Yellowstone research reignites debate over wolf-generated trophic cascade

BILLINGS – The theory was exciting and quickly grabbed headlines.

Fifteen years after wolves were reintroduced to Yellowstone National Park in 1995, some scientists pointed to the resurgence of aspen growth, an increase in songbirds and beavers as a sign that the wild canines had caused a trophic cascade.

A trophic cascade is when an apex predator, like the gray wolf, indirectly benefits plants by controlling the number of plant eaters, in this case elk.

The results of a 20-year study in Yellowstone, recently published by Colorado State University researchers Tom Hobbs and David Cooper, propose some of the earlier research and articles coming to such conclusions were “excessive simplifications of a more complex truth.”

“We conclude that the restoration of apex predators to Yellowstone should no longer be held up as evidence of a trophic cascade in riparian plant communities of small streams on the northern range,” the scientists write in their study published on Jan. 30 in the journal “Ecological Monographs.”

Looking back

Humans eliminated Yellowstone wolves in the 1920s. Prior to that, cougar and bear numbers were falling due to predator control tactics that included the use of poisons.

By the 1950s, beavers had abandoned streams they once occupied near Tower Junction. Beaver need taller willows and aspen for food and to build their dams. Without beaver dams to slow streamflows, some creeks ran faster, cutting deeper channels.

Elk will browse on trees like aspen and willow, although it’s a small part of their diet.

By the mid-1990s, the big ungulates’ population had risen on the park’s Northern Range to an estimated 17,000. Some willows growing in the region were chomped down to knee height.

In 1995, wolves were reintroduced. The predators found the Northern Range’s large elk population naive to their style of predation. At the same time, cougar and bear populations in the park were climbing, and Montana hunters were killing large numbers of reproductive-age cow elk when they migrated outside the park near Gardiner in the fall.

By 2003, the elk population plummeted to about 4,000 animals.

All of these factors combined over decades to slowly alter the landscape across Yellowstone’s Northern Range, Hobbs and Cooper contend, changing it from willow-dominated streams with abundant beaver populations in the 1920s to what they now see as an elk-grassland.

Beaver analog study

The two Colorado ecologists base their conclusions on a study that began in 2001. That’s when they started building fake beaver dams on a few streams in the Northern Range. Some of the dams were fenced off to prevent elk or bison from dining on willows.

Willows thrive in a moist environment, so beaver dams create a great place for them to grow. The dams slow a stream’s movement, creating ponds and allowing more water to percolate into the surrounding soil. When beaver dams are abandoned and dry up, the exposed soils are the perfect place for willow seeds to take root.

Hobbs and Cooper started their study with four sites along Elk Creek and the West and East Forks of Blacktail Deer Creek. Eight years later, they expanded the study to 21 sites in the park.

“The combined effects of the dam and fence treatment tripled the rate of increase in the height of willows in the experiment,” the researchers wrote. “It is notable that dam treatment and fence treatment had effects of similar magnitude. This suggests that well watered plants could tolerate relatively heavy browsing.”

Scientists comment

Dan MacNulty, a Utah State University associate professor of Wildland Resources, has worked on several Yellowstone studies involving wolves and plant growth following wolf reintroduction.

Overall, he said the CSU study was “rigorous and well-executed,” highlighting “how physical aspects of the environment, namely water availability, can limit the cascading effects of wolves and other large carnivores on plant growth.”

Robert Beschta, professor emeritus of Forest Ecosystems and Society at Oregon State University, along with fellow researcher William Ripple, were some of the first scientists to advocate in research published in 2010, for the “top-down” effect of wolf reintroduction positively affecting willow and aspen growth.

Hobbs and Cooper, in their paper, were critical of this earlier work, calling it limited and biased. When asked for his response to the CSU study, Beschta cited other studies he said his Colorado counterparts had overlooked for their interpretations and conclusions.

“Their focus was simply on willows growing in a certain setting and, because those willows had not ‘recovered’ (according to their definition), they downplay the ongoing changes in vegetation that are underway, thus misrepresenting what is happening,” Beschta wrote in an email.

