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