Listen to part of a lecture in an environmental science class. Professor: Today we're going to discuss a concept that's central to understanding ecosystem dynamics: the trophic cascade. A trophic cascade occurs when predators at the top of a food chain indirectly affect the abundance of organisms at lower levels. Perhaps the most famous example is the reintroduction of gray wolves to Yellowstone National Park in 1995. When wolves were absent for decades, elk populations had grown so large that they overgrazed young willow and aspen trees. This devastated streamside habitats, causing erosion and reducing biodiversity. After wolves returned, they not only reduced elk numbers but also changed elk behavior—the elk avoided certain areas, allowing vegetation to recover. The regrowth of trees stabilized riverbanks, shade cooled the water, and bird and fish populations increased. This shows that top predators can shape entire landscapes, not just by direct predation, but by altering the behavior of their prey. So, when we think about conservation, we must consider the ripple effects that removing or adding a species can have.
❓ Question What is the main point the professor makes about trophic cascades?
A. They occur only in large national parks.
B. They show that predators affect ecosystems through indirect effects.
C. They are caused primarily by changes in plant growth.
D. They result from the direct predation of herbivores by humans.
✅ Correct Answer
They show that predators affect ecosystems through indirect effects.
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