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, dramatically altering the ecosystem's structure and biodiversity. Classic examples include predators that control prey populations, thereby maintaining balance among multiple species. The concept highlights how certain organisms are critical for the stability of their ecological community, even if they are not the most numerous.
🎧 リスニング講義
Professor: So, let me give you a real example. In the Pacific Northwest, sea otters are a keystone species. They eat sea urchins. Without otters, urchin populations explode, and they devour kelp forests. Kelp provides habitat for fish, crabs, and other marine life. When otters were hunted to near extinction, the kelp forests disappeared, and the ecosystem collapsed. But when otters were reintroduced, they controlled the urchins, allowing kelp to regrow. Soon, fish and other species returned. So, you see, the presence or absence of just one species—the otter—completely changed the entire ecosystem. That's why we call it a keystone species.
❓ 質問 Question: Using the example from the lecture, explain the concept of keystone species.
📊 TOEFLスピーキング評価基準
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.
💡 解答のヒント
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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