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Recording One
录音一
Governments, private groups and individuals spend billions of dollars a year trying to root out non-native organisms that are considered dangerous to ecosystems, and to prevent the introduction of new intruders.
政府、私人团体以及个人每年都花费了大量的金钱来清除那些被认为会危害当地生态系统的外来生物，以及预防新入侵者的引入。
But a number of scientists question the assumption that the presence of alien species can never be acceptable in a natural ecosystem.
这一观点却受到了很多科学家的质疑，他们对这些外来物种永远也不会融入当地自然生态系统这一看法持怀疑态度。
They say that portraying introduced species as inherently bad is an unscientific approach.
他们说把外来物种描述成有害的是不科学的。
"Distinctions between exotic and native species are artificial," said Dr. Michael Rosenzweig, a professor of evolutionary biology at the University of Arizona,
亚利桑那大学进化生物学教授迈克尔·罗森茨维格博士说，外来物种和本地物种的区分是人为的。
"because they depend on picking a date and calling the plants and animals that show up after that date 'exotic.'"
“因为他们依赖于选择一个日期，把这一天结束后出现的植物和动物称为‘外来物种’”
Ecosystems free of species defined as exotic are, by default, considered the most natural.
默认情况下，没有外来物种的生态系统被认为是最自然的生态系统。
"You can't roll back the clock and remove all exotics or fix habitats," Dr. Rosenzweig said.
罗森·茨威格博士说：“你不能让时间倒流，消除所有外来物种，或者修复栖息地。”
"Both native and exotic species can become invasive,
“本土和外来物种都可能成为入侵物种，
and so they all have to be monitored and controlled when they begin to get out of hand."
因此，当它们开始失控时，所有物种都必须受到监视和控制。”
At its core, the debate is about how to manage the world's remaining natural ecosystems and about how, and how much, to restore other habitats.
这场辩论的核心是如何管理世界上现存的自然生态系统，如何以及多大程度上恢复其它栖息地。
Species that invade a territory can harm ecosystems, agriculture, and human health.
入侵物种可能会损害生态系统、农业和人类健康。
They can threaten some native species or even destroy and replace others.
它们可以威胁到一些本土物种，甚至摧毁和取代其它物种。
Next to habitat loss, these invasive species represent the greatest threat to biodiversity worldwide, many ecologists say.
许多生态学家表示，物种入侵仅次于栖息地的丧失，是全世界生物多样性的最大威胁。
Ecologists generally define an alien species as one that people, accidentally or deliberately, carried to its new location.
生态学家通常将外来物种定义为人们偶然或故意带到新地点的物种。
Across the American continents, exotic species are those introduced after the first European contact.
美洲大陆上的外来物种是在第一次接触欧洲后引进的。
That date, rounded off to 1500 AD, represents what ecologists consider to have been a major shift in the spread of species, including crops and livestock,
那一天，约在公元1500年，标志着生态学家所认为的物种（包括农作物和牲畜）传播的重大转变。
as they began to migrate with humans from continent to continent.
当他们开始和人类一起从一个大陆迁移到另一个大陆的时候。
"Only a small percentage of alien species cause problems in their new habitats,"said Don Smith, professor of ecology and evolutionary biology at the University of Tennessee.
“只有一小部分外来物种给新栖息地带来麻烦，”田纳西大学生态学和进化生物学教授唐·史密斯说。
"Of the 7,000 alien species in the United States-out of a total of 150,000 species-only about 10 percent are invasive," he pointed out.
他指出：“美国物种达15万，7000是外来物种，只有10%属于入侵物种。
The other 90 percent have fit into their environments and are considered naturalized.
另外的90%已经适应了环境，被归化了。
Yet appearances can deceive, ecologists caution,
然而，生态学家警告说，这些表象可能并不是真的，
and many of these exotics may be considered acceptable only because no one has documented their harmful effects.
而且，这些外来物种中有许多可能被认为是可接受的，只是因为没有人记载过它们的有害影响。
What is more, non-native species can appear harmless for decades, then turn invasive.
更重要的是，几十年内非本地物种可能会看起来无害，然后变得具有侵略性。
Questions 16 to 18 are based on the recording you have just heard.
根据你刚才听到的录音回答16-18题。
16. What assumption about introduced species is challenged by a number of scientists?
问题16：许多科学家对引进物种的假设提出了质疑。
17. What does Dr. Michael Rosenzwei think of exotic and native species?
问题17：迈克尔·罗森茨维格博士对外来和本地物种有何看法?
18. What does Professor Don Smith say about alien species?
问题18：关于外星物种，唐·史密斯教授说了些什么?
