Five times in the earth's history, some three-quarters of all living species disappeared forever-and within a short period of time. Those species losses stand for a major change in the planet's ecology . Clues to these changes can be seen in the fossils and rock layers. Today, human activities are driving species to die out at a rate never seen before. And this loss of species really does matter, said Anita Narwani. She works at the University of Michigan in Ann Arbor. That's because the diversity of species in an ecosystem provides people with all types of services we can't do without. Trees provide oxygen for us to breathe. At the same time, they remove carbon from the air which is a cause of global warming. Plants help clean air and water. Animals move seeds from one area to another. A broad range of living resources also provides people with food, shelter and medicine. Narwani wanted to find out how important the diversity of species is for people. So she teamed up with other ecologists to review more than 1,700 studies. They found that more diverse parts of the earth tend to do well in things like removing carbon and providing us with wood and other natural resources. _ were also better at keeping fisheries large and healthy so that they could feed many people. Unfortunately, the diversity of species is in steep drop. It is too soon to say that the earth is undergoing a sixth mass extinction, Narwani said. She said a mass extinction means the loss of 75 percent of species over 2 million years or less. We haven't lost that many - at least not yet. But according to her, if species losses continue so fast, such a mass extinction could occur in just 300 years. The new findings highlight the potential for humans to use up many of the resources on which we now depend. As people learn more, however, they can take steps to lessen those risks. People should protect ecosystems so that they can continue to provide us with their services. Narwani and her team found that  _  .
many people know the importance of protecting wildlife many species die out because of human activities making good use of natural resources benefits people rich diversity of species greatly benefits people