Bear-Human Encounters Are Expected To Rise Thanks To Climate Change

In Yellowstone National Park, climate change is helping fuel a decline in a key food source for bears, a scarcity which could lead to potentially dangerous situations between bears and tourists.

Whitebark pine trees, whose nuts are an important late-summer and fall food source for grizzly and black bears, have been killed by the thousands in recent years by mountain pine beetles, whose populations have exploded and range expanded with the onset of warmer weather and milder winters.

The beetles burrow under the trees’ bark, where they lay their eggs and infect the trees with blue stain fungus, an affliction which prevents the trees from using their own defenses to kill the beetles. The fungus blocks the trees’ flow of nutrients and water, and that, coupled with the feeding of the beetle larvae once they hatch, kills the trees within a few weeks. The first major outbreaks of the beetle were documented in Yellowstone in 2004, and since then, the beetles have ravaged whitebark pines — trees which can live up to 1,000 years and haven’t built up a natural defense against the beetles — in Yellowstone and across the U.S.

That’s bad news for the Yellowstone ecosystem — the whitebark pine is a keystone species in the park, its highly nutritious nuts providing food to multiple bird and animal species. But it’s also potentially dangerous for tourists, who often come to the park hoping to spot a bear from afar but not anticipating an encounter with them. Lack of whitebark pine seeds are driving bears to seek nourishment in more highly-populated tourist areas of the park, and in recent years, several bear attacks on visitors have been recorded near and around Yellowstone. In 2010, Wyoming had a record 251 bear-human “conflicts,” which include attacks on people, livestock and property.


“We are expecting an increase in human-bear encounters and we are reinforcing safety messages,” Yellowstone spokesman Al Nash said this week. Fatal bear attacks are still rare, but this fall, park officials are calling on visitors to keep an eye out for bears, carry bear spray and hike in groups.

As for the whitebark pine, there may be some hope: a U.S. Geological Survey report from 2012 found evidence that the rate of beetle-induced mortality among whitebark pines had slowed. Still, the trees’ populations are greatly reduced — from 2002 to 2012, 72.3 percent of the 190 whitebark pines monitored in transects by the USGS’s Interagency Grizzly Bear Study Team died.

And it’s not just the whitebark pine that’s disappearing and disrupting the bear’s eating routine — several other bear food sources are declining as well. Cutthroat trout are threatened by hydroelectric dams and may also be affected by higher water temperatures, earlier snowmelt and drought. In the springtime, when grizzly bears emerge from hibernation, carrion — carcasses of animals who died over the winter — are also an important food source for the weakened bears, but as winters become more mild, fewer animals are likely to die due to harsh conditions, leaving less carrion for bears.