| Abstract | Polar bears (Ursus maritimus) depend on sea ice for most aspects of their life history, including access to seals, their main prey. Climate warming induced losses in sea ice are therefore expected to result in a loss of feeding opportunities, with consequent reductions in polar bear body condition, survival, reproduction, and eventually population abundance. To date, no quantitative predictions for changes in body condition, reproduction, and survival under climatic warming exist despite long-term empirical research - largely because historic and predicted sea ice conditions differ substantially, making extrapolation from current observations difficult. Here, we develop a mechanistic dynamic energy budget model for adult female polar bears to quantitatively predict how their reproductive success, and specifically their litter size, would be affected by expected losses in sea ice and feeding opportunities. The model is based on a body composition model that allows estimating the amount of energy stored in the fat and protein reserves of individual bears, given their body mass and body length, and tracks changes in energy stores due to somatic maintenance, movement, and feeding. The litter size of pregnant females is predicted as a function of energy stores at maternity den entry, which in turn depend on previous feeding rates. We apply the dynamic energy budget model to the population of western Hudson Bay to predict changes in energy stores, and consequent changes in litter size, as a function of predicted changes in sea ice dynamics and feeding opportunities. We show that severe declines in litter size can be expected under climatic warming, but the precise rates of change can only be approximated due to a lack of data on current feeding rates. |
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