A conservation group wants the polar bear listed as a threatened species under the federal Endangered Species Act. The Center for Biological Diversity says the bears are threatened by the effects of global warming and could become extinct by the end of the century. The group says the Arctic sea ice habitat polar bears use for feeding, mating and maternity denning is breaking up earlier each spring and forming later in the autumn. The petition sets off a 90-day review by the US Fish and Wildlife Service and an evaluation of whether further study is warranted. The petition says other threats include Arctic oil and gas development and overhunting of some populations in Canada, Greenland and Russia.