Abstract 186 – A life-cycle based bioenergetic model can explain the decline of a polar bear population across four decades of sea-ice loss

Louise C Archer, Stephen N Atkinson, Nicholas J Lunn, Stephanie R Penk, Péter K
Molnár

The Arctic is the fastest warming region on Earth, threating the persistence of Artic species.
Polar bears are particularly vulnerable because warming leads to declines in their essential sea-
ice foraging habitat. Longer ice-free periods have been linked with declining metrics of
population health, but a data-validated framework quantifying the processes linking sea-ice
dynamics to polar bear vital rates is lacking, limiting our ability to quantify risk and proactively
manage populations. To determine the quantitative links between physiology, population
dynamics, and environmental change, we developed an individual-based bioenergetic model
that incorporates key energetic processes across a polar bear’s lifecycle. We used the model to
reconstruct the dynamics of polar bears in Western Hudson Bay across four decades under
observed sea-ice conditions. We then compared outputs from model hindcasts to long-term
population monitoring data (1979-2016) and found that the model was able to successfully
capture empirical trends in individual morphometrics, reproduction metrics, and overall
population size. The modelling framework demonstrates that an observed decline in a sentinel
population of polar bears can be mechanistically explained by the effects of sea-ice loss on
individual energy budgets, which scale up to influence population vital rates.