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IUCN red-listing criteria

The World Conservation Union (IUCN) has developed a clear set of categories and criteria to be used to assess the risk of extinction of a species. Assessed species are listed in a publication known as the IUCN Red List under categories that indicate the varying degrees of their probability of extinction. There are nine clearly-defined IUCN categories under which every species in the world can be classified. These categories are as follows:

Extinct (EX): A species is Extinct when there is no reasonable doubt that the last individual has died. A species is presumed Extinct when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual. Surveys should be over a time frame appropriate to the species life cycle and life form.

Extinct in the wild (EW): A species is Extinct in the Wild when it is known only to survive in cultivation, in captivity or as a naturalised population (or populations) well outside the past range. A species is presumed Extinct in the Wild when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual. Surveys should be over a time frame appropriate to the species life cycle and life form.

Critically endangered (CR): A species is Critically Endangered when the best available evidence indicates that it meets any of the criteria (a) to (e) for Critically Endangered (according to IUCN criteria), and it is therefore considered to be facing an extremely high risk of extinction in the wild.

Endangered (EN): A species is Endangered when the best available evidence indicates that it meets any of the criteria (a) to (e) for Endangered (according to the IUCN criteria), and it is therefore considered to be facing a very high risk of extinction in the wild.

Vulnerable (VU): A species is Vulnerable when the best available evidence indicates that it meets any of the criteria (a) to (e) for Vulnerable (according to the IUCN criteria), and it is therefore considered to be facing a high risk of extinction in the wild.

Near threatened (NT): A species is Near Threatened when it has been evaluated against the criteria but does not qualify for Critically Endangered, Endangered or Vulnerable now, but is close to qualifying for or is likely to qualify for a threatened category in the near future.

Least concern (LC): A species is Least Concern when it has been evaluated against the criteria and does not qualify for Critically Endangered, Endangered, Vulnerable or Near Threatened. Widespread and abundant species are included in this category.

Data deficient (DD): A species is Data Deficient when there is inadequate information to make a direct, or indirect, assessment of its risk of extinction based on its distribution and/or population status.

Not evaluated (NE): A species is Not Evaluated when it is has not yet been evaluated against the criteria.

Extinction is a chance process. Thus, a listing in a higher extinction risk category implies a higher expectation of extinction. All species listed as Critically Endangered qualify for Vulnerable and Endangered, and all listed as Endangered qualify for Vulnerable. Together these categories are described as ‘threatened’. The threatened categories form a part of the overall scheme. It will be possible to place all taxa into one of the categories, see the structure of the categories below (IUCN 2001).

Structure of the categories



IUCN (The World Conservation Union) Red Data Books explicitly document and highlight biodiversity losses at the species level and are important tools for guiding the conservation activities of governments and conservation organisations. Red Data books are furthermore widely recognised as the most comprehensive, apolitical evaluation of the conservation status of plant and animal species as well as measures of the success or failure of various conservation initiatives. IUCN assessments reflect extinction risk and not conservation priority, therefore other factors such as biological, political, socio-economic factors must be considered when setting such priorities.

How the lists are drawn up

Experts use the IUCN Red List Categories and Criteria to classify species according to levels of threat and risk of extinction. The IUCN Categories and Criteria were developed to improve the objectivity of Red List assessments. Species are placed in one of the three threat categories only if they meet specific quantitative thresholds. However, this does not mean that species for which there is very little information cannot be assessed. This is because the IUCN criteria allow the use of assumption, estimation, inference and projection so that assessors can make the best use of all available information on a species. It is often found that the process of making national red lists actually generates data and stimulates data collection in the field.

Technical definitions of the IUCN Red List Categories and Criteria are given in the IUCN Red List Categories and Criteria: Version 3.1 booklet and the Guidelines for using the IUCN Red List Categories and Criteria. These documents can be downloaded from the links below:

IUCN Red List Categories & Criteria Version 3.1
Guidelines on how to use the IUCN Red List Categories and Criteria










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