rarity The relative abundance of a species and, therefore, its vulnerability to extinction. The International Union for Conservation of Nature and Natural Resources 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’.

rarity The relative abundance of a species and, therefore, its vulnerability to extinction. The International Union for Conservation of Nature and Natural Resources 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’.

rar·i·ty / ˈre(ə)ritē/ • n. (pl. -ties) the state or quality of being rare: the rarity of the condition. ∎  a thing that is rare, esp. one having particular value as a result of this: to take the morning off was a rarity.