Tropical rainforests cover over an area of nearly 3 billion acres，or about 8.3 percent of the Earth‘s total 1and surface.These remarkable forests are shared by some 50 countries on five continents.Biologists believe that rainforests are the home of perhaps half the world’s biotic species，about five-sixths of which have not yet been described and named.
Throughout most of history，rainforests were considered to be remote，inaccessible，and unpleasant places，and as a result they were 1ittle affected by human activities.In the present century，however，rainforests have been exploited and ruined at a quickening pace，and in the last decade or so，tropical deforestation has become one of the Earth‘s most serious environmental problems.The rate of deforestation is spectacular—51 acres per minute;74，000 acres per day;27 million acres per year.More than half of the original African rainforest is now gone;about 45 percent of Asia’s rainforest no longer exists;the proportion in Latin America is approaching 40 percent.
As the forest goes，so goes its animal life.In the mid-1980s it was calculated that tropical deforestation was responsible for the extinction of one species per day;by 1990 it was estimated that the rate was one species per hour.
Moreover，loss of the forests contributes to increased soil erosion，drought，flooding，worsening water quality，declining agricultural productivity，and greater poverty for rural inhabitants.In addition，atmospheric carbon dioxide continues to be increased because there are fewer trees to absorb it and because burning of trees for forest clearing releases more to the air.
The irony of tropical deforestation is that the anticipated economic benefits are usually illusory.Much of the forest clearing，especially in Latin America，is in response to the social pressure of overcrowding and poverty in societies where most of the people are landless.The governments open up“new lands”for settlement in the rainforest.The settlers clear the land for crop growing or livestock raising.The result almost always is an initial one or two years of high soil productivity，followed by poor years as fertility declines and the soil begins to suffer from erosion.
The forests，of course，are renewable.If left alone by humans，they can regenerate，as long as there are seed trees in the neighborhood and the soil has not lost all its nutrients.The loss of biotic diversity，however，is much more serious.Extinction is an irrecoverable process.Valuable potential resources may disappear before they are even discovered.Natural genotypes that could be combined with agricultural crops or animals to resist disease，insects，parasites，and other environmental stresses may also be lost.Last，but not least，is the possibility that many small，isolated valuable groups of native people may be wiped out.
Much concern has been expressed about tropical deforestation，and some concrete steps have been taken.The development of agroforestry (planting crops with trees，rather than cutting down the trees and replacing them with crops) is being fostered in many areas.In Brazil，which has by far the largest expanse of rainforest，some 46，000 square miles of reserves have been set aside，and Brazilian law requires that any development in the Amazon region leave half of the land in its natural state.In 1985 a comprehensive world plan，sponsored by the World Bank，the World Resources Institute，and the United Nations Development Programme，was introduced.It proposes concrete，country-by-country strategies to combat tropical deforestation.It is an $ 8 billion，five-year project，dealing with everything concerning the protection of rainforests.
Meanwhile，the sounds of the axe and the chain-saw and the bulldozer continue to be heard throughout the tropical forest lands.
1. In the past，rainforests were nearly left intact because____.
[A]people then had a better sense of environmental protection
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