Modelling Future Impacts of Climate Change and Harvest on the
Reproductive Success of Female Polar Bears (Ursus maritimus)
Climate change and human harvest are among the most significant threats to polar bear
(Ursus maritimus) populations today. Climatic warming and resultant sea ice reductions
affect polar bears because they depend on this substrate for most aspects of their life history,
including access to seals, their main prey. Harvest is highly sex-selective, and males have
been reduced significantly in most Canadian populations, leading to concerns that males
might eventually be depleted to a point where many females become unable to mate
(a so-called Allee effect). Few studies have attempted quantitative predictions of polar bear
population dynamics under climate change, and all predictions are associated with large
uncertainty. The conditions that would lead to an Allee effect are similarly unclear, but sexselective harvest is ongoing. In this thesis I coupled mathematical models with empirical data
to understand and anticipate effects of climate change and human harvest on the reproductive
success of female polar bears. To predict conditions leading to an Allee effect, I developed a
mechanistic model for the polar bear mating system. The model described observed mating
dynamics well, predicts the proportion of mated females from population density and
operational sex ratio, and specifically outlines conditions for an Allee effect. Female mating
success was shown to be a nonlinear function of the operational sex ratio, implying sudden
reproductive collapse if males are severely depleted. The threshold operational sex ratio for
such an Allee effect depends on population density. To predict effects of climatic warming
on female reproduction, I first developed a body composition model that estimates the
amount of energy stored in the fat and protein reserves of a polar bear. Based on this model, I
developed a dynamic energy budget model that predicts changes in energy stores of both
fasting and feeding adults. Metabolic rates of adult polar bears were estimated using the
energy budget model, and corresponded closely to theoretically expected and experimentally
measured values. The models were then used to predict changes in litter size of pregnant
females in western Hudson Bay as a result of predicted losses in sea ice and feeding
opportunities, and consequent reductions in female storage energy. Severe declines in litter
size can be expected under climatic warming, although the precise rates of change depend on
current, to date unobserved, summer feeding rates. Behavioural adaptation towards terrestrial
feeding is unlikely to significantly compensate for expected losses in storage energy and
resultant reductions in litter size. The results of this thesis are a significant step towards a
predictive framework for polar bear populations, and aid optimal population management
and proactive direction of conservation efforts.