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 drastic reduction in biodiversity. These species play a critical role in maintaining the structure and balance of their habitat, much like a keystone in an arch—if removed, the entire structure collapses. Examples often include predators that control prey populations or species that provide essential resources for others.
🎧 Listening Lecture
Professor: Let me give you a classic example: the sea otter. In the kelp forest ecosystems along the Pacific coast, sea otters are a keystone species. They prey on sea urchins, which are herbivores that feed on kelp. Without sea otters, the urchin population explodes and they devour the kelp forests, leaving behind 'urchin barrens'—areas with almost no kelp. This destroys the habitat for fish, crabs, and other organisms that depend on kelp. However, when sea otters are present, they keep urchin numbers in check, allowing kelp to thrive. As a result, the entire ecosystem stays rich and diverse. So, the sea otter, though not the most numerous species, has a huge impact on its environment—exactly what defines a keystone species.
❓ Question 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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