As their traditional food supply disappears because of climate change and invasive species, Yellowstone National Park's grizzly bears (Ursus arctos horribilis ) are increasingly seeking sustenance outside their protected home—a move which more than ever puts them in the crosshairs.

An estimated 75 Yellowstone grizzlies where killed or "removed from the wild," as the U.S. Fish and Wildlife Service phrases it when animals are expatriated from their habitat, following conflicts with humans in 2010. Approximately 600 grizzlies live in Yellowstone, so this year's death-slash-removal rate represents as much as 12 to 13 percent of the animals inhabiting the park. This was the second-highest number of grizzly deaths since the species was listed in 1975 as "threatened" under the Endangered Species Act. The record high was in 2008 when an estimated 79 bears were killed or removed. This year, only three reported grizzly deaths resulted from natural causes.

Although some deaths were caused by hunters who confused the grizzlies for black bears, which are not a protected species, most occurred after grizzlies wandered outside the park in search of food. In 2010 this wandering caused a record number of human–bear conflicts as well as the mauling deaths of two people.

All these problems are driven by reductions in two of the grizzly's main food sources, which have been hit hard in recent years. The first of these is the Yellowstone cutthroat trout (Oncorhynchus clarkia bouvieri ), which is being squeezed out by nonnative lake trout (Salvelinus namaycush ). The much larger lake trout not only outcompetes the cutthroats for food, they eat the cutthroats before they can reach the shallow waters where grizzlies do their fishing.

Even worse off is the whitebark pine tree (Pinus albicaulis ), which produces high-fat nuts the grizzlies depend on as a major staple. Warming temperatures from climate change have brought the mountain pine beetle (Dendroctonus ponderosae ) into Yellowstone for the first time, leaving huge areas of dead trees in their wake . Cold temperatures once kept the beetles away, but now as much as half of Yellowstone's whitebark pine trees have been killed by the insects. According to Defenders of Wildlife , "As the whitebark pine declines grizzly bears are spending less time feeding at high elevations and more time searching for substitute food sources, often in closer proximity to people." And according to Louisa Willcox, a senior wildlife advocate at the Natural Resources Defense Council, "The best available science (mostly done by government scientists) shows unequivocally that whitebark pine directly or indirectly drives the reproductive success and survival of grizzly bears ."

In addition to the beetles, white pine blister rust (Cronartium ribicola ) from Europe is killing off many whitebark pine trees. The fungus first came to this country around 1900, and very few pine trees have resistance to it.

Grizzlies are omnivores, and some government scientists say the bears should be able to adapt to catch and eat more elk, but it costs a lot more calories to take down an elk than to eat nuts off a tree, which provide calorie-rich sustenance the bears sorely need before their hibernation period.

Grizzlies are protected in the lower 48 states and in Canada, where their combined population is estimated at 1,000 to 1,400. Another 30,000 grizzlies live in Alaska where they are not protected. Yellowstone grizzlies briefly dropped off the endangered species list in 2007, but a federal court ordered them back on in 2009. The U.S. government is currently appealing that decision.