One of the experts here is Dr. David Mattson of the US Geological Survey. He’s spent 20 years studying how grizzlies use the landscape with an eye to the long-term future of the bears. His research tells him that everything depends on the four foods the bears need most.

Dave Mattson: If you’re going to look at the bears’ future here in Yellowstone, really what it comes down to is looking at what might happen to the bears’ foods. And with all four of the foods, there’s either grave threats or at least considerable uncertainly as to what’s going to happen to them.

Yellowstone’s grizzlies rely on meat for 60 percent of their diet. But a devastating disease called Brucellosis may force wildlife managers to reduce bison herds by half, cutting back on vital food for the bears.

Cutthroat trout are losing ground to an aggressive predator. It’s an introduced trout from the Great Lakes that’s decimating the Cutthroat. The Park gillnets Yellowstone Lake all summer to keep the invading lake trout from taking over.

Moths face threats at both ends of their migration. Considered agricultural pests on the plains, they’re under attack from pesticides. More alarming is the warming trend at high elevations. The summer snowfields are disappearing. And climate experts predict that the alpine flower meadows where the moths come to feed will be overtaken by trees if temperatures continue to rise.

It’s the moths that bring the bears to the high country. If moths should fail, bears will be forced to find food elsewhere, sending them down into the path of people and potential conflicts.

But Mattson is most concerned about the White Bark pines. An alien pathogen called Blister Rust is killing the trees, and their seeds may be the bears’ most important food of all.

Dave Mattson: There’s very clear evidence when a female eats more White Bark pine seeds, she gives birth to more cubs the year after, and not only that, she’s more likely just to produce a litter.

Pine seeds may determine how many cubs are born each year. But they also determine how many of their mothers survive.

Dave Mattson: There’s a very strong relationship between seed availability and conflict. When we have few seeds available, we have high levels of conflict between humans and bears. When we have a big crop we have very low levels of conflict. It relates to where white bark pine grows. It only grows at elevations of about 8,400 ft. and that’s a long ways typically from where humans are, where our houses are, where our roads are, where we’re active. And so, when pine seed crops are large, most of the bears are up there and when they’re up there they’re quite safe from humans. On the other hand, when pine seed crops are small, there down low they’re operating a lot more near people and when they’re near people they die at about twice the rate they do when they’re far away.

In Yellowstone, the fate of the bears seems to be tied to natural foods that keep bears away from people – and all of them are in trouble.

Dave Mattson: If we stand to lose a significant part of the four foods that are currently really important, it sort of begs the question, what should we do to compensate? We can compensate for loss in our ability to support bears on any given square mile by allowing them to live on more square miles, over a larger area.

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