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  3. In many parts of the world, alien species are harming the environment and causing other problems. An alien species is a species

In many parts of the world, alien species are harming the environment and causing other problems. An alien species is a species

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问题     In many parts of the world, alien species are harming the environment and causing other problems. An alien species is a species of plant or animal that has moved from its original home to a new area. Sometimes people have intentionally introduced the new species; sometimes it arrives accidentally, as a side effect of international trade. In its new location, the alien species has no natural enemies, so it can grow and multiply without limit. Over time—sometimes decades, sometimes a few years—the new species takes living space and food away from the native plants and animals. The result can mean drastic change in the natural landscape and problems for people.

    The tiger mosquito is an example of an alien species. This kind of mosquito is common in Asia, but in recent years it has moved to parts of southern Europe. Scientists believe that the eggs of this insect probably arrived inside some car tires that were filled with water. In a few years, the mosquitoes multiplied and spread over large areas of Italy and other countries. Tiger mosquitoes can reproduce faster than common European mosquitoes, so they quickly replaced a large percentage of the native mosquitoes. Though they don ’ t cause serious disease in humans, tiger mosquitoes are a serious nuisance. They are far more aggressive than common European mosquitoes, and the effects of their bites are worse.

    Another example of a different kind of animal that has recently moved to another continent is the zebra mussel. This small shellfish was first discovered in the Great Lakes of North America in 1986. It may have come over from Rvrssia on a cargo ship. In a very few years, zebra mussels had spread over all the Great Lakes and into many important rivers. They have grown into thick masses, covering many areas of lakes or river bottoms. They have also covered and closed up pipes of power stations and water treatment centers.
    Among the many examples of alien species of mammals that have caused damage is the Indian mongoose. This animal was brought to Hawaii in the 1800s to kill rats in sugarcane fields. Since then, they have grown in numbers so that today they are a major threat to bird life on the islands. One of the mongoose’ s favorite foods, in fact, is birds’ eggs. Thus, the mongoose has caused great damage to poultry farmers in Hawaii, and it has greatly reduced the numbers of many native bird species. Many of these species, which are unique to the islands, are now at risk of extinction.
  In what ways does the alien species get to a new area?
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答案Sometimes people have intentionally introduced it; sometimes it arrives accidentally, as a side effect of international trade.
解析 文章第一段第三句提到，有时人们有意引进新物种；有时作为国际贸易的副作用，新品种会随着而来。由此可知本题答案。
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