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 gray wolves to Yellowstone National Park in the 1990s. After wolves were absent for about 70 years, the elk population had exploded, leading to overgrazing of willow and aspen trees along rivers. This caused erosion and a decline in beaver populations, as beavers rely on those trees for food and dam building. When wolves were reintroduced in 1995, they not only reduced elk numbers but also changed elk behavior—elk avoided river valleys where they were more vulnerable. This allowed vegetation to recover, which stabilized riverbanks, improved habitat for birds and fish, and even brought back beavers. The wolves also provided carcasses that fed scavengers like bears and eagles. This cascade of effects, known as a trophic cascade, shows how a single predator can reshape an entire ecosystem.
❓ Câu hỏi What is the main point the professor makes about the reintroduction of wolves to Yellowstone?
A. A) The wolves primarily reduced elk numbers, which allowed grasses to grow for livestock.
B. B) The wolves triggered a series of ecological changes that restored balance to the ecosystem.
C. C) The wolves caused a decline in beaver populations due to increased competition.
D. D) The wolves had little effect because elk simply moved to other areas.
✅ Đáp án đúng
B) The wolves triggered a series of ecological changes that restored balance to the ecosystem.
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
