In biology, 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 cause significant changes in the ecosystem's structure and biodiversity, often leading to the collapse of the entire system. These species play a critical role in maintaining the balance among other species, and their presence ensures the survival of many others.
🎧 Лекция
Professor: Consider the sea otter, a classic example of a keystone species. In kelp forest ecosystems along the Pacific coast, sea otters feed on sea urchins. Without otters, the urchin population explodes and consumes vast amounts of kelp, destroying the forest. This kelp provides habitat and food for many fish and invertebrates. When otters were hunted to near extinction in the 1800s, the kelp forests disappeared, and the fish populations crashed. But when otters were reintroduced, they controlled the urchins, and the kelp forests recovered, bringing back the entire ecosystem. So, the otter's presence or absence dramatically shaped the whole community, despite being relatively few in number compared to urchins and kelp.
❓ Вопрос Using the example from the lecture, explain the concept of keystone species.
📊 Критерии оценки 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.
💡 Советы
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
🎤 免费 AI 口语评分
基于 2800+ 托福真题评分库 · 无限次免费使用
⚠️ AI 自动评分基于托福官方口语三大维度（Delivery / Language Use / Topic Development）标准开发，结果仅供练习参考。
