In biology, the concept of 'keystone species' refers to a species whose presence and role in an ecosystem have a disproportionately large effect on other organisms within the ecosystem. Keystone species help maintain the structure and health of their environment. If a keystone species is removed, the ecosystem can dramatically change or even collapse, leading to the loss of many other species. Their influence often comes from their unique feeding habits or behaviors that shape the habitat for others.
🎧 Listening Lecture
Professor: Let me give you a classic example: the sea otter along the Pacific coast. Sea otters eat sea urchins, which in turn feed on kelp forests. Without otters, sea urchins multiply rapidly and destroy the kelp, creating 'urchin barrens' where few other organisms can survive. But when otters are present, they keep urchin numbers in check, allowing kelp forests to thrive. Those forests then provide food and shelter for fish, crabs, and other marine life. So, the otter's role is small in terms of its numbers, but its impact is huge—it literally maintains the entire ecosystem's balance. This shows how a keystone species can shape its environment.
❓ Question Using the example from the lecture, explain the concept of keystone species.
📊 TOEFL Speaking Task 3 Rubric
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.
💡 Answering Tips
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
🎤 免费 AI 口语评分
基于 2800+ 托福真题评分库 · 无限次免费使用
⚠️ AI 自动评分基于托福官方口语三大维度（Delivery / Language Use / Topic Development）标准开发，结果仅供练习参考。
