[简答题] Historians and philosophers agree that ever since the development of agriculture, human beings have had an effect on the natural habitat (动、植物生息地) of many plants and animal species. As human populations have increased, as many as 6,000 species are disappearing each year. A number 10,000 times greater than before man appeared on the planet. In one day alone, say scientists, some 45 kinds of plants and animals will die, some 17,500 species will be wiped out. In 1977, the United States Congress took an important step to help the ever-increasing number of endangered species. With the enforcement of the Endangered Species Act, some species have gained more public recognition than others. Elephants and whales, for example, have received far more media attention than have the smaller, less known, and less attractive species that are also endangered. Yet it is the tiny species, such as bugs and bacteria, that keep the planet in balance. Scientists are becoming even more concerned with