There’s a lovely film on Youtube called How Wolves Change Rivers. Narrated by the British writer and environmental campaigner George Monbiot, and largely drawn from his book Feral, it describes how the reintroduction of wolves to Yellowstone National Park in the US state of Wyoming in 1995 has precipitated a series of ecological benefits.
It wasn’t just that wolves were hunting and killing herbivores such as elk (animals closely related to European red deer), but were radically changing their behaviour.
- “At first, we were puzzled.” Scientists track 69 ravens through Yellowstone – and make remarkable discovery
- Stunning photos that reveal the incredible beauty and power of the world’s oldest national park
“Elk avoided valleys and gorges where they could be easily caught,” Monbiot says. “Immediately those places started to regenerate.”
This is the so-called ‘landscape of fear’ hypothesis, whereby herbivores come to be afraid in certain areas, and the young trees they might have browsed on survive.
“Bare valley sides became forests of aspen, willow and cottonwood,” continues Monbiot. “Then birds started moving in and beaver numbers increased because they like to eat the trees.”
The regenerating forests stabilised riverbanks, so they were less prone to collapse, and rivers became more fixed in their course.
“Wolves transformed not just the ecosystem but its physical geography.” The impact wolves are said to have had in Yellowstone is known as a trophic cascade.
- Remote cameras in Yellowstone capture cougars and wolves on the hunt. The footage reveals something important
- Watch amazing, jaw dropping footage of fearless elk chasing grey wolf in Yellowstone National Park
“People were the apex predator”
It’s a beautiful and uplifting notion but, according to some scientists, total nonsense. “It is the source of so much confusion,” says Dan MacNulty of Utah State University.
“Not just for the general public but among scientists and students, too. I gave a talk at the University of Oxford two years ago, to the biology department, and I had PhD students telling me afterwards that they felt they’d been deceived – which, frankly, they were.”
Tom Hobbs, an ecologist at Colorado State University, agrees with MacNulty, describing the idea of a widespread trophic cascade stemming from the wolves as a great idea that doesn’t hold up.
“There are two key points,” argues Hobbs. “Firstly, has vegetation dramatically improved everywhere? Absolutely not. There has been a very patchy, spatially variable change in woody deciduous plants.”
The second question, Hobbs goes on, is whether wolves are responsible for any changes. He says that wolves have only been part of the story, and possibly only a small part.
“During the decade after wolves were reintroduced, the decline in the elk population was not caused by wolves – there weren’t enough of them – but by human harvest outside the park. People were the apex predator, and that caused a 50 per cent reduction in the elk.”
Even today, there are only an estimated 100 wolves in Yellowstone. Of course, Monbiot’s assertions haven’t come out of thin air. As he points out in an email to BBC Wildlife, he took his cue from a series of papers by William Ripple and colleagues of Oregon State University.
Seminal research from 2012 described how browsing of the tallest aspens, in 97 stands surveyed, had decreased from 100 per cent in 1998 to between 20 and 25 per cent by 2010 – during the period almost immediately after wolves had been reintroduced, in other words.
“Wolves restored a trophic cascade with woody browse species growing taller and canopy cover increasing in some but not all places,” says the paper.
Luke Painter, who has published with Ripple, most recently in 2025, is also of the view the wolves have had a significant impact. He says the difference of scientific opinion is caused by whether you see the glass as half full or half empty. Aspen recovery has been patchy, but at least it’s happening.
Painter agrees with other scientists that just after the wolves were reintroduced, around 1997, a number of factors, including human hunting and some hard winters, considerably knocked back elk numbers.
But instead of rising again, as they would have done, there has been a sustained lower number inside the park since then.
“I think we can say confidently that wolves are a necessary part of maintaining these lower elk densities,” argues Painter. Grizzlies and cougars are important, too, he says, and it would probably be impossible to determine the precise contribution of the wolves, but they are certainly helping to keep herbivore numbers in check.
Yellowstone’s wolves and bears
Does whether, and to what extent, wolves have unleashed this trophic cascade really matter? All the scientists involved in researching their impacts at Yellowstone would agree there should be wolves in the park – they are a native predator, after all.
According to MacNulty, they have helped the recovery of grizzly bears in the Greater Yellowstone ecosystem. “Wolves support a more resilient grizzly bear population because they provide a food resource following predation on ungulates [such as elk],” he says.
“Bears are really good at pushing large packs of wolves off carcasses and taking them over – I’ve seen it myself numerous times.”
McNulty adds that the future of the bears in the area is in question due to the impacts of climate change, which is reducing the quantity of food sources such as the whitebark pine nuts that the bears have traditionally relied upon to fatten up in the autumn.
Over-promoting the wolves’ impact
But over-promoting wolves’ supposed benefits might have negative consequences, argues MacNulty. For example, a wolf reintroduction in Colorado was approved by a very small margin in a referendum, with promoters of the proposal arguing that they would see the same ecological benefits as claimed in Yellowstone.
“People are using predators as a management tool to restore ecosystems,” says MacNulty. “If they’re not having the effect that’s been attributed to them in Yellowstone, that’s problematic. It creates the potential for misguided management.”
MacNulty suggests that there’s a broader, arguably greater issue at stake. He points to a paper published a decade ago that predicts the northern area of Yellowstone – where almost all the relevant research has been carried out – will become increasingly inhospitable to aspen trees due to climate change.
Already, he says, there is evidence that aspen are growing at higher elevations. MacNulty mentions an interpretative display in an area called Lamar Valley that outlines the trophic cascade caused by the wolves and points to a regenerating aspen stand across the way.
“But what the sign doesn’t tell the visitor is about the other aspen stand that has completely vanished since the wolves were reintroduced,” he says. “It was a forest in 1954 and is absolutely gone now, and showing no signs of recovery.”
Even a strong advocate for the trophic cascade impact of wolves, such as Painter, accepts that what they can do is limited.
Often, where predators are restored, he suggests, humans keep their numbers suppressed because they might either want to maintain high levels of game (such as deer) to hunt for themselves, or because of the impact on livestock.
This has particular ramifications for some of the arguments made to justify reintroducing predators into the UK because, unlike many parts of the USA, there is nowhere in Britain that’s truly remote from livestock farming.
Even in many parts of Europe, where population densities are lower, wolves, lynx and bears are controlled to take account of their predation on deer, sheep and cattle.
There are, of course, arguments about whether this is necessary, but the point is that they are unlikely to be initiating any sort of trophic cascade. That’s not to say, as all the ecologists interviewed for this piece point out, that bringing back predators isn’t good for ecosystems.
As well as the benefit for bears, there is also what is called the ‘healthy herd hypothesis,’ which posits that carnivores help prey populations by picking off sick, old and injured animals, leaving the healthiest, strongest individuals to reproduce.
For example, a study of wolves and moose on Isle Royale National Park (an island in Lake Superior) found that after years of high kill-rates by the wolves, incidences of osteoarthritis in the moose declined.
And because osteoarthritis is a genetic condition, the scientists carrying out the study say that wolf predation may act as a selective force against genes associated with the condition.
Wolves and other large predators found in Yellowstone are creatures that any natural-history lover would want to see thriving in their natural habitat. Perhaps the lesson of their reintroduction to Yellowstone is that they are not miracle workers, and we shouldn’t expect them to be.
Top image: grey wolves chasing bison in Yellowstone National Park. Credit: Credit: Stan Tekiela Author / Naturalist / Wildlife Photographer / Getty Images
