The IUCN (International Union for the Conservation of Nature and Natural Resources) maintains an international list, published as the Red Data Book.

Red Data Book species are classified into different categories of perceived risk. Each Red Data Book usually deals with a specific group of animals or plants (for instance, reptiles, insects or mosses).

The original IUCN Red Data Book threat categories have been widely applied to hundreds of thousands of taxa of animals, plants, and fungi. Note that these categories refer to the conservation status of an organism in the wild, not to its presence in cultivation or captivity.

In 1994, IUCN adopted a revised set of Red List categories prepared by the IUCN Species Survival Commission (SSC). There are eight categories and three sub-categories. Unlike the original IUCN threat categories, these also include a series of criteria (population reduction, extent of occurrence, population size - mature individuals, probability of extinction) used to make the conservation assessment.
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