Government’s private groups and indivisuals spend billions of dollars a year trying to route 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 nevr be acceptable in a 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 Rosenspeek， a professor of evolutionary biology at the University of Arizona。
“Because they depend on pegging a date and calling the plans on animals that show up after the date， exotic。”
Ecosystems free of species to find as exotic are by default consider the most natural。
“You can’t roll back the clock and remove all the exotics or fix habitats，” Dr。 Rosenspeek 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 the hand ”
At its core， the debate is about how to manage the world’s remaining natural ecosystems and 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 habitat loss， these evasive species represents the greatest threat to bio-diversity 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 of to 15 AD， represents what ecologist considered to have been a major shift in the spread of species including crops and life stock as they began to migrate with humans from continent to continent。
（18）“Only a small percentage of alien species calls problems in their new habitats，”said Don Smith， professor of ecology and evolutionary biology at the Univiersity of Tenacy。
“Of the 7000 alien species in the United States， out of a total of 150 species， only about 10% are invasive，” he pointed out， “The other 90% have fit into their environments and are considered naturalized。 ”
“Yet appearances can deceive， ”ecologiest caution， “and many these exotics may be considered acceptable only because no one has documented their harmful affects
What is more， non native species can appear harmless then turn invasive。”
16.What assumption about introduced species is challenged by a number of scientists？
17.What does Dr。 Michael Rosenspeek think of exotic native species ？
18。 What does Professor Don Smith say about alien species？
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