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托福听力考试背景知识综合辅导(三十六)

来源：网络发布时间：2011-02-18

下面是托福听力背景知识，育路教育网特别为您搜集整理，内容如下：

KEYSTONE SPECIES

A keystone is the stone at the top of an arch that supports the other stones and keeps the whole arch from falling. A keystone species is a species on which the persistence of a large number of other species in the ecosystem depends.

If a keystone species is extirpated from a system， the species it supported also will disappear， as will the other dependent species. Keystone species may be top carnivores that keep prey in check， large herbivores that shape the habitat in which other species live， important plants that support particular insect species that are prey for birds， bats that disperse the seeds of plants， and many other types of organisms.

An example of a keystone predator is the sea otter， which is a keystone in the kelp forest ecosystem. Kelp forests are marine forests of seaweed that provide food and shelter for large numbers of species of fish and shellfish. They also protect coastlines from damaging wave action. Sea otters prey on sea urchins and keep their numbers in check.

If the sea otters are extirpated， large numbers of sea urchins feed heavily on the kelp forests， causing severe declines in the kelp forests and their associated species. Wherever otters have been reintroduced， kelp communities have returned. Ironically， many fishers resent sea otters because they view them as competitors for valuable commercial fish and shellfish， such as abalone. They do not realize that without the otters， the abalone would not have kelp forest habitat.

Protecting keystone species is a priority for conservationists. Unfortunately， the keystone functions of a species may not be known until it has been extirpated and the ecosystem changes. Keystone species may occur at any level of the ecosystem， from plants and herbivores （plant eaters）， to carnivores （meat eaters）， and detritivores （waste eaters）。 Where a keystone species has been identified， efforts to protect it also will help protect the other species in delicately balanced ecosystems.

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