In biology, the concept of 'keystone species' refers to a species whose presence and role within an ecosystem has a disproportionately large effect on other organisms within the ecosystem. The removal of a keystone species can cause a cascade of changes, leading to a significant decline in biodiversity. Classic examples include predators that control prey populations, thereby preventing any single species from dominating and allowing many others to thrive. Their influence is not based on abundance but on their ecological function.
🎧 Лекция
Professor: Let me give you a concrete example from a Pacific coast rocky shore. The keystone species there is the sea otter. Sea otters feed on sea urchins. Now, if you remove the otters, what happens? Sea urchin populations explode. They overgraze on kelp forests, destroying the habitat that shelters fish, crabs, and other invertebrates. Within a few years, the once-rich kelp forest becomes a barren area with almost no marine life. But when otters are present, they keep urchin numbers low, allowing kelp to grow tall and dense. This kelp forest then supports a diverse community of species. So, you see, the otter—though not the most abundant—has a massive impact on the whole ecosystem's health and variety.
❓ Вопрос Question: 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。
