Extinction is the disappearance of every living member of a species or other biological group. A species becomes globally extinct when its last individual dies, although the exact time is usually unknown. Extinction removes lineages from biodiversity and, together with speciation, shapes the changing diversity of life through evolution. It occurs naturally, but human activities have substantially increased extinction risks across many groups of organisms. (naturalhistory.si.edu)
Definitions and distinctions
Global extinction differs from local extinction, also called extirpation: a species disappears from a particular area but survives elsewhere. The IUCN Red List distinguishes Extinct (EX) from Extinct in the Wild (EW). The latter describes organisms surviving only in captivity, cultivation, or naturalized populations outside their former range. Declining abundance or a threatened classification does not, by itself, mean that extinction has occurred. (nc.iucnredlist.org)
An Extinct classification requires sufficient evidence that no individuals remain. Assessment considers searches of known and expected habitats, appropriate survey seasons, and the organism’s life cycle. Failure to observe a rare species is not automatically proof of its disappearance. “Possibly Extinct” is a supplementary tag applied to some Critically Endangered species, rather than a separate Red List category. (nc.iucnredlist.org)
Causes and population processes
Extinction can result from habitat loss, environmental change, competition, predation, disease, or catastrophic events. These influences often interact rather than operate independently. Human-driven pressures include land and sea use change, direct exploitation, climate change, pollution, and invasive species. Their relative importance differs among environments and groups of organisms. (nps.gov)
Population size affects vulnerability. In small populations, chance differences in births, deaths, and reproductive success—demographic stochasticity—can cause decline even when average conditions appear favorable. Environmental fluctuations add risks that may affect many individuals simultaneously. At low density, difficulty finding mates or maintaining cooperative defenses can reduce reproductive success or survival; such density-dependent disadvantages are called Allee effects. (bpb-us-w2.wpmucdn.com)
Genetic processes can reinforce these demographic problems. Genetic drift can remove beneficial variants by chance, while inbreeding can expose harmful recessive variants. Loss of genetic diversity may restrict future adaptive responses. Interactions between falling abundance, reduced reproductive performance, and genetic deterioration can produce an extinction vortex, although the contribution of each mechanism varies among populations. (bpb-us-w2.wpmucdn.com)
Extinctions may also lag behind environmental disturbance. Extinction debt refers to future species losses associated with earlier habitat destruction or fragmentation: organisms persist temporarily even though changed conditions no longer support their long-term survival. Consequently, the presence of a species after disturbance need not indicate a stable population. (nature.com)
Background and mass extinction
Background extinction is the ongoing disappearance of species outside exceptional global crises. Over geological time, extinction and the formation of new species continually alter biological diversity. A mass extinction is an unusually large loss of biodiversity over a geologically short interval, affecting multiple groups across broad geographical areas. (nps.gov)
Five particularly prominent events are conventionally called the “Big Five”: the end-Ordovician, Late Devonian, end-Permian, end-Triassic, and end-Cretaceous extinctions. They differed in duration, affected organisms, and environmental causes; the Late Devonian crisis, for example, involved multiple episodes rather than one instantaneous event. (nps.gov)
The end-Permian extinction, approximately 252 million years ago, was the largest of these events. Evidence connects it with massive volcanism in Siberia and associated warming and atmospheric changes. The end-Cretaceous extinction, approximately 66 million years ago, eliminated non-avian dinosaurs and numerous marine organisms. Its principal cause was the environmental disruption following a large asteroid impact near present-day Mexico. Birds survived, so dinosaurs did not disappear in their entirety. (naturalhistory.si.edu)
Mass extinctions reshape evolutionary opportunities. Surviving groups may diversify into ecological roles previously occupied by vanished organisms. This subsequent diversification changes community composition; it does not restore the particular lineages that were lost. (nps.gov)
Evidence and measurement
Paleontology reconstructs ancient extinctions from the fossil record. Fossil preservation is uneven, and organisms with durable skeletons are disproportionately represented. Much of the evidence for the Big Five therefore comes from marine invertebrates. Estimates based on species, genera, or families measure different levels of loss and should not be treated as interchangeable. (nps.gov)
For recent extinctions, evidence includes field surveys, specimens, historical observations, and records of captive individuals. A well-documented example is the passenger pigeon: Martha, the last known individual, died at Cincinnati Zoo on September 1, 1914. Commercial hunting had devastated a species formerly numbering in the billions. (si.edu)
Extinction rates can be expressed as extinctions per million species-years, allowing comparisons among groups of different sizes. Estimates remain uncertain because many species are undescribed, poorly monitored, or difficult to detect. Recorded extinctions are therefore not a complete inventory of actual losses. (ipbes.net)
Contemporary assessment and conservation
In its 2019 global assessment, IPBES estimated that approximately one million animal and plant species were threatened with extinction. This was an estimate of risk, not a count of species already extinct. It drew on assessed threat levels and estimates of total species diversity, rather than a complete examination of every species. (ipbes.net)
Conservation biology studies how species persist and how interventions affect extinction risk. Its work includes habitat management, population monitoring, captive breeding, and analysis of demographic and genetic processes. Red List assessments supply a standardized account of threat, while population research examines the mechanisms behind decline. Preserving living populations and recording confirmed extinctions are distinct tasks: one concerns persistence, the other establishes that a lineage has disappeared. (naturalhistory.si.edu)