The IUCN Red List and Extinction Case Studies

What the Red List is for. The International Union for Conservation of Nature maintains the Red List, a global inventory of species assessed against standard criteria and placed in categories of extinction risk: Least Concern, Near Threatened, then the three threatened categories — Vulnerable, Endangered, Critically Endangered — then Extinct in the Wild (surviving only in captivity) and Extinct. Its purpose is to make risk comparable across species and countries, so that limited conservation money can go where it prevents the most loss, and to track whether the situation is improving. It is the evidence base behind trade bans, protected-area decisions and funding.

The criteria, and why they matter. A species is placed in a category if it meets any one of five quantitative criteria: A, rate of population decline (Critically Endangered if 80% or more in ten years or three generations); B, small and shrinking or fragmented geographic range; C, small and declining population; D, very small population or very restricted range (fewer than 50 mature individuals for CR); E, a quantitative analysis giving a probability of extinction. Other factors considered: the number and nature of threats, degree of endemism, ecological role, and whether the species is protected. The traps: Data Deficient is not "safe" — many species have gone extinct before being assessed; and a species can be Least Concern globally while locally extinct.

Three case studies you must carry. Extinct: the dodo of Mauritius, flightless and unafraid of humans, was hunted by sailors and its eggs eaten by introduced rats, pigs and monkeys; it was gone within a century of discovery. Or the Tasmanian tiger (thylacine), hunted with a government bounty as a livestock threat, the last dying in a zoo in 1936; with it went the island's top marsupial predator. Critically Endangered: the Javan rhinoceros, fewer than 80 individuals in a single park in Indonesia, threatened by poaching for horn, an invasive palm that crowds out its food, disease from cattle, and a single tsunami or eruption because there is only one population; management is armed patrols, habitat management and a planned second population. Recovered: the humpback whale, driven to a few thousand by commercial whaling, has recovered to over 80 000 since the 1986 moratorium — an example of a level-1 response that worked because the threat was single and stoppable, and because the population had time to breed.

What each case teaches. Extinction usually combines several pressures — hunting plus introduced species plus a trait such as flightlessness — and island species are the most vulnerable. A single population is a single point of failure, so recovery plans create a second one. Recovery is possible when a threat is removed in time and the species breeds fast enough; it is slow or impossible for slow breeders with lost habitat. Examiners reward mechanisms and numbers, not the species name.