The U.S. Fish and Wildlife Service says the whitebark pine, a keystone species in the mountains of western North America, faces an “imminent” risk of extinction because of outbreaks of pests and disease caused by rising temperatures. The agency’s decision is significant because it marks the first time the U.S. government has cited global warming as a driving factor behind the possible disappearance of a wide-ranging tree species. But while it declared that climate change was steadily pushing the whitebark pine to oblivion, the agency said budget cutbacks and staff shortages meant that it did not have the resources to undertake the studies needed to place the tree on the endangered species list; the agency said a listing was “warranted but precluded” at this time. Whitebark pine, found at higher elevations in the Rocky Mountains and other mountain ecosystems in the West, are a critical part of those environments, providing food to species such as the grizzly bear. But increasingly warmer winters have led to outbreaks of pests, such as the mountain pine beetle, that have destroyed 70,000 square miles of pine forest. A recent study found 80 percent of whitebark pine forests in the Greater Yellowstone ecosystem are dead or dying.