Extinction and Extinction Risk
How and why species go extinct, the rates at which they are being lost today, and the traits and processes that make some species more vulnerable than others.
Definition
Extinction is the permanent loss of a species, occurring when its last individual dies. Extinction risk is the probability that a species or population will go extinct within a given period, shaped by population size, range, threats, and intrinsic biological traits.
Scope
Covers the process of extinction and the assessment of extinction risk: background versus current extinction rates, mass extinctions in the geological record, the traits that predispose species to extinction, and the small-population processes that drive declining populations toward loss. Includes the extinction vortex concept. Excludes the specific external drivers treated in sibling topics and the genetic-rescue responses treated under conservation genetics.
Core questions
- How do current extinction rates compare to the natural background rate?
- What is a mass extinction, and is one underway now?
- Which traits make species especially vulnerable to extinction?
- How do small-population processes accelerate decline toward extinction?
Key concepts
- Background extinction rate
- Mass extinction
- Extinction vortex
- Demographic and environmental stochasticity
- Extinction-prone traits
- Minimum viable population
Key theories
- Background versus elevated extinction rates
- By comparing modern documented losses with the rate inferred from the fossil record, researchers estimate current extinction rates are far above background, supporting the case that human activity has triggered a sixth mass extinction.
- Extinction vortex
- Small populations are caught in self-reinforcing feedbacks among demographic stochasticity, inbreeding, and loss of genetic variation that progressively reduce numbers and adaptability, spiralling toward extinction.
Clinical relevance
Quantifying extinction risk underlies the IUCN Red List and triage decisions about where to direct scarce conservation resources. Identifying extinction-prone traits — large body size, small range, low reproductive rate, specialization — helps anticipate which species need intervention before they decline to critical levels.
History
The reality of extinction was established in the early nineteenth century by Cuvier. Identification of the 'Big Five' mass extinctions and the asteroid hypothesis for the end-Cretaceous event in 1980 framed extinction as a recurrent macroevolutionary process. From the 1990s, conservation biologists quantified modern rates and articulated the small-population paradigm and the extinction vortex.
Debates
- How reliable are modern extinction-rate estimates?
- Because most species are undescribed and many extinctions go unrecorded, estimates depend on extrapolation and assumptions; debate continues over the precise magnitude, even as most analyses agree rates are far above background.
Key figures
- David Raup
- Anthony Barnosky
- Gerardo Ceballos
- Stuart Pimm
Related topics
Seminal works
- ceballos2015
- pimm2014
- primack2014
Frequently asked questions
- What is the extinction vortex?
- A downward spiral in which small populations suffer from chance fluctuations in births and deaths, inbreeding, and loss of genetic diversity. These factors reinforce one another, further shrinking the population until it goes extinct.
- Are we really in a sixth mass extinction?
- Many scientists argue yes: current extinction rates appear to be hundreds to a thousand times the natural background rate. The exact figure is debated because of undescribed species and unrecorded losses, but the evidence points to an exceptional, human-driven extinction episode.