Review of the Yellowstone Trophic Cascade: The Effect of Wolves on Aspen Regeneration Exaggerated, Study Results


BILLINGS – Following the reintroduction of wolves in Yellowstone National Park in 1995-97, aspen regrowth became a global story highlighting the importance of large predators.

The wolves ate moose that grazed on aspens. As the numbers of elk fell, the aspen stocks recovered and birds and beavers returned. Such environmental impacts caused by the addition or removal of a spike predator are known as trophic cascades.

Although some scientists long questioned the direct effect in Yellowstone, a recently published study shows that aspen regrowth was not as robust as originally advertised.

To learn

Elaine Brice and Dan MacNulty of the Department of Wildland Resources and Ecology Center at Utah State University and Eric Larsen of the Department of Geography and Geology at the University of Wisconsin Stevens Point outlined their research in the Nov. 8 publication of Ecology Letters.

At its core, the research questions the methodology used to measure aspen trees for previous research, saying that the choice of trees to include skewed the study.

“Previous studies have examined the recovery of aspen in Yellowstone by measuring the five largest young aspen within a stand,” the researchers wrote. “The idea was that the tallest young aspen trees would be a ‘leading’ indicator of the future recovery of the entire aspen population. But that’s not the case – the sample of only the largest young aspens estimated a recovery rate that was significantly faster than the random sample of all young aspens within the stock. “

MacNulty et al. found that when selecting the five tallest young trees to represent the stand, researchers tended to select “the top performing 10-15% young aspen in stands that have survived the 20th and 21st centuries”.

“It’s a complicated story,” MacNulty said in a telephone interview. “The effect that has been described in numerous publications is an exaggerated effect as they measured the best performing plants.”

He compared it to a teacher who only picked students who got ones and said they represented the entire classroom.


One of the problems with relying only on the tallest young aspens is that moose will recognize which aspen they are eating. They prefer trees no taller than their shoulders – those less than 6½ feet tall – a fact confirmed by wildlife cameras – but they continue to eat trees up to 13 feet tall.

As the trunk of a young aspen grows taller than the shoulder of an adult moose, it becomes less and less likely to be eaten, MacNulty said.

“This means that the largest young aspens grow faster because they’re bigger, not because wolves reduce moose hunting,” he said in a statement.

Rather than relying on the five largest young aspens in a rack, scientists should have randomized their samples, claim MacNulty and colleagues. According to the principle of chance, fewer aspen trees exceed the height that the researchers describe as the preferred browsing height.

“These are extremely complex systems and they are challenging to understand because they are difficult to sample properly,” said Brice.


Using a random sample, however, the researchers found evidence of a trophic cascade. After all, the moose population in the park has fallen from around 19,000 in the Northern Range, where wolves were concentrated after their release in 1995, to around 6,200 this year.

Such a large decrease in the number of large animals grazing and grazing will inevitably appear in the environment. The effect is much weaker than some of the earlier analyzes showed, MacNulty and his colleagues found.

The randomization of the trees included in the study also indicated that “more productive resource conditions may have contributed to faster annual height growth and overdrive recruitment … faster height growth could in turn have contributed to a faster decline in browsing rate.”

The study concluded: “Understanding how ecosystems respond to the loss and / or addition of large predators is critical to solving broader debates about the forces that structure food webs, determine biodiversity and provide ecosystem services. Our study, which focused on a textbook example of the extinction and reintroduction of large predators, shows how deviations from basic sampling principles can skew this understanding. Non-random samples have overestimated the strength of a trophic cascade in the system we examined, but can underestimate cascade effects in other systems. “

Matt Kauffman of the Wyoming Cooperative Fish and Wildlife Research Unit has also done studies in Yellowstone, including one looking at the effects of browsers on aspens. He praised MacNulty, Brice, and Larsen for their work “correcting this important ecological story.

“Ecological research is messy and often requires long-term data and rigorous analysis to get the story right,” he wrote in an email. “Studies like this are vital, especially when we are talking about science that could influence decisions about how to deal with such a controversial species as wolves in the northern Rockies.”


Bob Beschta, a professor emeritus at Oregon State University, worked with colleague William Ripple on many of the earlier studies at Yellowstone that document trophic cascades. In their research, the scientists used the five largest young aspens, the sampling method that MacNulty and his colleagues find less accurate. But Beschta doesn’t see it that way.

He said it took MacNulty and his staff two decades to double-check what he and Ripple had previously claimed that the loss of elk led to the regeneration of the aspens. He said the use of the five tallest trees turned out to be “a little prophetic”.

“There are many ways to measure plant communities,” he said. “To say that only one is right is a bit presumptuous.

“We decided to work with the five biggest because we could see that something was happening.”

Beschta said he and Ripple were careful not to say their numbers represented the average aspen, but instead referred to them as “leading indicators.”

“Your results confirm that. Our results showed that this was coming,” he said.

Although MacNulty and colleagues point to a number of other factors that can affect plant growth, Beschta said these are all pale in comparison to the effects of grazing. Reducing grazing allows the plants to grow, he added. Without such a cut, plants will not thrive.


It is difficult to distinguish cause and effect in a landscape inhabited by so many species that include other grazing animals – like a growing bison population – and as many top predators as grizzly bears, cougars, and black bears. In addition, there is the threat of climate change and the ecological stew is becoming even more murky.

Regardless of the outcome, the studies have all highlighted the importance of aspens and predators, both of which are in decline.

As the climate warmed, Yellowstone forests burned and elk and wolf populations rose and fell; Aspen groves have also changed. Analysis of historical landscape photographs from the 1880s found estimates that aspen once covered 4 to 6% of the park’s Northern Range. In the 1990s, aerial photographs showed that this had dropped to around 1%.

“Aspen forests are important for biodiversity; They support a greater variety of plant associations than the typical coniferous forests of the area, and add bird species richness and overall abundance, ”wrote John Klaptosky, a biological science technician in the Vegetation Division of the Yellowstone Center for Resources.

“Northern Yellowstone is right on the brink of aspen moisture requirements,” MacNulty said. “There are some stalls that have disappeared, probably due to moisture stress.”

He praised his fellow student Eric Larsen for building a database on aspens in Yellowstone from 1999 onwards.

“He has faithfully visited almost all of these properties almost every year,” said MacNulty. “This enabled us to make a real, direct comparison that others couldn’t.”


Forty-one wolves were eventually reintroduced to Yellowstone. In the moose-rich environment, their population exploded to 174 animals by 2003 before declining. In January, the number of wolves in the park was 123.

Wolves aren’t the only predators of elk. From 1995-2002, late moose hunting near Gardiner, just outside of Yellowstone’s northern border, removed between 940 and 2,465 moose per year.

“The total number of elk harvested by hunters in the winter of 1996-1997 represented the second largest elk removal (3,320 animals) in the era of natural regulation,” wrote park researchers.

“We tend to say that the change in herds of moose was due to hunting or predation, but both forces acted together,” said MacNulty.


Comments are closed.