Not many years ago,some farmers were worried because hawks were taking many of their chickens.The farmers did not know what to do.Finally they went to the officials and asked for help.
"Kill the hawks,"the officials said."We'll even pay you for them."
So the farmers began to think of ways to kill hawks.
The farmers killed many hawks.Before long they didn't have to warry about their chickens.But they now had a new worry.Field mice ate a lot of the farmer's grain.How did this happen?Hawks eat not only chickens but also field mice.They eat more field mice than chickens.But the farmers did not know this.When they killed a lot of hawks,they changed the balance of nature.
When people move into a new place,they often destroy many wild plants.Many of these plants are food for the animals.If the animals cannot find enough plants to eat,they will starue(饿死)and have to leave the place.In one part of the United States,for example,the deer there like to eat a certain kind of wild rose.The mountain lions there eat the deer.The number of deer,mountain lions and wild roses does not change much if people leave things as they are.
But people killed many mountain lions in order to protect the deer.Soon there were so many deer that they ate up all the wild roses.Finally the deer began to eat the green leaves of young trees.These trees were important to the farmers.
So the farmers thought of ways to protect their trees. Now the deer had nothing to eat,and many of them died.This was another lesson from nature.
It is importaint for us to keep the balance of nature.This the lesson we should remeber.