Do wolves really change rivers?
Part One
The complicated story of Yellowstone trophic cascades
By Jack Rabe
Editor’s note: This is part one of a two-part story. The second part will run in the Spring 2025 edition of International Wolf.
Graphic from National Geographic March 2010 “Wolf Wars” Issue
The first time I heard about trophic cascades, I was sitting in an undergraduate “Intro to Environmental Science” course. I can only assume we were talking about ecosystem food webs and how they’re connected, because all that I remember from that day, what’s now seared into my brain, are two PowerPoint slides. On one slide of the towering screen at the front of the 500-seat lecture hall was a photo of several biologists carrying a crated wolf through deep snow that would soon be released into Yellowstone National Park. This alone caught my attention, but what I saw next blew me away.
On the following slide was a Nat Geo graphic of a Yellowstone landscape before and after wolf reintroduction. The differences were astounding. The “before wolves” depiction showed a degraded landscape riddled with elk, vegetation chewed down to nubs, fast-flowing rivers with eroded banks, and little to no other wildlife. “After wolves” showed a thriving ecosystem with fewer elk: lush, green vegetation lining slowly meandering rivers full of trout, and flourishing wildlife including beavers, songbirds, wolves, grizzly bears, amphibians, and more. As a 19-year-old college kid just diving into the field of wildlife ecology, I was hooked. How could one species have such a profound impact on an ecosystem? Fast-forward 10 years, after I’ve spent the last eight years studying large carnivores and elk in Yellowstone, I now realize the better question isn’t how, but did wolves have such an impact?
An Era of Control
Our story begins about 100 years ago, when wolves were extirpated from Yellowstone and the American West after decades of intensive hunting, trapping, and government-issued bounties and poisoning. These predator removal efforts also eliminated cougars from the park, and with the establishment of trash dumps near roads and developed areas that attracted bears, the functional extinction of grizzly and black bears as predators. With charismatic large predators gone from Yellowstone, there was only one thing preventing the northern Yellowstone elk herd from exploding in number.
From 1935 to 1968, hunters in the state of Montana killed over 40,000 elk as they migrated north of the park boundary. Park officials also culled over 15,000 elk in Yellowstone and transplanted over 10,000 elk to other states and countries looking to restore or introduce elk. Still, elk numbers remained relatively high (9,000–16,000) until the 1960s when park culling efforts increased, driving the population down to just 3,000–4,000 elk. But in 1969, park management shifted from a mentality of human intervention to letting the elk herd “naturally regulate” itself. With the end of park culling and a drastic reduction in elk hunting outside the park, elk numbers surged to ~20,000 individuals by the mid-1980s and remained high for a decade. During this time, it became increasingly apparent that overabundant elk were overgrazing and degrading northern Yellowstone. Something had to be done.
Expanded trophic cascade link from Beschta et al 2016 (NRC Research Press).
How wolves change rivers
According to the popular narrative, that something was the reintroduction of 41 wolves from Canada and northwest Montana to Yellowstone from 1995–1997, which caused a dramatic recovery of the ecosystem through a trophic cascade. It started with wolves hunting and killing elk, which did two things: 1) it reduced the number of elk; and 2) it changed elk foraging behavior. The fewer elk that remained could no longer spend all their time out in the open valleys, casually eating vegetation — they now had to hide from wolves. With fewer elk eating less vegetation, woody plants like aspen and willow could now thrive, growing faster and taller. More plants meant more songbirds, beavers, and other species. And more beavers and vegetation meant less stream bank erosion and the recovery of wetland and riparian species. This is a textbook example of a trophic cascade: a predator alters the behavior or size of a prey population, thereby limiting the prey’s consumption of vegetation, leading to the predator having widespread indirect effects on vegetation communities by cascading throughout the trophic levels of a food web.
This example has been taught in classrooms all over the globe, shared by tens of millions on the internet, broadcast on countless nature documentaries, and used to justify recovery efforts of wolves and other large carnivores around the world. I imagine most folks reading this have told family and friends about the remarkable effect that wolves have had in Yellowstone. I know I did after first learning about it in that college classroom a decade ago. That story is a major reason I became an ecologist, and why I ended up in Yellowstone. It’s why untold others became ecologists, or donated to conservation efforts, or simply gained a greater appreciation for the wonders of the natural world and vowed to help protect it for their children and their children’s children. But this story isn’t quite what happened, rather, it’s an exaggerated and oversimplified version of a much more complicated story. So where did the popular story come from?
