Food for Thought
What does the future hold for Yellowstone's grizzlies?
To paraphrase an old joke: What does a 500-pound bear eat? Answer: Anything it wants.
This line wouldn't get many laughs around Yellowstone National Park these days. While the park's grizzly bears currently have plenty to eat, food problems may loom on the horizon. The main ursine edibles all face possible declines, largely because of alien invaders. Ensuing food shortages could result in fewer and smaller ursine litters and more bears killed by people.
According to at least one researcher, the future of the park's grizzlies is uncertain. "There's a significant risk that Yellowstone bear numbers will decline," says David Mattson, a biologist with the U.S. Geological Survey (USGS). Mattson's analysis of grizzly numbers from 1975 to 1995 shows that the population may decline during lean years for whitebark pine seeds, one of the bear's key food sources.
Others say it's too soon to begin sounding the alarm. "The prognosis for the Yellowstone grizzly bear is not any more uncertain than that for any other grizzly population, or condor population, or wolf population," says Chris Servheen, grizzly bear recovery coordinator for the U.S. Fish and Wildlife Service (FWS). "The population is continuing to increase and the bears are occupying areas they have been absent from for decades."
As omnivores, bears eat almost anything--from grass to insects to elk to garbage. For much of this century, many of Yellowstone's bears foraged on tourist trash, often while visitors watched from bleachers. In the 1970s, however, authorities closed the rubbish buffet, and bruins returned to their natural cuisine.
By official accounts, the change has served the bears well. Between 400 to 600 grizzlies now roam the Yellowstone region, according to FWS--about twice as many as in 1975 when they were declared a threatened species. "Even researchers used to have a hard time finding grizzlies in Yellowstone, but today tourists are disappointed if they don't see a bear along the road in Hayden Valley at high noon," says Steve French, a researcher with the Yellowstone Grizzly Foundation.
The natural diet of Yellowstone's grizzlies consists largely of pine seeds and three other foods: spawning cutthroat trout, army cutworm moths and carrion (primarily bison and elk). Since each of these delicacies is generally available only a few months per year and only in certain parts of the park, individual bears may rely more on one food than another, but together this foursome forms the backbone of ursine fare. But an odd set of coincidences has seasoned the bruins' diet with doubt and raised questions about the bears' future.
"Loss of one of these foods could have an adverse impact on Yellowstone's bears, and if two foods are lost, the bears could be in a much more precarious situation," says biologist and National Wildlife Federation bear expert Sterling Miller.
Some biologists fret most about whitebark pines, hardy alpine trees with cones that contain nutritious corn-kernel-sized seeds that bears relish. In bumper cone crop years, Yellowstone bruins eat little else during their autumn preparation for hibernation.
Most bears get pine seeds by raiding squirrel caches containing hundreds of cones. How bruins extract and eat the tiny seeds remained a mystery until Kate Kendall, now a USGS ecologist, gave cones to two grizzlies at a zoo in Boise, Idaho. The bears stomped and/or bit the cones to pieces. Then, with their agile tongues, they lapped up the pieces. "It looked like a human couch potato eating sunflower seeds--only the bears were neater," Kendall says. In a good cone year, a wild bear might consume hundreds of thousands of these seeds.
The main problem for whitebark pine is blister rust, a fungus that arrived in the United States around 1900 on seedlings from Europe. With no natural defense against the invader, America's whitebark pine has been decimated. (The rust affects other pines, too, but whitebark pine is its biggest victim.) In the tree's northwest range, between 40 and 100 percent of the whitebark pines in many areas have been killed, and most of the rest are infected. "Blister rust is devastating," says Kendall. "Whitebark pine is now functionally extinct in many areas."
So far, the Yellowstone ecosystem, with about a 10-percent infection rate, has largely escaped this scourge--thanks mostly to a drier, rust-unfriendly climate. But that may change. For her master's degree from Yale, Laurie Koteen examined Yellowstone's historic climatic data, then used computer models to help predict global warming's possible future effect. Her 1999 study, funded by the National Wildlife Federation, found that wetter, rust-friendly conditions already occur often enough in the Yellowstone area to facilitate rust's spread. And her computer modeling of expected future climate changes also augurs ill for whitebark pine. "There's every reason to expect that blister rust is really going to take hold in Yellowstone," she says.
A U.S. Forest Service computer model comes up with very different results, however. This model predicts that whitebark pine will increase with global warming as more frequent fires eliminate competing tree species, says Servheen. The FWS biologist adds that, even if whitebark pine did disappear from the park, grizzlies would just change their habits. "This animal has one of the most adaptive and diverse diets of almost any species," Servheen says. "Remember that whitebark pine is extinct now in many areas where it once was an important grizzly food, such as Glacier park, and grizzlies still live in these areas in substantial numbers."
Meanwhile, a second alien invader threatens another primary bear food: cutthroat trout. Every spring and summer, thousands of cutthroats end up in bear bellies when the fish leave Yellowstone Lake for spawning runs up many small streams. For weeks, some bears may eat nothing but fish.
Typically, a bear standing in shallow water pins the fish down with a paw, then locks on with its jaws. The trout may be eaten in mid-stream or taken to dry ground for more leisurely dining. In the 1980s, French and his researcher wife Marilyn kept count all day as a female grizzly caught a cutthroat every few minutes while fishing. "She ate at least 100 fish, and that didn't count what she got at night," says French. "Her 24-hour total could have hit 200."
