How Wolves Change Rivers


One of the most exciting scientific findings of the past half century has been the discovery of widespread trophic cascades. A trophic cascade is an ecological process
which 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 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 vegitation
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 muscrats and ducks and fish and reptiles and amphibians. The wolves killed coyotes and as a result
of that, the number 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 vegitation
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. Subtitles by the community


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“When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” – John Muir

When wolves were reintroduced to Yellowstone National Park in the United States after being absent nearly 70 years, the most remarkable “trophic cascade” occurred. What is a trophic cascade and how exactly do wolves change rivers? George Monbiot explains in this movie remix.

If you likes How Wolves Change Rivers, check out How Whales Change Climate:

For more from George Monbiot, visit and for more on “rewilding” visit and/or check out George Monbiot’s book Feral: rewilding the land, the sea and human life:

NOTE: There are “elk” pictured in this video when the narrator is referring to “deer.” This is because the narrator is British and the British word for “elk” is “red deer” or “deer” for short. The scientific report this is based on refers to elk so we wanted to be accurate with the truth of the story.

B-Roll Credits:
“Greater Yellowstone Coalition – Wolves” (
“Wolf Mountain” (
“Primodial – Yellowstone” (
“Timelapse: Yellowstone National Park” (
“Yellowstone” (
“Howling Wolves – Heulende W├Âlfe” (
“Fooled by Nature: Beaver Dams” (

Music Credits:
“Unfoldment, Revealment, Evolution, Exposition, Integration, Arson” by Chris Zabriskie (

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Author: dhobson