Polar bears have evolved to feed on seals, specifically seal fat, the highest calorie food source possible. The bears prey on both ringed and bearded seals. Ringed seals, which are smaller, are the most accessible, especially to younger bears and females. Male polar bears also hunt bearded seals, which are larger. When hunting is good, polar bears eat only the blubber in order to build up the fat reserves they need to sustain themselves between meals. They leave the carcass for scavengers, such as arctic foxes, ravens, and younger bears.
All the other foods that polar bears may eat are opportunistic feedings. Most of these foods, with the exception of beached whales, don't provide enough calories to sustain the polar bear's massive body size or to build up the bear's own fat reserves.
What is the polar bear's place in the food chain?
Right at the top of the arctic food chain. Polar bears balance nature by preventing an overpopulation of seals.