An understanding of man's effect on the balance of nature is crucial(关键的)to being able to find the appropriate remedial action. It is a very common belief that the problems of the population explosion are caused mainly by poor people living in poor countries who do not know enough to limit their reproduction. This is not true. The actual number of people in an area is not as important as the effect they have on nature. Developing countries do have an effect on their environment，but it is the populations of richer countries that have a far greater impact on the earth as a whole. For example，tropical forests are known to be essential to the balance of nature，yet we are destroying them at a great rate. They are being cleared not to benefit the natives of that country，but to satisfy the needs of rich countries. Central American forests are being destroyed for grassland to make pet food in the United States cheaper；in Papua New Guinea，forests are destroyed to supply cheaper cardboard packaging for Japanese electronic products；in Burma and Thailand，forests have been destroyed to produce mole attractive furniture in Singapore and Hong Kong. Therefore，a rich person living thousands of miles away may cause more tropical forest destruction than a poor person living in the forest itself. In short，it is everybody's duty to safeguard the future of mankind-not only through population control，but by being more aware of the effect his actions have on nature. Nature is both fragile and powerful. It is very easily destroyed；on the other hand，it can easily destroy its most dangerous enemy-man. According to the first paragraph，people usually believe that______.