Listen to part of a lecture in an environmental science class. Professor: Today we're discussing the role of 'keystone species' in ecosystems. A keystone species is one that has a disproportionately large effect on its environment relative to its abundance. The classic example is the sea otter. In kelp forest ecosystems along the Pacific coast, sea otters prey on sea urchins. Without otters, urchin populations explode and overgraze on kelp, destroying the habitat for many fish and invertebrates. When otters are present, they keep urchin numbers in check, allowing kelp forests to thrive. This, in turn, supports biodiversity. Interestingly, recent studies show that the reintroduction of otters in some areas has led to a recovery of kelp forests within a few years. However, it's not always simple. In some places, orcas have begun preying on otters, causing a cascade effect. So, the concept reminds us that removing or adding a single species can trigger dramatic changes across the whole ecosystem. Any questions?
❓ Вопрос What is the main point the professor makes about keystone species?
A. A) They are usually the most abundant species in an ecosystem.
B. B) They have a large impact on the ecosystem despite their relatively low numbers.
C. C) They only affect marine environments like kelp forests.
D. D) Their removal rarely causes changes in other species.
✅ Правильный ответ
B) They have a large impact on the ecosystem despite their relatively low numbers.
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
