听力原文：

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. ［16］ But a number of scientists question the assumption that the presence of alien species can never be acceptable in the natural ecosystem. They say that portraying introduced species as inherently bad is an unscientific approach. ［17］ “Distinctions between exotic and native species are artificial,” said Dr.Michael Rosensberg, a professor of evolutionary biology at the University of Arizona. Because they depend on picking a date and calling the plants and animals that show up after that date exotic. “Ecosystem’s free of species to find as exotic are by default considered the most natural. You can’t roll back the clock and remove all exotics or fix habitats,” Dr.Rosensberg said. Both native and exotic species can become invasive and so they all have to be monitored and controlled when they begin to get out of hand. 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. Species that invade a territory can harm ecosystems, agriculture and human health. They can threaten some native species or even destroy and replace others. “Next to habitat loss, these invasive species represent the greatest threat to biodiversity worldwide,” many ecologists say. Ecologists generally define an alien species as one that people accidentally or deliberately carried to its new location. Across the American continents, exotic species are those introduced after the first European contact. That date rounded up to 1500 AD represents what ecologists consider to have been in major shift in the spread of species including crops and livestock, as they begin to migrate with humans from continent to continent. ［18］ “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 Tenacy. “Of the 7,000 alien species in the United States, out of a total of 150,000 species, only about 10% are invasive,” he pointed out. The other 90% have fit into their environment and are considered naturalized. Yet appearances can deceive, ecologists caution, and many of these exotics may be considered acceptable only because no one has documented their harmful effects. What is more, non-native species can appear harmless for decades then turn invasive.
Questions 16 to 18 are based on the recording you have just heard.
16.What assumption about introduced species is challenged by a number of scientists?
17.What does Dr.Michael Rosensberg think of the exotic and native species?
18.What does Professor Don Smith say about alien species?

解析：浏览四个选项，根据选项中出现的 native species，region’s biodiversity， local ecosystem，crossbreed 等词可以推测，该题应该和物种、生态系统有关。题干问的是 一些科学家质疑关于外来物种的什么假设。讲座中提及，许多科学家质疑在自然生态系统中 外来物种的存在是不可接受的假设，说明他们并不认为外来物种会构成威胁，故答案为 C(它 们会对当地生态系统构成威胁)。A 项(它们会使本土物种都灭绝)、B 项(它们促进了当地的生 物多样性)和 D 项(它们会和本土物种杂交)均不符合讲座内容，故排除。

浏览四个选项，根据选项中出现的 their，classifications，interactions，definitions，distinctions 等词可以推测，该题应该涉及某 群体的分类、定义或区别等。题干问的是迈克尔-罗森博格博士如何看待外来物种和本土物 种。讲座中提及，亚利桑那大学的进化生物学教授迈克尔-罗森博格博士认为，外来物种与 本土物种之间的区别是人为定义的，故答案为 D 项(它们之间的区别是人为定义的)。文中提 到，罗森博格博士认为本土物种和外来物种都可能成为入侵物种，由此可知它们的分类是没 有意义的，故排除 A 项(它们之间的分类是有意义的)。B 项(它们之间的相互作用很难界定) 和 C 项(它们的定义是可以改变的)原文未提及，故排除。

浏览四个选项，根据选项中出现的 cause problems to native species，benefit the local environment，survive，naturalized 等词可以 推测，该题可能和外来物种有关。题干问的是唐·史密斯教授提到关于外来物种的什么内容。 讲座中提及，生态学和进化生物学教授唐·史密斯认为，只有少量的外来物种会给它们的新 栖息地造成问题，故答案为 A(它们之中只有少量物种会对本土物种造成问题)。B 项(它们可 能会对当地环境有益处)在文中并未提及，故排除。C 项(它们之中只有很少一部分能在新栖 息地生存下去)和 D 项(只有 10％的物种能被驯化)不符合讲座内容，故排除。