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托福听力学科背景知识：KEYSTONE SPECIES

2018-07-23 11:30:40来源：网络

　　托福考试中，听力部分是难度最大的部分。想要拿到这部分的高分成绩，大家在复习备考中，还是需要掌握一些提分的方法。今天新东方在线托福网为大家整理的是关于托福听力学科的背景知识，辅助大家更好地掌握这些重点知识，为接下来的听力备考做好基础。

【点击查看托福听力学科背景知识汇总】

　　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.

　　以上就是为大家整理的“托福听力学科背景知识：KEYSTONE SPECIES”，更多精彩内容请关注新东方在线托福频道!


本文关键字: 托福听力背景 托福听力学科 托福听力

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