Question 17 is based on the recording you have just heard.

Recording One

    (16) 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. But a number of scientists question the assumption that the presence of alien species can never be acceptable in a natural ecosystem. They say that portraying introduced species as inherently bad is an unscientific approach. (17) “Distinctions between the exotic and native species are artificial,” said Dr. Micheal Rosenzweig, 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’.” Ecosystems free of species defined as exotic are, by default, considered the most natural. “You can’t roll back the clock and remove all exotics or fix habitats,” Dr. Rosenzweig 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 off to 1500 AD, represents what ecologists consider to have been a major shift in the spread of species, including crops and livestock, as they began 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 Tennessee. “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. The other 90 percent have fit into their environments 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 there harmful effects. What is more, non-native species can appear harmless for decades, then turn invasive.

17. What dose Dr. Micheal Rosenzweig think of exotic and native species?

解析：D。本题目为细节题。问题为：Micheal Rosenzweig博士对外来物种和本土物种的看法是什么？根据原文可知，Micheal Rosenzweig博士说，外来物种和本地物种之间的区别是人为的。故正确答案为D。