In biology, the term 'keystone species' refers to a species whose impact on its ecosystem is disproportionately large relative to its abundance. Removing a keystone species can cause a cascade of changes, often leading to the collapse of the ecosystem's structure. Classic examples include predators that control prey populations, thereby maintaining the balance among multiple species. The concept highlights how a single organism can be vital for the survival of many others.
🎧 Listening Lecture
Professor: Let me give you a classic example from the Pacific coast. The sea otter is a keystone species. In areas where sea otters are present, they eat sea urchins. Sea urchins, in turn, feed on kelp forests. Without otters, urchin populations explode, and they devour the kelp, turning the lush underwater forest into a barren area. This destroys the habitat for fish, crabs, and other organisms that rely on kelp. But when otters are present, they keep urchin numbers in check, allowing kelp to thrive, which supports a diverse community. So, a relatively small number of otters has a huge effect on the entire ecosystem's health and biodiversity.
❓ 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
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