of Arctic Environments. Edited by: S. Woodin and M. Marquiss. Blackwell Scientific Publications Ltd., Oxford, 167-181.

12. Wiig, Ř. 2005 Are polar bears threatened? Science 309: 1814-1815.

13. Taylor, M.K., DeMaster, D.P., Bunnell, F.L. and Schweinsburg, R.E., 1987. Modeling the sustainable harvest of female polar bears. J. Wildl. Manage. 51: 811-820.

14. Lee, J. and Taylor, M., 1994. Aspects of the polar bear harvest in the Northwest Territories, Canada. Int. Conf. Bear Res. and Manage. 9: 237-243.

15. Derocher, A.E., Stirling I. and Calvert, W., 1997. Male-biased harvesting of polar bears in Western Hudson Bay. J. Wildl. Manage. 61: 1075-1082.

16. Clark, C.W. and Tait, D.E. 1982. Sex-selective harvesting of wildlife populations. Ecological Modelling 14: 251-260.

Last Modified:2009-06-22