托福口语 Task 3

In biology, the term 'keystone species' refers to a species whose presence and activities have a disproportionately large effect on its ecosystem, such that its removal would cause significant changes in the structure and function of the community. Unlike dominant species, which are abundant and common, keystone species may be relatively low in numbers but play a critical role in maintaining biodiversity and ecological balance. The concept was introduced by ecologist Robert Paine in the 1960s, and it highlights how certain organisms, often predators, can influence the survival of many other species in their habitat.

Professor: So, let me give you a classic example of a keystone species. Think about sea otters living in kelp forests along the Pacific coast. These otters feed on sea urchins. Now, if the otters are removed—say, due to hunting or disease—the sea urchin population explodes. Sea urchins graze on kelp, and with so many urchins, they eat the kelp down to the roots. This destroys the entire kelp forest, which is home to many fish, crabs, and other marine life. Without the kelp, those species lose their habitat and food, so their numbers drop dramatically. But when otters are present, they keep urchin numbers in check, allowing kelp to thrive. This maintains a rich, diverse ecosystem. So, otters, though not the most numerous animals in the forest, are essential—they exert a huge influence far beyond what their size or numbers would suggest.