托福口语 Task 3

In biology, the term 'keystone species' refers to a species that has a disproportionately large effect on its environment relative to its abundance. Removing a keystone species from an ecosystem can cause a cascade of changes, often leading to the collapse of the ecosystem's structure. The classic example is the sea otter, which controls sea urchin populations. Without otters, urchins overgraze kelp forests, destroying habitat for many other marine species.

Professor: So, let me give you a real example from a study I worked on. In a coastal area, researchers removed all the starfish from a tidal pool. At first, nothing seemed to happen. But within a few months, the mussel population exploded because starfish normally prey on mussels. The mussels then crowded out other species like barnacles and algae. Within a year, the diversity of the pool dropped dramatically. When the starfish were reintroduced, the mussel numbers decreased, and the other species gradually returned. This shows how a single predator, though not the most numerous, can hold the whole system in balance.