A close look at the most important foods for grizzly bears in the Greater Yellowstone Ecosystem shows that these key foods are at various levels of risk - complicating the long-term prospects for the species, according to biologists.

"There are four basic foods," said Dave Mattson, a biologist for the Biological Resource Division of the U.S. Geological Survey, who has investigated grizzly behavior and habitat for the past 19 years. The four foods are:

- Meat from ungulate herbivores, such as elk, moose and most importantly, bison.

- Whiteback pine seeds.

- Cutthroat trout.

- Army cutworm moths.

The conference was hosted by the Natural Resources Defense Council, a national conservation group.

Meat

Mattson said Yellowstone grizzlies get 70 percent of their meat by scavenging and 30 percent by predation. Elk meat is 53 percent of the meat diet for grizzlies, followed by bison at 24 percent and moose at 18 percent.

Elk calves are an important food source, Mattson said, from May to July, with the average adult female averaging one calf kill per season, while the average adult male kills three. Grizzlies "go after the bull elk and moose during rut," when the bulls are distracted and not very cautious, he said.

"What was surprising is that the volume of bison meat is about three times more than what we expected given bison numbers," Mattson said. Winter killed bison provide a lot of meat to grizzlies when they emerge from hibernation in the spring, Mattson said.

Fears that bison may spread the disease brucellosis to cattle herds in the area led to a management plan that sometimes calls for the capture and killing of infected bison. If federal and state managers opt for large-scale culling of bison herds, as a way of reducing the threat of brucellosis, Mattson fears there could be a significant impact on bear populations.

All true, said Chuck Schwartz, head of the Inter-agency Grizzly Bear Study Team, but he's more optimistic than Mattson, about the fate of bison and other ungulates. "Those species are important for more reasons than they're food for bears," said Schwartz, in a telephone interview from his Bozeman, Mont., office.

Trout

Cutthroat trout are an important food source for 10-30 percent of the grizzly bears, said Mattson. "Bears prefer the small streams," Mattson said, where the native trout can be scooped out or trapped against a bank.

In the wake of the great die-off of bears when the park dumps were closed in 1970, grizzly bears have dramatically increased their cutthroat fishing since then, he said.

Yellowstone cutthroat are threatened by the lake trout, introduced sometime within the last 20 years. Left unchecked, biologists believe the lake trout could knock the cutthroat population down by 70 percent over the next few decades, depriving bears of a valued food source. Lake trout are not a substitute food, however, because they live and spawn in the depths of Yellowstone Lake, where few predators can reach them.

Schwartz acknowledged that wildlife managers are "on a treadmill" when it comes to controlling lake trout populations with an extensive and expensive gill netting operation on Yellowstone Lake.

Moths

Every summer, many millions of army cutworm moths take flight from the Great Plains and fly up into the rugged mountains of the Greater Yellowstone Ecosystem, said Mattson. "They turn into little fat bombs," Mattson said, feeding on the nectar of mountain flowers at night, then taking refuge under rocks of nearby talus slopes. The moths are 50-75 percent fat, said Mattson, and a hungry bear can eat as many as 40,000 moths in a single day, for 20,000 calories.

Considered an agricultural pest, the cutworm moth is threatened by agricultural pesticide controls, as well as major habitat changes that may arise from global warming, he said.

The cutworm is only targeted with pesticides when it hits fairly high densities of population, Schwartz countered, and then with pesticides that break down fairly quickly in the environment.

Whitebark pine

The seeds of the whitebark pine provide a highly concentrated fat source for Yellowstone's grizzlies. Red squirrels harvest the cones and hide them in large middens or caches. "The bears listen for the chatter of the squirrels and home in on that," Mattson said.

The pine nuts are hugely important to female grizzly bears, Mattson said, as they fatten up for hibernation in late fall. "When we have good whitebark pine nut crops, we have more cubs the following spring," Mattson said. A good crop can produce a 7 percent population increase, he said, while a poor crop can produce a 5 percent population decrease, through female mortality and fewer cubs, he said.

In addition, good whitebark pine crops tend to keep bears up in the high country and away from humans, Mattson said. Conversely, a poor crop causes bears to scatter down and out from the mountains, looking for food and often getting into trouble with humans. Conflicts go down when crops go up, while conflicts rise when crops are bad, he said.

Whitebark pine is threatened by the blister rust fungus, which has exterminated whitebark forests elsewhere in the country. Although Yellowstone hasn't been hit hard yet, Mattson said, blister rust is certain to damage or destroy this food source in the next few decades.

Schwartz agrees that the whitebark pine is in trouble, but notes extensive research by the Forest Service to identify and grow rust-resistant whitebark pines for future replantings. "Blister rust has been in the Yellowstone ecosystem since the 1940s and we don't have a good handle on the rate of loss," Schwartz said. Many trees that are infected with the rust continue to function very well and produce large amounts of seeds, Schwartz said.

Bear's future?

"If you look at all the key foods, there's a lot of uncertainty," said Mattson. The bison herd could be severely reduced in an effort to eradicate brucellosis, he said. The lake trout is hammering the cutthroat trout.

"You need a management effort in perpetuity," he said, just to control the lake trout. The army cutworm moth is an agricultural pest and may be decimated by new pesticides. The whitebark pine has a 99 percent mortality rate when hit by the blister rust fungus and will ultimately be lost in Yellowstone, Mattson said.

Global warming could result in a 90 percent loss of both whitebark pines and cutworm moths, he said, though it could also result in the growth of mushroom, ant, Gambel's oak and hornet populations in the Greater Yellowstone. None of the potential species could replace any of the threatened four food groups for the grizzly bear diet, he said.

"We're going to lose high energy foods, gain low energy foods and that will result in a smaller carrying capacity for the ecosystem," Mattson said.

Schwartz acknowledges the importance of the grizzly bear diet. "I'm not as pessimistic," he emphasizes. There are real threats and there are speculative threats, he said, and much work needs to be done to get a better understanding about grizzly bear foods and habitat.