单项选择题

Continue to Protect or Destroy Ecosystem
Biosphere Ⅱ was a spectacular failure. The gleaming glass-and-concrete habitat sprawling across the desert in Oracle, Arizona, was supposed to support eight human "biospherians" for two years. But the seal has to he broken before the experiment ended in 1993. Oxygen had fallen to levels normally seen at an elevation of 17,500 feet. Nitrous oxide had risen to the point where it threatened to cause brain damage. The fresh water supply became contaminated, and vines smothered (厚厚地覆盖) food plants. Insect pollinators (传授花粉的生物) and many other species became extinct. By the end, Biosphere Ⅱ was overrun with swarms of ants and cockroaches.
Scientists who gathered recently to review the Biosphere Ⅱ experiment reached a disturbing conclusion: "No one yet knows how to engineer systems that provide humans with the life-supporting services that natural ecosystems produce for free."
The problem is that these ecosystems are undergoing wrenching changes. Water and air quality, while improving in some regions, are deteriorating in many others. Carbon dioxide levels in the atmosphere arc climbing. The world’s population could reach 10 billion by 2050. And famed Harvard biologist Edward O. Wilson says the current rate of species losses puts us "in the midst of one of the great extinction spasms (突然进发) of geological history." All of which makes many ecologists wonder whether humans too will soon become extinct. It’s an incredibly important but incredibly difficult question. If we continue on this course, we’re heading for a world in which we will have to engineer services we’ve always received for free from nature. That’s why the failure of Biosphere Ⅱ was so disturbing: it proves that we don’t yet know how to do that.
The Biosphere Ⅱ experience demonstrated that maintaining human life is a tricky proposition-especially if we can no longer rely on the services provided by natural ecosystems. If we are currently living through a mass extinction, as Wilson believes, we should consider the past. In the great Permian extinction 245 million years ago, 96 percent of species perished. Eventually, the Earth was repopulated with a rich collection of new species, but it took 100 million years. "That should give pause to anyone who believes that what Homo sapiens (现代人) destroys, nature will redeem," Wilson says. "Maybe so, but not within any length of time that has meaning for contemporary humanity." Edward O. Wilson believes that the mass extinction happened in the past ______.