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rarity


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The relative abundance of a species and, therefore, its vulnerability to extinction. The International Union for the Conservation of Nature and Natural Resources (IUCN), formerly also known as the World Conservation Union (WCU), measures the vulnerability of a species according to five criteria: (a) the rate at which its numbers are observed, inferred, or projected to be declining; (b) in association with (a), whether the species occurs as a single, small population or a few, small, fragmented ones; (c) in association with (a), whether the species occupies a small geographic range or area; (d) the size of the population; (e) a mathematical estimate of the predicted risk of extinction within a specified time. From this assessment, species are allocated a position on a continuum of increasing threat with three categories: as ‘critical’, ‘endangered’, or ‘vulnerable’. Species that are known to be at risk of extinction, but fail to qualify for any of the main categories, are classified as ‘susceptible’.

http://cms.iucn.org/about/work/programmes/species/red_list/index.cfm IUCN and Species Survival Commission website containing the red data lists of endangered species.

Subjects: Zoology and Animal Sciences — Ecology and Conservation.


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