Revisiting Yellowstone’s trophic cascade: Wolves’ effect on aspen regeneration exaggerated, study finds

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

The wolves ate elk, which browsed on aspen. When elk numbers fell, aspen stands rebounded and birds and beavers returned. Such ecological effects caused by the addition, or removal, of a top predator are called trophic cascades.

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

Study

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

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

“Previous studies evaluated aspen recovery in Yellowstone by measuring the five tallest young aspen within a stand,” the researchers wrote. “The reasoning was that the tallest young aspen trees represent a ‘leading edge’ indicator of the future recovery of the entire aspen population. But this is not the case – sampling only the tallest young aspen estimated a rate of recovery that was significantly faster than was estimated by random sampling of all young aspen within the stand.”

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

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

He compared it to a teacher only picking students who get A’s and saying they represented the entire classroom.

Aspen

One of the problems of relying on only the tallest young aspen trees is that elk are discerning about which aspen they eat. They prefer trees that are no taller than their shoulders, ones less than about 6½ feet tall – a fact verified by game cameras – but will keep eating trees as tall as 13 feet.

As the trunk of a young aspen grows taller than an adult elk’s shoulder, it is decreasingly likely to be eaten, MacNulty said.

“This means that the tallest young aspen grow faster because they are taller, not because wolves reduce elk browsing,” he said in a statement.

Instead of relying on the five tallest young aspen in a stand, the scientists should have made their sampling more random, MacNulty and his colleagues assert. When randomized, fewer aspen trees exceed what the researchers call preferred browsing height.

“These are extremely complex systems, and understanding them is a major challenge because they are difficult to properly sample,” Brice said.

Random

Using a random sample, the researchers still found evidence of a trophic cascade. After all, the elk population in the park plummeted from around 19,000 on the Northern Range, where wolves concentrated after being released in 1995, to about 6,200 this year.

That large of a drop in the number of big animals that browse and graze is bound to show up in the environment. But the effect was much weaker than some of the earlier analyses showed, MacNulty and his colleagues found.

Randomizing the trees included in the study also showed that “more productive resource conditions may have contributed to the faster annual height growth and overstory recruitment … Faster height growth could have in turn contributed to the faster decrease in browsing.”

The study concluded, “Understanding how ecosystems respond to the loss and/or addition of large predators is vital to resolving broader debates about the forces that structure food webs, determine species abundance and deliver ecosystem services. Our study, which focused on a textbook example of large predator extirpation and reintroduction, demonstrates how deviations from basic sampling principles can distort this understanding. Nonrandom sampling overestimated the strength of a trophic cascade in the system we studied, but it may underestimate cascading effects in other systems.”

Matt Kauffman, of the Wyoming Cooperative Fish and Wildlife Research Unit, has also conducted studies in Yellowstone, including one that looked at browsers’ effects on aspen. He praised MacNulty, Brice and Larsen for their work to “correct this important ecological story.

“Ecological research is messy and often requires long-term data and rigorous analyses to get the story right,” he wrote in an email. “Studies like these are critically important, especially when we are talking about science that might inform the decision of how to manage a species as controversial as wolves in the Northern Rockies.”

Counterpoint

Bob Beschta, professor emeritus at Oregon State University, partnered with his colleague William Ripple on many of the earlier studies in Yellowstone documenting trophic cascade. In their research, the scientists used the five tallest young aspen, the sampling method MacNulty and his colleagues find less accurate. Yet Beschta doesn’t see it that way.

He said it took MacNulty and his co-workers two decades to verify what he and Ripple had already asserted, the loss of elk led to aspen regeneration. He said using the five tallest trees proved to be “a bit prophetic.

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

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

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

“Their results confirm that. Our results indicated that was coming,” he said.

Although MacNulty and his colleagues reference a number of other factors that can affect plant growth, Beschta said those all pale in comparison to the effect of browsing. Reducing grazing allows plants to grow, he added. Without such a cutback, plants aren’t going to thrive.

Interwoven

The difficulty of separating out cause and effect on a landscape inhabited by so many species that includes other grazers – like a growing population of bison – and one that includes so many top predators such as grizzly bears, cougars and black bears – is difficult. Throw in the threat of climate change and the ecological stew becomes even cloudier.

No matter the outcome, the studies have all highlighted the importance of aspen and predators, both of which are in decline.

As the climate has warmed, Yellowstone’s forests have burned and elk and wolf populations have surged and ebbed; aspen groves have also varied. Analysis of historic landscape photographs dating back to the 1880s provided estimates that aspen once covered 4% to 6% of the park’s Northern Range. By the 1990s, aerial photography showed that had decreased to around 1%.

“Aspen forests are important for biodiversity; they support a greater variety of plant associations than the typical conifer forests of the area, as well as increase bird species richness and total abundance,” wrote John Klaptosky, a biological science technician in the vegetation department at the Yellowstone Center for Resources.

“Northern Yellowstone is right at the margin of moisture requirements for aspen,” MacNulty said. “There are some stands that have disappeared, probably because of moisture stress.”

He praised his colleague in the study, Eric Larsen, for building a database on aspen in Yellowstone beginning in 1999.

“He’s faithfully visited nearly all these plots nearly every year,” MacNulty said. “That allowed us to do a real direct comparison that others weren’t able to make.”

Wolves

Forty-one wolves were eventually reintroduced to Yellowstone. In the elk-rich environment, their population exploded to 174 animals by 2003 before declining. In January, the park’s wolves numbered 123.

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

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

“We tend to want to say that the change in elk herds is because of hunting or because of predators, but both of these forces were acting together,” MacNulty said.