A、Only a few of them cause problems to native species. B、They may turn out to benefit the local environment. C、Few of them can s

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?