Smoldering pastureland cleared for cattle from the Amazon rainforest, Rondonia State, Brazil

Photo by Michael Nichols/National Geographic/Getty Images

Extinction Today

Based on analysis of the fossil record, researchers estimate that most species on Earth have an overall life span of about 10 million years. It's an ongoing cycle of plants, animals and microscopic organisms appearing in the fossil record, remaining for about 10 million years and disappearing. Most likely, this is the natural state of life on Earth, regardless of exactly what species are alive.

However, live forms are dying out much faster today than they appear to at any point in the fossil record. As we discussed earlier, the background rate of extinction is somewhere between one and five species per year. But today, the extinction rate appears to be anywhere from 100 to 1,000 times greater than that [source: Holsinger]. Exact numbers are hard to pin down. No one knows exactly how many species are alive on Earth today. On top of that, it can be difficult to impossible to determine whether a plant or animal has died out. Even after exhaustive searches, researchers have declared some animals extinct only to find new specimens later.

The major cause of these extinctions isn't global warming or acid rain -- it's habitat loss. As the human population grows and more of the planet becomes industrialized, natural habitats for plants and animals disappear. As species that lived in these habitats die, the level of biodiversity decreases. The loss of plant and animal species can lead to everything from food shortages to poor soil quality. The loss of microscopic organisms can also play a role. For example, one theory about the Permian-Triassic extinction is that helpful marine bacteria became extinct, and the bacteria that flourished as a result produced hydrogen sulphide and caused acid rain.

Human behavior is also causing other ecosystem stresses, such as pollution, which could threaten species with extinction. Global changes,such as global warming, play a role in extinction as well. In theory, addressing these issues might slow the rate of extinction, but it's unclear how long it might take animal and plant populations to return to normal.

Regardless of whether a mass extinction is looming, researchers agree that the loss of biodiversity has a negative impact on ecosystems. To learn more about whether to expect a mass extinction in the near future, read Will we soon be extinct? If you'd like to know more about biodiversity, conservation and related topics, see the links on the next page.