Polar bears rely heavily on sea ice for traveling, hunting, resting, mating and, in some areas, maternal dens. But because of ongoing and potential loss of their sea ice habitat resulting from climate change–the primary threat to polar bears. Arctic-wide–polar bears were listed as a threatened species in the US under the Endangered Species Act in May 2008. The largest bear in the world and the Arctic's top predator, polar bears are a powerful symbol of the strength and endurance of the Arctic. Polar bears are listed as vulnerable to extinction, according to the International Union for Conservation of Nature (IUCN) Red List, and there are only around 36,000 individuals left in the wild. But some studies have suggested that the species could vanish by the end of the century due to the effects of climate change.