How Wolves Change Rivers | movie | 2014 | Official Clip - video Dailymotion

When wolves were reintroduced to Yellowstone National Park in the United States after being absent nearly 70 years, the  | dG1feXNhNU9CaFh6LVE

Transcript

00:00One of the most exciting scientific findings of the past half century has been that the

00:29discovery of widespread trophic cascades. A trophic cascade is an ecological process

00:37which starts at the top of the food chain and tumbles all the way down to the bottom.

00:43And the classic example is what happened in the Yellowstone National Park in the United

00:47States when wolves were reintroduced in 1995. Now, we all know that wolves kill various

00:55species of animals, but perhaps we're slightly less aware that they give life to many others.

01:06Before the wolves turned up, they'd been absent for 70 years, that the numbers of deer, because

01:10there was nothing to hunt them, had built up and built up in the Yellowstone Park and

01:14despite efforts by humans to control them, they'd managed to reduce much of the vegetation

01:20there to almost nothing, they'd just grazed it away. But as soon as the wolves arrived,

01:26even though they were few in number, they started to have the most remarkable effects.

01:31First, of course, they killed some of the deer, but that wasn't the major thing. Much

01:36more significantly, they radically changed the behaviour of the deer. The deer started

01:41avoiding certain parts of the park, the places where they could be trapped most easily, particularly

01:47the valleys and the gorges, and immediately those places started to regenerate. In some

01:52areas, the height of the trees quintupled in just six years. Bare valley sides quickly

01:59became forests of aspen and willow and cottonwood. And as soon as that happened, the birds started

02:07moving in. The number of songbirds and migratory birds started to increase greatly. The number

02:13of beavers started to increase because beavers like to eat the trees. And beavers, like wolves,

02:19are ecosystem engineers, they create niches for other species. And the dams they built

02:24in the rivers provided habitats for otters and muskrats and ducks and fish and reptiles

02:31and amphibians. The wolves killed coyotes and as a result of that, the number of rabbits

02:37and mice began to rise, which meant more hawks, more weasels, more foxes, more badgers.

02:44Ravens and bald eagles came down to feed on the carrion that the wolves had left. Bears

02:49fed on it too and their population began to rise as well, partly also because there were

02:54more berries growing on the regenerating shrubs. And the bears reinforced the impact of the

03:00wolves by killing some of the calves of the deer. But here's where it gets really interesting.

03:09The wolves changed the behaviour of the rivers. They began to meander less, there was less

03:17erosion, the channels narrowed, more pools formed, more riffle sections, all of which

03:22were great for wildlife habitats. The rivers changed in response to the wolves. And the

03:28reason was that the regenerating forests stabilised the banks so that they collapsed

03:35less often, so that the rivers became more fixed in their course. Similarly, by driving

03:41the deer out of some places and the vegetation recovering on the valley sides, there was

03:46less soil erosion because the vegetation stabilised that as well. So the wolves, small in number,

03:55transformed not just the ecosystem of the Yellowstone National Park, this huge area

03:59of land, but also its physical geography.