Narrator: Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a fascinating example of ecological restoration: the reintroduction of wolves to Yellowstone National Park in the mid-1990s. When wolves were absent for about 70 years, the elk population exploded, and they overgrazed the young willow and aspen trees along streams. This caused erosion and a decline in biodiversity. In 1995, park officials reintroduced 31 wolves from Canada. The effects were dramatic. The wolves not only reduced the elk population but also changed their behavior. Elk began to avoid open river valleys, which allowed vegetation to recover. As trees grew, beavers returned and built dams, creating habitats for fish and amphibians. This cascade of effects, known as a trophic cascade, even altered the physical geography of the rivers, as restored vegetation stabilized the banks. So, this case shows how a single species can have a profound impact on an entire ecosystem.
❓ 문제 What is the main point of the lecture?
A. A. Wolves are the most important predator in Yellowstone National Park.
B. B. The reintroduction of wolves caused unexpected negative effects on the ecosystem.
C. C. The reintroduction of wolves triggered a chain of positive ecological changes.
D. D. Elk populations in Yellowstone are primarily controlled by human hunting.
✅ 정답
C. The reintroduction of wolves triggered a chain of positive ecological changes.
TOEFL Listening Tips
Skim the questions first to predict the content
Watch for signal words in lectures and conversations (however, actually, in fact)
For main idea questions, focus on the beginning; for detail questions, focus on examples and data
For attitude questions, pay attention to the speaker's tone and word choice
Take notes on keywords; don't try to write down every word