BOX 3.1 Categories of the IUCN Red List
The following categories are used by the World Conservation Union to classify species for the IUCN Red List1, a global compilation of species of concern to conservationists.
Extinct (EX)
A taxon is Extinct when there is no reasonable doubt that the last individual has died. The great auk, Carolina parakeet, thylacine, and almost 800 other species are known to have become extinct since 1600.
Extinct in the Wild (EW)
A taxon is Extinct in the Wild when it is known only to survive in cultivation, in captivity, or as a naturalized population well outside the past range. Dozens of species are currently found only in captivity (e.g. the Guam rail and several tree snails) or used to be Extinct in the Wild until they were successfully reintroduced (e.g. the wisent and Przewalski’s horse).
Species that fall in the next three categories are collectively called Threatened. Note that the US Fish and Wildlife Service uses “threatened” as a category of jeopardy one step below “endangered.”
Critically Endangered (CR)
A taxon is Critically Endangered when available scientific evidence indicates that it meets any of the criteria A to E in Box 3.2, and it is therefore considered to be facing an extremely high risk of extinction in the wild. Well‐known examples include the Sumatran, Javan, and black rhinoceroses, the Philippine eagle, California condor, Atlantic sturgeon, and hawksbill turtle.
Endangered (EN)
A taxon is Endangered when available scientific evidence indicates that it meets any of the criteria A to E in Box 3.2, and it is therefore considered to be facing a very high risk of extinction in the wild. Many high‐profile endangered species fall in this group: for example, giant pandas, tigers2, snow leopards, Eastern gorilla, Bornean orangutan, chimpanzees, Asian elephants, blue and fin whales, whooping and Japanese cranes, loggerhead and green turtles, and dawn redwood3.
Vulnerable (VU)
A taxon is Vulnerable when available scientific evidence indicates that it meets any of the criteria A to E in Box 3.2, and it is therefore considered to be facing a high risk of extinction in the wild. Most threatened species are listed as Vulnerable; examples include the lion, cheetah, hippopotamus, sperm whale, snail darter, and most bear species (Asiatic black, polar, sun, sloth, and spectacled).
Near Threatened (NT)
A taxon is Near Threatened when it has been assessed against the criteria and 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. Also included here are taxa that are the focus of a conservation program, the cessation of which would result in the taxon qualifying for one of the threatened categories. Jaguars and maned wolves are listed as Near Threatened because their status is of some concern, but they do not meet any of the criteria listed below. White rhinos and black caimans are listed as Near Threatened species because their survival depends on conservation programs.
Least Concern (LC)
A taxon 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 taxa are included in this category.
Data Deficient (DD)
A taxon 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. Listing of taxa in this category indicates that more information is required and acknowledges the possibility that future research will show that threatened classification is appropriate. Many mollusks, fishes, and nocturnal birds and mammals have been evaluated, but could not be listed as Threatened because there was not enough information.
Not Evaluated (NE)
A taxon is Not Evaluated when it has not yet been assessed against the criteria. Most of the world’s species, notably all the invertebrates and other small life‐forms, fall into this category.
1 The World Conservation Union maintains the Red List with a consortium of other conservation groups. The wording used here follows that of the IUCN with minor differences. For the exact and latest wording see the Red List website: www.iucnredlist.org.
2 In some cases a species is assigned to one category overall, while various subspecies or populations may be designated differently. For example, tigers are Endangered as a species but the Sumatran and South China subspecies are considered Critically Endangered.
3 The examples used here are dominated by animals because relatively few threatened plant species are widely known.
BOX 3.2 Quantitative criteria for assessing threatened status
The IUCN Red List of threatened species now requires that a species meet at least one of five quantitative criteria. Shown below are the five criteria for Critically Endangered. The primary differences for Endangered and Vulnerable are certain key numbers; these are shown in parentheses as values for EN and VU. The specificity of these criteria may seem rather naive given the uncertainty that often surrounds these kinds of data; see Akçakaya et al. (2000) for a system for dealing with this uncertainty. See Mace et al. (2008) for further discussion of the assessment process.
A taxon is Critically Endangered when the best available evidence indicates that it meets any of the following criteria (A to E), and it is therefore considered to be facing an extremely high risk of extinction in the wild:
1 Reduction in population size based on any of the following:An observed, estimated, inferred, or suspected population size reduction of ≥90% (EN 70%; VU 50%) over the last 10 years or three generations, whichever is the longer, where the causes of the reduction are clearly reversible AND understood AND ceased, based on any of the following:Direct observation.An index of abundance appropriate for the taxon.A decline in area of occupancy, extent of occurrence, and/or quality of habitat.Actual or potential levels of exploitation.The effects of introduced taxa, hybridization, pathogens, pollutants, competitors, or parasites.An observed, estimated, inferred, or suspected population size reduction of ≥80% (EN 50%; VU 30%) over the last 10 years or three generations, whichever is the longer, where the reduction or its causes may not have ceased OR be understood OR be reversible,