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Reflecting on 40 years of change in Yellowstone following wolf reintroduction

BILLINGS – More than 40 years ago, a young Mark Boyce landed his first teaching job at the University of Wyoming’s Department of Zoology and Physiology.

In that inaugural year, he made his initial visit to nearby Yellowstone National Park and soon after began teaching a field course there.

“Suddenly, I was in the thick of things,” he said.

Now 68 and a professor of Ecology at the University of Alberta in Edmonton, Boyce was honored in 2017 by the American Society of Mammalogists with the C. Hart Merriam Award.

As part of the award, Boyce presented a paper reflecting on his 40 years of work – as well as the work of some of his students and colleagues – on the reintroduction of wolves to the Greater Yellowstone Ecosystem.

That paper, while questioned by some scientists, gives insight into the history of a complicated and heated debate about the impact of wolves in Yellowstone and beyond.

Boyce’s involvement with wolves began when he was asked by the park to develop a computer population model for elk to understand what would happen if wolves were reintroduced to Yellowstone. Boyce went to work, borrowing information from scientific studies to calculate how the wolf and elk populations would expand or contract. By 1988, he produced the first model, which was revised in 1992. The work was included in information submitted to Congress to bolster the National Park Service’s argument that wolves should be returned to Yellowstone.

“This was very unpopular in Wyoming at the time,” Boyce said. “The Wyoming Farm Bureau Federation wrote to the president of the University of Wyoming demanding I be fired. Fortunately, the president believed in academic freedom.”

In 1995, 14 wolves were released in Yellowstone. A year later, another 17 wolves followed, Boyce wrote.

For the first 10 years after wolf reintroduction, Boyce said he was “pretty smug” about how well his model followed what happened in the park. But when he dug into the model a third time, he found that some of the parameters were wrong.

Wolves, he had predicted, were supposed to dine on other species like deer, moose and the occasional bison, but elk proved to be by far the most popular item on the big canines’ menu; 95 percent of the prey was elk in the first decade.

“He makes some really good points about how hard it is to predict ecological conditions,” said Merav Ben-David, a professor and head of the University of Wyoming’s Zoology and Physiology department. “I was impressed he was willing to admit his mistakes.

“We have to be careful when we model because ecological ecosystems are complicated and we can’t predict how things will change,” she said.

Boyce also discovered that wolves and human hunters influenced elk populations in different ways.

“Another shortcoming was failure to recognize the strong age selectivity by wolves and hunters,” Boyce wrote. “Hunters preferentially kill bulls, but when they kill cow elk, they primarily kill prime-age females of high reproductive value. Cows learn to avoid hunters to the extent that individuals older than about 9-10 are essentially ‘bullet proof.’ ”

Yellowstone wolf biologist Doug Smith has often referred to the park’s Northern Range herd as some of the toughest elk around.

“In contrast, wolves killed primarily young and old (elk), and as a consequence the per capita influence of hunters on elk populations was much greater than for those killed by wolves.

“Even after making these adjustments, however, during the second decade after wolf reintroduction, the number of elk averaged lower than predicted by the simulation model.

“I believe that this is because bear and cougar predation was higher than we had anticipated. In particular, grizzly bears have been shown to be highly effective predators on elk calves, reducing recruitment by about seven calves/100 cows.”

Other studies have shown that after cutthroat trout numbers in Yellowstone plummeted and with the reduction in whitebark pine seeds, grizzly bears compensated by eating more meat, often elk calves in the spring.

“One thing he left out, that I and several other researchers believe is important, is the indirect effect of wolves on elk pregnancy rates (the presence of a predator on the landscape reduces prey pregnancy rates – not direct killing),” wrote John Winnie Jr., an associate professor of Ecology at Montana State University in Bozeman, in an email.

Since the reintroduction of wolves, elk populations – which some biologists will argue were unnaturally high in the largest Northern Range herd that migrates into Montana in the winter – have fallen from about 19,000 in 1995 to about 5,000 the last three years. Likewise, wolf numbers have dropped from a high of about 170 in 2003 to roughly 100 now, mirroring a decline in the availability of their main prey – elk.

