Not many years ago, some American farmers began to complain about hawks . The hawks were killing chickens. The farmers did not know what to do. They went to the government officials and asked for help. The officials said, " We'll pay you for each hawk you kill." The farmers began to kill the hawks. Many hawks were killed, but the farmers now had another problem. The field mice were growing in number and they were eating up the farmers' corn. Hawks eat not only chickens but also field mice. They eat more field mice than chickens. But if the farmers had known that, they would not have upset  the balance of nature. In one part of the USA, there are deer which eat a kind of wild rose. The deer are eaten by mountain lions. The number of deer, mountain lions, and wild rose does not change much if people stay away from them. If there are too many deer, mountain lions will increase rapidly in number and kill many deer. If there are too many mountain lions, there will be fewer deer and there will be more roses. It is always necessary for us to keep the balance of nature. The government once killed almost all the mountain lions to protect the deer. Soon there were so many deer that they ate up all the wild roses. Then they began to eat the green leaves of young trees which were important to the farmers. So the farmers protected their trees from the deer. The deer had nothing to eat, and many of them died. The government learned a valuable lesson from nature. Why did American people have to protect their trees?
Because the mountain lions destroyed the trees. Because there were so many deer that they began to eat the green leaves. Because they wanted the trees to grow well. Because the trees are more important than the deer.