In ecology, the concept of 'keystone species' refers to a species whose impact on its ecosystem is disproportionately large relative to its abundance. Removing a keystone species can trigger a cascade of changes, often leading to a dramatic loss of biodiversity or habitat structure. Such species play a critical role in maintaining the balance of the community, often through predation, competition, or providing resources that other organisms depend on.
🎧 Listening Lecture
Professor: Let me give you a classic example. In the Pacific Northwest, sea otters are a keystone species. They feed on sea urchins. Without otters, sea urchin populations explode. The urchins then overgraze on kelp forests, which provide habitat and food for fish, crabs, and other marine life. Once the kelp is gone, those species disappear, and the entire underwater ecosystem collapses. But when otters are present, they keep urchin numbers in check, allowing kelp forests to thrive. This supports a rich diversity of species. So, even though otters aren't numerous, their presence has a huge effect on the whole ecosystem.
❓ 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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