Narrator: Listen to part of a lecture in an environmental science class. Professor: Today we're going to talk about a concept that's crucial for understanding how ecosystems function—keystone species. Now, a keystone species is one that has a disproportionately large effect on its environment relative to its abundance. Think of it like the keystone in an arch—if you remove it, the whole structure collapses. A classic example is the sea otter. In the kelp forest ecosystems off the Pacific coast, sea otters prey on sea urchins. Sea urchins feed on kelp, and if their population explodes, they can decimate the kelp forest. Kelp forests provide habitat and food for many marine species. When otters were hunted to near extinction in the 19th century, the urchin population soared, and the kelp forests disappeared. But when otters were protected and reintroduced, the kelp forests recovered, and biodiversity increased. This shows how a single predator can shape an entire ecosystem. Now, can anyone think of another example? Maybe a terrestrial one?
❓ 题目 What is the main point the professor makes about keystone species?
A. A) They are the most numerous species in an ecosystem.
B. B) They have a small effect despite their large numbers.
C. C) They have a large impact on their ecosystem out of proportion to their numbers.
D. D) They are always top predators like sea otters.
✅ 正确答案
C) They have a large impact on their ecosystem out of proportion to their numbers.
