问答题 《复合题被拆开情况》 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.