IUCN Red List Assessment

The IUCN Red List classifies species according to their risk of extinction in the wild. Assessors apply standardized evidence and five quantitative criteria concerning population change, geographic range, population size and modelled extinction probability to determine an extinction-risk category.

Evidence used in assessment

Assessors compile the best available information on the species across its range, including abundance, distribution, trends, habitat, threats and life history. Data may come from field surveys, monitoring, scientific literature, museum records, remote sensing and expert knowledge, with uncertainty documented rather than ignored.

• The assessment uses a defined time horizon, often linked to generation length, because population decline has different significance in short-lived and long-lived species.
• An assessment records its evidence, assumptions, geographic scope and justification, and is reviewed through the IUCN Red List process.

Five quantitative criteria

A species is tested against all applicable criteria. If it meets more than one category, the highest extinction-risk category supported by any criterion is assigned; satisfying any one criterion is sufficient.

• Criterion A measures population reduction over a specified period.
• Criterion B examines restricted geographic range together with fragmentation, decline or extreme fluctuations.
• Criterion C considers a small population that is continuing to decline.
• Criterion D covers very small or geographically restricted populations.
• Criterion E uses quantitative analysis, such as population viability analysis, to estimate extinction probability.

Categories and their meaning

The framework arranges species by their estimated risk of extinction in the wild. The quantitative criteria distinguish the threatened categories, while the other categories indicate lower risk, extinction, inadequate data or absence of evaluation.

• Critically Endangered, Endangered and Vulnerable indicate, respectively, an extremely high, very high and high risk of extinction in the wild; together they constitute the threatened categories.
• Near Threatened applies when a species does not currently qualify as threatened but is close to qualifying or is likely to qualify in the near future.
• Least Concern applies when an evaluated species does not qualify for Critically Endangered, Endangered, Vulnerable or Near Threatened.
• Extinct in the Wild means that the species survives only in cultivation, captivity or as a naturalized population outside its past range; Extinct means that there is no reasonable doubt that its last individual has died.
• Data Deficient means that available information is inadequate for assessing risk; it does not mean that the species is safe or threatened.
• Not Evaluated means that the species has not yet been assessed against the criteria.
• Red List status measures biological extinction risk; it does not by itself create legal protection or prescribe a particular conservation action.