Listening Script Vocabulary
(Section 4: You will hear a lecture discussing the reintroduction of wolves to Yellowstone Park. First, you will have some time to look at questions 31 to 40 [20 seconds]. Listen carefully and answer questions 31 to 40.)
L: Hello everyone. Welcome to this morning’s lecture on the subject of the reintroduction of wolves to Yellowstone Park. This is one in a series of lectures organized for the university by the Environmental Society. My name’s Brenda Johnston and I am an environmental scientist with a particular interest in this initiative to regenerate Yellowstone Park.
One of the most exciting scientific findings of the past half-century has been the discovery of trophic cascades. A trophic cascade is an ecological process that starts with an animal at the top of the food chain and makes an impact on creatures at the bottom of the chain. A 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 not aware that they also give life to many others. Before the wolves were reintroduced - they'd been absent for 70 years - the numbers of deer had increased in the Yellowstone Park because there had been no bigger animals to hunt them. Those deer had reduced the vegetation there to almost nothing. They had just grazed it away – eaten it. But as soon as the wolves arrived, things started to change.
First, of course, the wolves killed some of the deer but that wasn't the major thing. Much more importantly, they changed the behavior of the deer. The deer started avoiding certain parts of the park - the places where they could be trapped most easily by the wolves - and immediately those places started to regenerate. In some areas, the height of the trees increased by five times in just six years.
And as soon as that happened, the birds started moving in. The number of songbirds started to increase greatly. The number of beavers also started to increase because beavers like to eat the trees. And the beavers built dams in the rivers that provided habitats for ducks, fish and reptiles.
But here's where it gets really interesting. The wolves also changed the behavior of the rivers. Because there were now trees and plants with strong roots holding the soil together, that prevented soil erosion. The river narrowed. More pools formed. This was important for wildlife habitats. The rivers changed in response to the wolves.
So the wolves, small in number, transformed not just the ecosystem of the Yellowstone National Park but also its physical geography.