Despite decades of scientific research, no

问题：

Despite decades of scientific research, no one yet knows how much damage human activity is doing to the environment. Humans are thought to be responsible for a whole host of environmental problems, ranging from global warning to ozone depletion. What is not in doubt, however, is the devastating effect humans are having on the animal and plant life of the planet.

Currently, an estimated 50,000 species become extinct every year. If this carries on, the impact on all living creatures is likely to be profound, says Dr. Nick Middleton, a geographer at Oxford University. " All species depend in some way on each other to survive. And the danger is that, if you remove one species from this very complex web of interrelationships, you have very little idea about the knock-on effects of other extinctions. "

Complicating matters is the fact that there are no obvious solutions to the problem. Unlike global warning and ozone depletion—which, if the political will was there, could be reduced by cutting gas emissions—preserving biodiversity remains an intractable problem.

The latest idea is " sustainable management " , which is seen as a practical and economical way of protecting species from extinction. This means humans should be able to use any species of animal or plant for their benefit, provided enough individuals of that species are left alive to ensure its continued existence.

For instance, instead of depending on largely ineffective laws against poaching, it gives local people a good economic reason to preserve plants and animals. In Zimbabwe, there is a sustainable management project elephants. Foreign tourists pay large sums of money to kill these animals for sport. This money is then given to the inhabitants of the area where the hunting takes place. In theory, locals will be encouraged to protect elephants, instead of poaching them—or allowing others to poach them—because of the economic benefit involved.

This sounds like a sensible strategy, but it remains to be seen whether it will work. With corruption endemic in many developing countries, some observers are skeptical that the money will actually reach the people it is intended for．Others wonder how effective the locals will be at stopping poachers.

There are also questions about whether sustainable management is practical when it comes to protecting areas of great-bio-diversity such as the world’s tropical forests. In theory, the principle should be the same as with elephants—allow logging companies to cut down a certain number of trees, but not so many as to completely destroy the forest.

Sustainable management of forests requires controls on the number of trees which are cut down, as well as investment in replacing them. But because almost all tropical forests are located in countries which desperately need revenue from logging, there are few regulations to do this. Moreover, unrestricted logging is so much more profitable that wood prices from managed forests would cost up to five times more—an increase that consumers, no matter how " green " , are unlikely to pay.

For these reasons, sustainable management of tropical forests is unlikely to become widespread in the near future. This is disheartening news. It’s estimated these forests contain anything from 50 to 90 percent of all animal and plant species on Earth. In one study of a five-square-kilometer area of rain forest in Peru, for instance, scientists counted 1,300 species of butterfly and 600 species of bird. In the entire continental United States, only 400 species of butterfly and 700 species of bird have been recorded.

Scientist Professor Norman Myers sees this situation as a gigantic " experiment we’re conducting with our planet " . " We don’t know what the outcome will be. If we make a mess of it, we can’t move to another planet…It’s a case of one planet, one experiment.

Why is sustainable management of tropical forests unlikely to be used in the near future（）

A. Because tropical forests contain a majority of animal and plant species on Earth

B. Because we have to wait until the experiment in Peru and the U. S. is finished

C. Because we have no idea about its effect, and if it fails, our planet will be destroyed

D. Because besides tropical forests, other areas such as the U. S. also have biodiversity