Extinction is the end of existence of a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the ability to breed and recover may have been lost before that time. It is often difficult to determine the exact date of an extinction because a species' potential range can be very large.
Extinction, as with evolution, is a natural and nearly inevitable process. Isolated extinctions are quite common, but mass extinctions, in which large numbers of species become extinct in the span of a few years or centuries, are relatively rare events. More than 99 percent of the estimated five billion or more species that ever lived have become extinct during the roughly four billion years that life has existed on the earth. Natural causes include climate change, changes in sea levels, volcanic activity, meteorites, diseases and the spread of invasive species.
However, the rate of extinctions has been increasing rapidly during the past several hundred years as a result of human activity, and the incidences of extinctions caused by humans are vastly greater than those occurring naturally. Main factors in this human-caused mass extinction, termed the Holocene extinction, include overpopulation, habitat destruction, pollution and human-caused climate change. Some researchers forecast that up to half of presently existing plant and animal species could become extinct by the end of this century unless current trends in habitat destruction, pollution and climate change are reversed.