Forest studies human pressure on high-elevation grizzly bear feeding sites where moths congregate | Cable | Panda Anku

BILLS, Mon. — Even on steep mountainsides over 11,000 feet, hikers in the Greater Yellowstone Ecosystem are encountering grizzly bears — sometimes on purpose.

In the middle of summer, some of the large brown bears scour rocky slopes in search of a high-calorie food source, armyworm moths.

The moths hide in the cool shade of the rocks in large clusters that can attract up to 50 bears feeding relatively close together.

People either attempt to scale a peak unaware of the grizzly bears’ presence, or purposely seek out the assembled bears to photograph, film, and observe.

In response to this potentially dangerous crossbreed of humans and animals, Shoshone National Forest staff determined in their 2015 planning process that the agency needed to learn more about bear feeding sites.

It has also turned down requests for permits from commercial film crews until forest workers better understand the situation.

However, learning more is not easy.

“The logistics were complex,” says forest biologist Andy Pils. “Even getting to the construction sites was a real challenge,” made more difficult by the dangerous terrain and unpredictable weather.

To learn

Thirty-one moth feeding sites have been discovered throughout the GYE, including in Yellowstone and Grand Teton National Parks.

Of these, two have been identified on the east side of Shoshone Forest for a more intensive study to quantify bear usage, human activities where the two creatures intersect in the landscape, bear responses to human presence, and other baseline data.

Erika Nunlist, a graduate student at Montana State University, led a group of four technicians to survey the two sites in 2017 and 2018.

Her master’s thesis 2020 summarized the work and pointed out possible management options for the forest.

It was a lung-wrecking task.

During the field season, crew members hiked nearly 1,000 miles and gained more than 851,000 feet in altitude.

A monitoring site required a three-day backpacking trip to reach it.

Dan Tyers, grizzly bear habitat coordinator for the Forest Service at GYE, oversaw Nunlist’s work.

“We are concerned about the environmental integrity of these sites,” he said, especially as more people are using the hinterland and the moth sites are such an unusual phenomenon.


Moth locations were relatively unknown in the GYE until the 1980s. Few studies have been conducted into their importance to bears.

However, the moths are known to migrate hundreds of miles away from the lowlands to congregate at higher, cooler elevations in New Mexico, Colorado, Montana and Nevada, and Wyoming, Nunlist reported.

Tyers calls the migration surreal, especially considering there is no generational overlap, so every moth must know instinctively that they must fly to higher altitudes with cool, humid environments to survive.

“It’s just a weird construct that moths migrate by the billions,” he said.

The moths are so important as a food source because they feed on mountain flowers before returning to the lowlands in mid-August.

It has been estimated that a bear can consume up to 40,000 moths per day.

Consequently, the fat bugs are considered one of the most energy-dense food sources for grizzly bears and other species. Other moth eating animals include black bears, ravens, coyotes, a variety of birds, bats, mice and spiders.

“A study in the early 1990s estimated that up to 44% of the grizzly (Greater Yellowstone Ecosystem) population used these moth sites, including about half of known sow and young groups,” Nunlist wrote.

Moths aren’t the only attraction, Pils noted.

The areas may also contain vegetation such as biscuit root, which the bears feed on.

Buffalo berries grow further down the trails.

Grizzlies have been known to eat more than 260 foods.

Kerry Gunther, Yellowstone National Park’s chief bear biologist, said that seeing a dozen bears on a rocky bluff feeding on moths together was the most unique grizzly interaction he’s had in his 39 years working at the park.

“When they saw that many bears were so close together without conflict, they had worked out a hierarchy,” he said.

Early presentations by Steve and Marilyn French to officials documenting and studying the sites were “very intriguing and intriguing because of the novelty at the time,” Tyers said.

Similar areas where bears have concentrated around a food source include the now-prohibited dumps of Yellowstone National Park, cumin-covered fields in Tom Miner Basin, and salmon spawning streams in Alaska.

When McDonald Peak in the Mission Mountains was identified as a bear feeding ground for moths, the Confederated Salish and Kootenai Tribal Council closed a popular hiking trail to the mountaintop from mid-July to October to avoid disturbing the bears.

A similar use has also been documented in Glacier National Park.


Since 2007, Shoshone National Forest employees have seen increased interest in special purpose filming permits and outfitters looking to capitalize on the unique situation.

A summit log kept on one of the high mountain peaks showed entries increasing from less than one per year before 2003 to about 60 in 2016.

Nunlist and her group documented 86 hikers in the first year and 96 in the second. Most focused on one site mentioned on online peakbagging sites.

Groups were typically small, around three people or fewer, and ranged from hunters to hikers to photographers.

The most popular route begins on a deer trail that Nunlist and her colleagues dubbed the Bear Highway because it was covered in grizzly droppings.

Although the crew was careful to avoid nuisance grizzlies, they documented close encounters with bears after the summit climb.

The closest encounter reported by other hikers was 9 metres.

However, few of them knew of the bears’ presence beforehand or carried bear spray as a deterrent.

Even when bear encounters were not close, some grizzlies ran long distances to avoid humans. Such activities use up calories and interfere with food intake.

Bears that care little about humans are also of concern, since human-adapted grizzly bears are more likely to get into trouble when migrating to agricultural or more populated areas.


So what options does the forest have to protect bears and humans? Nunlist went through a number of options including: education, trail closures, diversion of trails or traffic, a permitting system, and seasonal closures.

Although establishing bear viewing areas is an option, Nunlist said this action could set “a dangerous precedent and a designation that will be difficult to remove.”

With the help of Nunlist’s work, Shoshone Forest is developing a moth area management plan, which is still in its infancy.

Pils said the agency recognizes that monitoring will be an important part of the plan, but the agency doesn’t have the resources to conduct intensive, time-consuming fieldwork like Nunlist and her colleagues were doing.

Among the measures the forest has in place is the ability to promote bear safety among backcountry users while trying to discourage human use during key feeding times, Pils said.

The plan may take another two years to complete.

“We know these sites are truly unique ecological phenomena and they are really important to grizzly bears, at least in the East (Greater Yellowstone Ecosystem) and we think they are very worthy of our attention,” he added. “I hope this plan will help us better manage permitted activities, send messages to the public about responsible bear stewardship, and develop some effective monitoring methods so we can track things in the future.”


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