Listen to part of a lecture in an environmental science class. Professor: Today we're going to discuss a fascinating example of ecosystem restoration—the reintroduction of wolves to Yellowstone National Park in the 1990s. After wolves were eliminated from the park in the 1920s, the elk population exploded. Overgrazing by elk devastated young trees, especially willows and aspens along rivers. This caused erosion and reduced habitat for birds and beavers. In 1995, scientists reintroduced wolves. The wolves not only reduced elk numbers but also changed elk behavior—they avoided open river valleys, allowing vegetation to recover. Trees grew taller, which stabilized riverbanks. The return of willows provided food for beavers, and their dams created wetlands that attracted amphibians and fish. This cascade of effects shows how a single predator can reshape an entire ecosystem. However, the success wasn't immediate—it took decades, and some argue that other factors, like drought, also played a role. Still, it's a powerful example of trophic cascades.
❓ Question What is the main point the professor makes about the reintroduction of wolves?
A. Wolves quickly solved all the ecological problems in Yellowstone.
B. The reintroduction had a slow but significant cascade effect on the ecosystem.
C. Drought was the primary cause of vegetation recovery in Yellowstone.
D. Wolves mainly helped by increasing the number of beavers.
✅ Correct Answer
The reintroduction had a slow but significant cascade effect on 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
