Extinction

A species is considered (globally) extinct once none of its members remain alive anywhere on Earth; if individuals survive only in captivity or under other human-controlled conditions, the species is described as extinct in the wild, and in both situations the species is treated as globally extinct in practical terms. A narrower category, locally extinct, applies when a species is no longer found in an area it once inhabited but continues to survive elsewhere in the wild. Across the roughly 450 million years of life on Earth, there have been five mass extinctions, each of which eliminated at least 50% of the flora and fauna species present at the time — extinction generally traces back to drastic environmental change combined with a population's own characteristics. The chapter identifies three types: natural extinction is a slow process in which existing species are gradually replaced by better-adapted ones as environmental and evolutionary conditions shift, or as predators and disease take their toll, with small populations especially vulnerable to going extinct sooner than larger ones through inbreeding depression (reduced adaptability and genetic variation); mass extinction results from major environmental catastrophes, such as the event roughly 225 million years ago in the Permian period that wiped out 90% of shallow-water marine invertebrates; and anthropogenic extinction is driven directly by human activity — hunting, habitat destruction, over-exploitation, urbanisation and industrialisation — with the Dodo of Mauritius and Steller's sea cow of Russia as historical examples, and amphibians today flagged as at particularly high risk because of ongoing habitat destruction. Extinction is considered the most serious aspect of biodiversity loss precisely because it is irreversible: the unique genetic information carried in a s …