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  3. "Exotic and Endangered Species" When you hear someone bubbling enthusiastically about an exotic species, you can safely bet t

"Exotic and Endangered Species" When you hear someone bubbling enthusiastically about an exotic species, you can safely bet t

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问题 "Exotic and Endangered Species"
   When you hear someone bubbling enthusiastically about an exotic species, you can safely bet the speaker isn’t an ecologist. This is a name for a resident of an established community that was deliberately or accidentally moved from its home range and became established elsewhere. Unlike most imports, which can’t take hold outside their home range, an exotic species permanently insinuates itself into a new community.
   Sometimes the additions are harmless and even have beneficial effects. More often, they make native species endangered species, which by definition are extremely vulnerable to extinction. Of all species on the rare or endangered lists or that recently became extinct, close to 70 percent owe their precarious existence or demise to displacement by exotic species. Two examples are included here to illustrate the problem.
   During the 1800s, British settlers in Australia just couldn’t bond with the koalas and kangaroos, so they started to import familiar animals from their homeland. In 1859, in what would be the start of a wholesale disaster, a northern Australian landowner imported and then released two dozen wild European rabbits (Oryctolagus cuniculus). Good food and good sport hunting—that was the idea. An ideal rabbit habitat with no natural predators was the reality.
   Six years later, the landowner had killed 20,000 rabbits and was besieged by 20,000 more. The rabbits displaced livestock, even kangaroos. Now Australia has 200 to 300 million hippityhopping through the southern half of the country. They overgraze perennial grasses in good times and strip bark from shrubs and trees during droughts. You know where they’ve been; they transform grasslands and shrublands into eroded deserts. They have been shot and poisoned. Their warrens have been plowed under, fumigated, and dynamited. Even when all-out assaults reduced their population size by 70 percent, the rapidly reproducing imports made a comeback in less than a year. Did the construction of a 2,000-mile-long fence protect western Australia? No. Rabbits made it to the other side before workers finished the fence.
   In 1951, government workers introduced a myxoma virus by way of mildly infected South American rabbits, its normal hosts. This virus causes myxomatosis. The disease has mild effects on South American rabbits that coevolved with the virus but nearly always had lethal effects on O. cuniculus. Biting insects, mainly mosquitoes and fleas, quickly transmit the virus from host to host. Having no coevolved defenses against the novel virus, the European rabbits died in droves. But, as you might expect, natural selection has since favored rapid growth of populations of O. cuniculus resistant to the virus.
   In 1991, on an uninhabited island in Spencer Gulf, Australian researchers released a population of rabbits that they had injected with a calcivirus. The rabbits died quickly and relatively painlessly from blood clots in their lungs, hearts, and kidneys. In 1995, the test virus escaped from the island, possibly on insect vectors. It has been killing 80 to 95 percent of the adult rabbits in Australian regions. At this writing, researchers are now questioning whether the calcivirus should be used on a widespread scale, whether it can jump boundaries and infect animals other than rabbits (such as humans), and what the long-term consequences will be.
   A vine called kudzu (Pueraria lobata) was deliberately imported from Japan to the United States, where it faces no serious threats from herbivores, pathogens, or competitor plants. In temperate parts of Asia, it is a well-behaved legume with a well-developed root system. It seemed like a good idea to use it to control erosion on hills and highway embankments in the southeastern United States. A   With nothing to stop it, though, kudzu’s shoots grew a third of a meter per day. Vines now blanket streambanks, trees, telephone poles, houses, and almost everything else in their path. Attempts to dig up or burn kudzu are futile. Grazing goats and herbicides help, but goats eat other plants, too, and herbicides contaminate water supplies. B   Kudzu could reach the Great Lakes by the year 2040.
   On the bright side, a Japanese firm is constructing a kudzu farm and processing plant in Alabama. The idea is to export the starch to Asia, where the demand currently exceeds the supply. C   Also, kudzu may eventually help reduce logging operations. D  At the Georgia Institute of Technology, researchers report that kudzu might become an alternative source for paper.
Why does the author mention "mosquitoes and fleas" in paragraph 5?
选项 A、Because they are the origin of the myxoma virus
B、Because they carry the myxoma virus to other animals
C、Because they die when they are infected by myxoma
D、Because they have an immunity to the myxoma virus
答案B
解析 "Biting insects, mainly mosquitoes and fleas, quickly transmit the virus from host to host." Choice A is not correct because South American rabbits are the normal hosts for the myxoma virus. Choice C is not correct because it is the O. cuniculus rabbit that dies when infected. Choice D is not correct because resistant populations of O. cuniculus rabbits, not fleas, have an immunity to the virus.
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