1. In the last century, because sparrows were listed as one of the four pests, the whole people launched a vigorous campaign to completely eliminate sparrows. Besides poisoning, catching and killing, we also use the method of bombing. Because of the loss of sparrows, crop pests have caused disasters, resulting in crop failure, followed by hunger, and human beings have tasted the bitter fruit of destroying the biological chain.

2. China introduced water hyacinth as feed. Water hyacinths seize the water surface all over the country, affecting shipping, choking fish, endangering health, and then flooding, especially in Fujian, Taiwan Province, Guangdong and other places.

In order to protect deer, a place in America captured and killed all the wolves in the forest. After the wolves were eliminated, the whole forest was destroyed because of the massive reproduction of deer in the forest. Finally, for various reasons, deer without natural enemies are healthy.