How Wolves Change Rivers

What happened in the Yellowstone National Park when wolves were reintroduced in 1995?

The story of how wolves changed the course of the rivers of Yellowstone National Park is a story that shifts the narrative of how nature itself works. In a very real way, the story of how wolves change rivers animates the web of life in a way that is impossible to ignore. Instead of viewing each individual species separate from the whole, people begin to see the orchestra of nature come alive. Something about this story just feels true. With the original video on YouTube having been seen over 43 million times, this video has been credited with changing the hearts and minds of people about wolves, helping to turn them from a sinister villain to an ecosystem engineer.

Beyond ecosystems and wolves, this video shows that the world is far more interconnected and co-dependent that we imagined. Only when all of life thrives can humanity thrive. There is no separation.

One of the most exciting scientific findings of the past half-century has been the discovery of widespread trophic cascades. Atrophic cascade is an ecological process that starts at the top of the food chain and tumbles all the way down to the bottom.

And the classic example is what happened in the Yellowstone National Park in the United States when wolves were reintroduced in 1995. Now, we – we all know that wolves kill various species of animals, but perhaps we’re slightly less aware that they give life to many others. Before the wolves turned up – they’d been absent for 70 years – the numbers of deer (because there had been nothing to hunt them) had built up and built up in the Yellowstone Park and despite efforts by humans to control them they’d managed to reduce much the vegetation there to almost nothing. They had just grazed it away.

But as soon as the wolves arrived, even though they were few in number they started to have the most remarkable effects. First, of course, they killed some of the deer but that wasn’t the major thing. Much more significantly, they radically changed the behavior of the deer. The deer started avoiding certain parts of the park – the places where they could be trapped most easily – particularly the valleys and the gorges and immediately those places started to regenerate. In some areas, the height of the trees quintupled in just six years. Bare valley sides quickly became forests of aspen and willow and cottonwood. And as soon as that happened, the birds started moving in. The number of songbirds and migratory birds started to increase greatly. The number of beavers started to increase because beavers like to eat the trees. And beavers, like wolves, are ecosystem engineers. They create niches for other species. And the dams they built in the rivers provided habitats for otters and muskrats and ducks and fish and reptiles and amphibians. The wolves killed coyotes and as a result of that, the number of rabbits and mice began to rise which meant more hawks more weasels more foxes more badgers. Ravens and bald eagles came down to feed on the carrion that the wolves had left. Bears fed on it, too. And their population began to rise as well partly also because there were more berries growing on the regenerating shrubs. And the bears reinforced the impact of the wolves by killing some of the calves of the deer.

But here’s where it gets really interesting. The wolves changed the behavior of the rivers. They began to meander less. There was less erosion. The channels narrowed. More pools formed. More riffle sections. All of which were great for wildlife habitats. The rivers changed in response to the wolves. And the reason was that the regenerating forests stabilized the banks so that they collapsed less often. So the rivers became more fixed in their course. Similarly, by driving the deer out of some places, and the vegetation recovering on the valley side, there was less soil erosion because the vegetation stabilized that as well.

So the wolves, small in number, transformed not just the ecosystem of the Yellowstone National Park – This huge area of land… but also, its physical geography.

“Wolves have long been persecuted here in my region to the point of complete eradication by the 1930s. Much of that persecution was based on hate and fear of the species. The conflicts were dramatically overblown as a means for justifying extermination campaigns. A common phrase here: the only good wolves are dead ones. Your film How Wolves Change Rivers has done more to liberate the wolf from historic bias than anything else I’ve seen working in the field for wolf conservation for over 30 years. It has given millions of viewers a chance to reframe their image of wolves from the horrific beast of myth to an ecologically key driver of ecosystem health. I’m so deeply in gratitude for your work for all species but it is the wolf that has suffered so much in my region from our lack of understanding and compassion. Thank you. Howls from the northlands.”

“I believe this video was the sole reason I started looking into more ecology.”

“This video makes you realize how we living organisms are really all connected.”

“My science teacher earned +10 respect for sending me this.”

“Sometimes when I’m feeling particularly emotional about the workings of the world, I come back and watch this video. the peaceful music and the wonder in the narrator’s voice does something to my soul.”

“Just a beautiful video showing us how important it is to look after and protect animals like the Wolves. Not to persecute and hunt them to extinction. Put simply, they bring balance and stability.”


Scientific References:

  • Chase, Alton (1986). Playing God in Yellowstone – The Destruction of America’s First National Park. Boston, MA: The Atlantic Monthly Press
  • Murie, Adolph (1940). Fauna of the National Parks of the United States-Ecology of the Coyote in the Yellowstone National Park (Report). US. Department of the Interior
  • Ripple, W; Beschta R: “Wolves and the Ecology of Fear: Can Predation Risk Structure”, page 761, “Bioscience”, 2004 Vol. 54 No. 8
  • Urbigkit, Cat (2008). Yellowstone Wolves: A Chronicle of the Animal, the People, and the Politics.Blacksburg, VI: cDonald & Woodward Publishing.
  • The Reintroduction of Gray Wolves to Yellowstone National Park and Central Idaho-Final Environmental Impact Statement (PDF) (Report). Denver, CO: US. Fish and Wildlife Service. 1994
  • “Technical Publications on Wolves, 1995-2004” (PDF). Yellowstone Science 12 (1): 42-43. Winter 2005.
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