Keystone Species
Within a habitat, each species depends on other species, and contributes to the overall stability of that ecosystem. However, some species do more than others by providing essential services. Without the influence of these key species, the habitat changes significantly. Scientists refer to these important players in an ecosystem as keystone species. When a keystone species disappears from its habitat, the habitat changes dramatically. Their disappearance can then trigger the loss of other species. As some species vanish, others move in or become more abundant. The new mix of species changes the habitat's appearance and character.
听力材料：
Professor: Let's take the elephant, for example. Elephants are an important species in the African grasslands. Without them, the grasslands actually stop being grasslands at all if you can believe it—they change to forests. What happens is that in the grasslands some types of seeds other than grasses can sprout and begin to grow, which ... if they're left alone... they could eventually grow into shrubs or trees. But what happens is that elephants come along and eat the sprouting plants ... or the plants get crushed under the elephants' feet. And even if a plant or two manage to survive, it won't last long because sooner or later the elephant will knock it over or pull it out of the ground.
So what if the elephants weren't there and these plants were allowed to grow to maturity? Well pretty soon there'd be whole clusters of trees. Their branches and leaves would shade the grasses ... and without the sunlight, the grasses won't survive. So pretty soon the grass disappears, trees grow in its place and eventually the whole grassland changes to forest.
And as you can imagine the elephant has an impact on other animal species in the habitat as well. A lot of animals in this habitat rely on the grasses for food, for example. When the grasses disappear—when their food source disappears—these animals areeventually forced to leave. Gradually, some new species come into the habitat—species that are better suited to life in the forest. These new species replace the ones that left. So you can see the influence of the elephant on the environment is significant.
Question：
The professor gives examples of the effects of elephants on the African grasslands habitat. Using the examples from the talk, explain why elephants are considered a keystone species.