The Five Tallest Stems
Some of the first evidence of a trophic cascade following wolf reintroduction came from a series of studies on aspen stands in northern Yellowstone. These studies documented reduced browsing of aspen in areas heavily used by wolves as they recovered, leading to a significant increase in the growth and regeneration of young aspen. Sounds like strong enough evidence for a trophic cascade, so most popular media outlets, nature documentaries, and even fellow scientists ran with it; but there’s a caveat. These studies didn’t randomly sample aspen. Instead, they measured the five tallest aspen in each stand. It was believed that measuring the five tallest aspen, which are easily identified, would represent a “leading edge” indicator that captured the future condition of the stand. Basically, if these five trees were showing increased growth, then it was only a matter of time before the other trees followed suit. This seems perfectly reasonable; however, non-random sampling like this inevitably brings biased results, leading to a few potential problems with this method and aspen sampling overall.
Aspen regrowth in Soda Butte Valley through time. Brice et al. 2022 (Ecology Letters)
First, by measuring the five tallest stems in a stand, one is selecting the most successful stems that may be doing well for reasons independent of wolves or elk, particularly if all the other stems are continuing to do poorly. For example, it turns out that the five tallest stems were often already taller than the preferred browsing height of elk or were tall enough to escape elk browsing entirely. The problem is that this creates a positive feedback loop: tallest stems surpass preferred browsing height elk stop feeding on tallest stems tallest stems grow even taller measured effect of wolf-caused trophic cascade also grows.
As I hinted above, this approach creates another problem: it fails to account for stems that aren’t regenerating at all. If a stand has five stems close to or exceeding the preferred browsing height of elk, then all other stems that fail to regenerate due to suppression by elk are left unaccounted for, exaggerating the strength of the trophic cascade.
Fortunately, while this research was going on, another group of scientists studied long-term aspen growth using both random sampling and the “five tallest” method. As expected, the five tallest stems grew at much faster rates than the randomly sampled stems, overestimating aspen regeneration by a factor of 4-7. Interestingly, there was still regeneration in the randomly sampled aspen, indicating there was still evidence of a trophic cascade, just that the effect of the cascade on aspen was considerably weaker than originally claimed.
Finally, sampling individual aspen from stands is biased by the simple fact that there must be aspen to sample. This approach may fail to capture more widespread declines or stands that vanish entirely. With the advent of better technology, satellite imagery of Yellowstone’s Northern Range show that aspen are still in major decline since wolf reintroduction, dropping from about 4% cover in the early 1900s to 1% in the mid-1990s and 0.25% by 2021. This decline is likely a combination of continued ungulate browsing of saplings and environmental conditions becoming increasingly inhospitable (e.g., hotter and drier). So, it’s established that aspen as a whole are declining, but that young aspen within active stands are recovering, albeit less than the popular trophic cascades narrative. But aspen isn’t the only plant species that supposedly benefited from wolf reintroduction.
Experimenting with Willows
Willows in riparian communities were another species claimed to have been restored by a wolf-caused trophic cascade. Yet, recent experimental work published in 2024 shows that may not be the case. In 2001, researchers set up experimental plots in willow stands. Within each plot, there was a control and three different treatments. While the treatments either contained a fence around the willows, a dam just below the willows, or both, the control represented unaltered (undammed and unfenced) willows. The idea was that fencing would show the effect that browsing ungulates like elk had on willow growth and recovery, and a dam would demonstrate the effect that available groundwater had on willows while also simulating beavers. Along with the experimental plots, researchers also measured willow growth in areas with and without beaver dams. They found that willow growth was heavily reliant on dams (natural or artificial) and fencing: dammed and fenced willows grew more than willows that were only dammed or only fenced, and willows in the control grew the least. In other words, the unaltered willows, which represented Yellowstone’s typical willow community, were still being suppressed by intense ungulate browsing and limited by available water. While there was evidence of some willow growth from 2001 to 2020, researchers concluded that willows had not yet recovered from prior degradation.
In conclusion, recovery of aspen and willow communities in Yellowstone since wolf reintroduction has not been as strong or widespread as claimed. Still, there has been some recovery, suggesting a trophic cascade has unfolded. But what’s causing this cascade? Earlier, we mentioned two possibilities that have divided the scientific community: wolves killing elk vs. wolves altering elk behavior. Understanding which mechanism drove the trophic cascade can be important for managing and conserving wildlife and ecosystems like Yellowstone.
This article was originally published in the Winter 2024 edition of International Wolf magazine, published quarterly by the International Wolf Center. The magazine is mailed exclusively to members of the Center.
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About the author: Jack Rabe is a Ph.D. candidate in conservation sciences at the University of Minnesota researching large carnivore competition and predator-prey interactions in Yellowstone. In following the lives of wolves, cougars and elk in the Yellowstone backcountry, he has hiked enough miles to walk coast-to-coast across America.
