Energetic constraints drive the decline of a sentinel polar bear population.

Human-driven Arctic warming and resulting sea ice loss have been associated with declines in several polar bear populations. However, quantifying how individual responses to environmental change integrate and scale to influence population dynamics in polar bears has yet to be achieved. We developed an individual-based bioenergetic model and hindcast population dynamics across 42 years of observed sea ice conditions in Western Hudson Bay, a region undergoing rapid environmental change. The model successfully captured trends in individual morphometrics, reproduction, and population abundance observed over four decades of empirical monitoring data. Our study provides evidence for the interplay between individual energetics and environmental constraints in shaping population dynamics and for the fundamental role of a single limiting mechanism-energy-underpinning the decline of an apex Arctic predator.