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How Wolves Change Rivers 4:33min

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This is video from Sustainable Man Youtube Channel about how wolves changed Yellowstone National Park.  Note:I do not own the rights to this video.  It’s a fast-paced video but worth listening and watching a few times — it’s an encouraging story, as well as a feast for the eyes. The narrator is George Monbiot, who writes for the Guardian newspaper (UK).


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 re-introduced in 1995 (nineteen ninety-five)

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 – that the numbers of deer, because there was 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 of the vegetation there to almost nothing, they’d 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 forest of aspens and willows 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, um, 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 in 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 river

They began to meander less, there was less erosion, the channels narrowed and more pools formed, more ripple sections, all of which was great for wildlife habitat

The rivers changed in response to the wolves and the reason was that the regenerating forest stabilized the banks so that they collapsed less often, so that the rivers become more fixed in their course

Similarly, by driving the deer out of some places and the vegetation recovering, on the valley sides, 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

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