In biology, the term 'keystone species' refers to a species that has a disproportionately large effect on its environment relative to its abundance. The presence of a keystone species helps maintain the structure and diversity of an ecosystem. If such a species is removed, the ecosystem can undergo dramatic changes, often leading to a loss of biodiversity. Classic examples include predators that control prey populations, or species that provide critical resources for others.
🎧 Conferencia de audio
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. When otters are present, they keep the urchin population in check. Urchins eat kelp, which forms underwater forests that shelter many fish and invertebrates. Without otters, urchin populations explode and they devour the kelp, creating 'urchin barrens'—areas with no kelp and much lower biodiversity. When otters were reintroduced, kelp forests recovered, and fish populations increased. So the otter's effect is huge relative to its numbers, exactly what we mean by a keystone species.
❓ Pregunta Using the example from the lecture, explain the concept of keystone species.
📊 Rúbrica TOEFL Speaking Task 3
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.
💡 Consejos
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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