In biology, the term "keystone species" refers to a species that has a disproportionately large effect on its environment relative to its abundance. Removing a keystone species can cause a cascade of changes, drastically altering the ecosystem's structure and diversity. These species often maintain balance by controlling prey populations or providing critical resources for other organisms. Their presence is vital for the survival of many other species in the community.
🎧 듣기 강의
Professor: Let me give you a classic example—the sea otter in the kelp forest ecosystem. Sea otters are not numerous, but they play a huge role. They prey on sea urchins, which feed on kelp. Without otters, urchin populations explode and devour the kelp forests. Kelp provides habitat and food for fish, crabs, and other marine life. When kelp disappears, those species decline, and the whole ecosystem becomes barren. But when otters are present, they keep urchin numbers low, allowing kelp to thrive and supporting high biodiversity. So, the otter is a keystone species because its impact is far greater than its small population size would suggest.
❓ 문제 Using the example from the lecture, explain the concept of keystone species.
📊 TOEFL 말하기 평가 기준
Delivery: Fluency, pronunciation, intonation
Language Use: Grammar, vocabulary, sentence variety
Topic Development: Accurately explain the reading concept and restate the lecture examples
Scored 0-30 based on holistic scoring (original 0-4, scaled to 30). 30s preparation, 60s response.
💡 답변 팁
Note the core concept definition from the reading (usually 1-2 sentences)
Record 1-2 specific examples the professor uses in the lecture
Start with the concept definition, then explain with lecture examples
Use template phrases: "The reading defines... The professor illustrates this by..."
Keep within 60s; ensure clarity and fluency
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