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 ecosystems: the keystone species. Now, a keystone species is one whose impact on its community is disproportionately large relative to its abundance. In other words, even though there might not be many of them, they play a critical role in maintaining the structure and health of the ecosystem. The classic example is the sea otter. In kelp forest ecosystems along the Pacific coast, sea otters prey on sea urchins. Without otters, sea urchins multiply and overgraze the kelp, destroying the habitat for many fish and invertebrates. When otters were hunted to near extinction in the 19th century, the kelp forests declined dramatically. But when otters were protected and their populations recovered, the kelp forests bounced back. So the presence or absence of a single predator can have a cascading effect on the entire ecosystem. This idea challenges the traditional view that ecosystems are controlled primarily by food availability from the bottom up. Instead, it shows that top-down control by predators can be just as important.
❓ คำถาม What is the main point the professor makes about keystone species?
A. They are usually the most numerous species in an ecosystem.
B. They have a large influence on the ecosystem despite being few in number.
C. They primarily affect ecosystems through bottom-up control.
D. They are only found in marine environments like kelp forests.
✅ คำตอบที่ถูกต้อง
They have a large influence on the ecosystem despite being few in number.
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