A debate between scientists continues as to whether or how much the reduction in elk has helped revive tree species like willows and aspen, whose tender shoots are often winter elk food.

Winnie said researchers Scott Creel, of MSU, and Dave Christianson, of the University of Arizona, “pointed out that a decline in elk numbers was sufficient to explain an increase in willow growth noted” by scientist Hawthorne Beyer, of the University of Queensland, Australia, in a study published in 2007.

“Beyer was claiming that changes in elk distribution on the landscape due to the risk of wolf predation was leading to willow being released. Boyce argues that (Creel and Christianson) are wrong because elk numbers didn’t decline during the course of Beyer’s study, citing Beyer (2007) to support this argument. Beyer’s figure 2 clearly shows a nearly 50 percent decline in the elk population during the course of his study, making (Creel and Christianson’s) criticism reasonable.”

Winnie also noted that the 1988 fires and regrowth of aspens in Yellowstone, along with the expansion of grizzly bear ranges to outside the park, have been other notable changes over the past 40 years.

“So overall, I think Boyce has the bigger picture right, but I think he’s wrong about some of the details,” Winnie said.

Another change has been the bison population’s growth on the Northern Range, replacing elk as the primary grazers. This coincides with more elk have spending winters outside the park in the safer confines of the Paradise Valley, where wolves can be shot by hunters.

That was not anticipated by Boyce’s predator-prey models.

As bison populations have grown on the Northern Range, Yellowstone officials have struggled with how to manage winter bison migrations into Montana near the North Entrance at Gardiner. Some Montana politicians, area ranchers and the state Department of Livestock have made it clear that bison are not welcome except in small corners of the state next to the park’s north and west entrances. An agreement between the state and park service instructs the federal agency to reduce bison numbers through capture and slaughter. The bison herd reduction did not sit well with the past park superintendent, Dan Wenk, who was recently replaced. He cited his bison stance as one reason for his dismissal.

Wenk had worked with Montana tribal officials to set up a bison quarantine program to ship some of the Yellowstone bison to the reservations as an alternative to slaughter. The main roadblock is that a majority of the bison have been exposed to brucellosis, a bacteria that can cause young cattle to abort. Many Yellowstone elk also carry the disease, prompting Montana to establish a Designated Surveillance Area around the park in an attempt to keep elk and cattle separated. As elk populations have expanded outside the park, infected animals have been found farther away from Yellowstone’s boundary.

A couple of academics contacted for comments on Boyce’s paper declined, saying it was one-sided and narrow. Ben-David, of the University of Wyoming, said the paper doesn’t give “the full picture of what’s going on.” For example, she noted some elk in the Cody area no longer migrate into the park, choosing instead to live next to agricultural fields.

A 20-something Mark Boyce could never have foreseen how Yellowstone would change in the past 40 years, but he has at least one prediction of what may be to come.

“The most immediate threat to this park policy is the increasing bison population that has precipitated political pressure to limit their abundance for fear that heavy grazing and browsing by bison might ‘damage’ vegetation,” Boyce wrote. “We do not know how bison will affect Yellowstone, but surely we will learn a great deal more if we allow the bison population to take its course rather than intervening in a fashion that will be arbitrary to the underlying ecological system.”

Boyce said one result of increasing bison numbers may mean wolves will kill more yearling bison, a tactic wolves in Canada’s Wood Buffalo National Park have refined to the point that bison are a main prey source for the canines. Wolf studies in the park have shown the Mollies pack, which lives mainly in the remote Pelican Valley, already concentrates on killing bison in the winter, when snow is deep and the animals are more vulnerable.

“It’s very difficult to kill a bison, even a young animal,” Ben-David said.

A change in Yellowstone wolves’ appetite for bison might be even more accentuated if additional elk begin residing outside the park, giving wolves fewer prey options, Boyce noted.

“It remains to be seen what will happen there,” Boyce said. “I should probably bet a case of beer on how things will roll out.”


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