Cutthroat prospects sank, however, on July 30, 1994, when a human angler at Yellowstone Lake hauled in a 19-inch lake trout. These fish are not native to the lake, and this sighting shocked biologists. Apparently, someone had illegally released lake trout there, possibly decades earlier. Larger and more aggressive than their cousins, lake trout could gobble up 90 percent of the cutthroats in Yellowstone Lake. (Lake trout predation on native fish has caused problems outside of Yellowstone, too.) This would be bad news for bears, since the invaders spawn in deep lake water, where ursine fishers can never reach them. Indeed, some stream counts suggest cutthroat numbers may already be falling. "Lake trout appear to be affecting the cutthroats, and without human intervention, cutthroats will cease being a viable food for Yellowstone's bears," says park biologist Dan Reinhart.
Authorities have taken to removing as many lake trout as possible with gill nets. Tens of thousands have already been nabbed, but hundreds of thousands probably remain. "We'll never get them all," says Reinhart, "but we may be able to keep them from decimating the cutthroats."
Yet another ursine cuisine uncertainty involves meat. Topping the meaty menu are bison and elk, often in the form of winter-killed carcasses. A single bison carcass can weigh as much as a ton and can provide food for a dozen bears for as long as two weeks. First dibs generally go to the biggest bears, but French once saw a male grizzly at one end of a carcass, a female at the other end, and a cub straddling the middle. "They apparently hadn't read the rule book," he says.
Yellowstone bison numbers could plummet at any time, however. For a couple decades now, bison have been leaving the park and entering Montana in search of winter food. The more severe the winter, the greater the exodus. Some bison carry the calf-killing disease brucellosis (caused by a bacterium species not native to the park), and Montana livestock interests fear that cattle may become infected. So Montanans have taken to killing many errant bison.
Through the spring of 2000 almost 3,200 bison had been killed--1,100 in the winter of 1996-97 alone. A park herd that once numbered about 4,000 now stands at 2,500, and the next severe winter could send many more bison to their deaths. Naturally--or more correctly, unnaturally--this leaves less carrion for bears to eat.
Finally, there are the army cutworm moths, which travel in teeming springtime flights hundreds of miles from prairies to Rocky Mountain peaks. Biologists aren't sure exactly where the moths come from, but it's probably Wyoming, Montana, South Dakota and Nebraska. The moths journey to talus-covered slopes to mate and consume alpine flower nectar, then in late summer they return to the plains to lay their eggs. Researchers discovered in the 1980s that bears consider these bugs tasty morsels, and 20 or more bruins sometimes gather on a single slope to turn over moth-concealing rocks and lap up insects.
Providing as many as nine calories per gram, moths are a concentrated source of energy for some Yellowstone bears, and on a good day a bruin may eat 20,000 of them. "A bear can get nearly half its annual energy just by eating moths," says Hillary Robison, a University of Nevada graduate student studying moths in Yellowstone. In moth-abundant years, some bears may remain on the rocky slopes for two or three months, feeding almost exclusively on the insects.
But as an ursine staple, moths can be unreliable. "Some years, moths are mostly absent from Yellowstone," says Robison, "but we don't know why." From 1993 through 1995, for example, few moths appeared in the Yellowstone area. Observers suspect the cause was weather, but no one can be certain.
Another uncertainty stems from the dual lives these insects lead--important bear foods in Yellowstone and serious agricultural pests on the plains. In their springtime larval (caterpillar) stage, these ravenous insects can devastate alfalfa, corn, oats, barley and other crops. The "army" part of their name stems from their ability to march through a field wreaking destruction. While bear proponents hope for heavy moth flights, prairie farmers work to wipe them out with insecticide. "And there is some concern that moths might convey pesticides directly to the bears," says Robison.
All these potential food problems worry some biologists. While bears are not likely to starve, poor nutrition could reduce the number of litters born and the number of cubs in each litter, potentially big problems for slow-reproducing bruins. Also, the habitat would likely support fewer bears, since each animal would need more space in which to make a living.
The biggest blow, however, would likely come from people. A bruin foraging alpine terrain for moths and pine seeds is not likely to raid garbage cans or chase livestock. But when hunger forces bears to lower elevations, conflicts with people become almost inevitable. "The number of dead bears nearly doubles during years with poor whitebark pine seed crops," says Mattson.
A food shortage also might affect the grizzly's status under the Endangered Species Act. With the official estimate of grizzly numbers at Yellowstone rising, the FWS is working on a plan for managing the park's bears if they recover and are removed from the threatened species list. Those who worry that grizzlies might be "delisted" have long cited threats to habitat around the park: energy exploration, mining, ranching and housing development. Now, food stability must also be considered when discussing the future status of the species.
"Although it is important to be vigilant about problems that might arise in the future," says Miller, "it is equally important to acknowledge that management efforts have resulted in significant progress in the status of grizzly bears in the Yellowstone area over the last two decades."
Gary Turbak wrote about American dippers in the June/July National Wildlife Magazine issue.
NWF Priority: Saving Grizzlies
The Federation is working with other groups to reintroduce the species to former habitat in the Selway-Bitterroot ecosystem in central Idaho. This joint effort would give local citizens a voice in managing transplanted grizzlies and restore bears to the largest roadless wilderness habitat in the lower 48 states.
In addition, NWF is helping authorities develop new management plans for bear populations that have grown beyond Yellowstone park's boundaries.