On the other hand, Hobbs and Cooper wrote, “The restoration of apex predators did not allow the return of the beaver–willow state because changes in the disturbance regime and the morphology of streams that occurred while predators were absent eliminated the environmental conditions needed for willows to establish and grow and because herbivory remained sufficiently intense to suppress willow growth.”

It’s complicated

The many studies generated by scientists conducting research in Yellowstone National Park over many decades emphasize the ecosystem’s complexity. So the discussion over trophic cascades is understandably nuanced, complex and controversial.

“We think it’s more accurate to represent this as a multipredator, density-mediated trophic cascade,” said Dan Stahler, chief scientist for the Yellowstone Wolf Project, during the Yellowstone Summit.

Chris Geremia, the park’s chief bison biologist, said during the summit that conflicting studies over trophic cascades are confusing for the general public.

“You can’t have a narrow vision of what a system should look like,” he said. “You need to embrace the complexity of this place to understand what’s happening.”

Retired Yellowstone wolf biologist Doug Smith said Hobbs’ and Cooper’s study is “top shelf” and used the “gold standard” for science in its design. He called Hobbs a brilliant scientist and a great writer. The study was also peer-reviewed.

The problem, Smith said, is that the study was conducted in a small area of northern Yellowstone using a specific set of conditions.

“Does that tell you what’s happening on a landscape scale?” Smith questioned, adding that willow communities had surged in the Slough Creek drainage where Hobbs and Cooper did not have any test plots.

Counterpoint

Cooper, co-author of the most recent study, said many scientists and visitors to the park see what they want to when it comes to willow growth.

“If you evaluate historic photos of the Yellowstone landscape it’s clear that many willow and aspen stands have disappeared from a combination of excessive herbivory and likely climate changes,” he wrote in an email. “So overall there is much less willow and aspen now than existed in the past.”

Yet earlier research, published in a 1997 Yellowstone Center for Resources journal titled “Yellowstone’s Northern Range: Complexity and Change in a Wildland Ecosystem,” asserts it is a misconception the region was ever beaver-willow habitat. The park’s scientists noted in the publication, “99% of the park’s willow communities are found in areas that are above 7,000 feet and/or receive more than 20 inches of annual precipitation, which excludes most of the Northern Range.”

While photos from the 1890s may show portions of the range with taller willows, the trees’ decline has been more closely associated with drought than large elk populations, the scientists added. That willow decline was especially significant during the Dust Bowl’s extended drought in the 1930s.

Cooper disagreed.

“The Northern Range has always been a beaver-willow landscape, probably for more than 10,000 years,” he wrote. “The changes that we and others have documented have occurred in the past 100 years, and are the combination of apex predator removal and heavy browsing.”

The Yellowstone scientists concluded, “How the park’s willows ‘should’ look is not simply a matter of comparing one land management style with another, but of understanding a complex suite of influences that shape both wild and manipulated willow communities.”

Likewise, they contended in the 1997 publication that larger populations of beavers documented on the Northern Range by E.R. Warren in 1926 – an early, although narrow study – was an anomaly.

“The reason that Warren was invited to conduct his study was that park managers and naturalists were alarmed over the population irruption of beaver, and were fearful that the beaver were going to kill all the park’s aspen,” the scientists said.

A study by Smith published in 2012 showed a slight uptick in beaver colonies on the Northern Range between 1996 and 2009, climbing from one to 12. Yet Cooper said although there may be a “few beavers around” there is no suitable habitat for them.

“Beaver populations are tiny on the Northern Range today,” he said.

Smith agreed, the reintroduction of wolves to Yellowstone has not moved the Northern Range ecosystem back to the beaver-willow state that existed 100 years ago. But in places where there were willows more recently when elk numbers declined, the trees are doing better, beavers are back and so are some willow-obligate bird species, he said.

“Those are signs of willow recovery he completely left out,” Smith said in reference to the Hobbs and Cooper study.