Demographic and Ecological Perspectives on the Status of Polar Bears
Written by Dr. Mitchell Taylor and Dr. Martha Dowsley
March 2008 For the Full Report in PDF Form, please click here.
Dr. Taylor and Dr. Dowsley are not affiliated with SPPI]
[Illustrations, footnotes and references available in PDF version]
Polar Bear Biology and Ecology
Morphology and Genetics
Habitat and Prey
Life Cycle and Reproduction
Physiology and Contaminants
Home Ranges, Movements, and Dispersal
Current Population Status based on Population Demography
Polar Bear Status Determination Based on Climate Model Projections
Projected Loss of Arctic Sea Ice and Anticipated Impacts on Polar Bears
Demographic versus GCM Perspectives on Polar Bear Persistence
Anecdotal Information and “The Science”
Although two polar bear subpopulations (Western Hudson Bay and Southern Beaufort Sea) no longer appear to be viable due to reduction in sea ice habitat, polar bears as a species do not appear to be threatened by extinction in the foreseeable future from either a demographic or an ecological perspective.
Ecological perspectives that suggest the reductions to survival and recruitment rates for two populations (Western Hudson Bay and Southern Beaufort Sea) have occurred because of a long-term decline in sea ice due to climate warming. These populations occur where summer ice coverage is seasonal (WH) or divergent (SB). The perspective that the impacts of sea ice reductions experienced in WH and SB subpopulations can be generalized to the remainder of the polar bear subpopulations depends entirely on the IPCC GCMs that predict continued reductions to sea ice due to CO2 driven climate change.
Current and historical polar bear subpopulation performance demonstrates that viable polar bear subpopulations have persisted and generally increased throughout the current period of climate warming. The mean generation time of polar bears as defined by the IUCN/SSC Redbook criteria and the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) is 12 not 15 years. The time-frame for three generations for polar bears is 36 not 45 years as indicated by the IUCN/SSC Polar Bear Specialists Group.
Based on the assumption of a linear relationship of population numbers to sea ice habitat, extrapolation of IPCC GCM sea ice predictions over a thirty-six year interval does not support the contention that polar bears are threatened with extinction over the next three generations. Extrapolation of IPCC GCM sea ice predictions over a hundred year interval does not support the contention that polar bears are threatened with extinction in the foreseeable future.
Population viability analyses (PVA), using demographic estimates from polar bear populations where the data are sufficient, indicate that population status is affected by both anthropogenic removals and vital rates. PVAs that employ current demographic and removal rates indicate that most polar bear populations could sustain the current removal rate indefinitely. Management action for populations where removal rates exceed the estimated sustainable levels has occurred and is ongoing.
The popular notion that polar bears are declining or already expatriated worldwide has been initiated and perpetuated by environmental organizations and individuals who apparently believe that current subpopulation numbers and trends are an insufficient basis for an appropriate status determination. These individuals and organizations suggest that an ecological consideration constitutes more appropriate methodology to assess status of polar bears and presumably all species.
Observations of natural mortality, intra-specific aggression, poor condition, and even healthy bears in good condition on ice floes have been cited as evidence of a population impacts on polar bears due to declining sea ice. Anecdotal information, although useful and interesting, is not equivalent to scientific information based on valid statistical analysis of sample data. Simultaneously, traditional ecological knowledge (TEK) from Inuit has been largely ignored because TEK is mostly oral, and because TEK generally does not support the assertion that polar bear populations are in general, or even local decline.
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