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, often leading to a collapse in biodiversity. Such species play a critical role in maintaining the structure of an ecological community, often through predation, competition, or providing resources for other species.
🎧 Listening Lecture
Professor: Let me give you a classic example. In the Pacific Northwest, the sea otter is a keystone species. Sea otters feed on sea urchins, which in turn graze on kelp forests. When otters were hunted to near extinction, sea urchin populations exploded, and they decimated the kelp forests. Without kelp, many fish and invertebrates lost their habitat and food sources, so the entire ecosystem collapsed. But when otters were reintroduced and protected, they controlled the urchin population, allowing kelp forests to recover. This brought back the fish and other species that depended on kelp. So the otter, though not abundant, has a huge impact on the whole community.
❓ 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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