WHO, WHAT, WHY?
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David Attenborough's long-beaked echidna has been found to be alive and well, despite fears it had become extinct. So how are species classified as dying out?
The Siberian Tiger is classed as "critically endangered"
When extinct species are rediscovered they are, aptly, called Lazarus species. They include the New Zealand storm petrel and a freshwater fish from Madagascar called the Rheocles sikorae, in recent years.
The long-beaked echidna, named after the famous naturalist, is not among them because its status was the category below, newly entitled "possibly extinct".
That status will now have to be revised after villagers in Papua New Guinea told scientists from the Zoological Society of London they had sighted and eaten the echidna. And holes in the ground suggested the creature had been hunting worms.
The World Conservation Union carries out detailed surveys of each species
Only 41,000 of 1.8m known species have been classified
The classifications are made by the World Conservation Union (IUCN) and documented on its Red List showing the conservation status of animals and plants.
Currently there are 844 extinctions, made up of 784 documented extinctions plus 60 species classed as extinct in the wild, but surviving in captivity.
The notion that extinction status is gained by a species not being sighted for 50 years is a myth, says Craig Hilton-Taylor, manager of the Red List Unit based in Oxford.
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"The Red List is quite short because it takes a long time to accumulate enough evidence before we can say 'It's extinct'."
A detailed survey has to be carried out in the species' natural habitat, and its findings reviewed every four or five years. It looks for either documented evidence of the animal or plant on the site or circumstantial evidence that it might be there, taking account of local opinions.
"To get a survey done in a remote area can be quite tricky and can take several years before we can finally say 'It's not there anymore'," says Mr Hilton-Taylor.
"Sometimes one survey may be enough. We have to weigh up the balance of evidence. We need to be conservative about listing them as extinct and even the ones we have listed may then turn up again."
Despite the odd species being rediscovered, the number of extinct organisms is going up slowly. But of major concern is the large quantity moving into the endangered or critically endangered category.
A review of the coral species, for instance, has 40% under threat, due to global warming and coral bleaching, says Mr Hilton-Taylor.
Habitat destruction is the biggest cause of extinction. Others include over-exploitation, alien species and pollution. The current overall extinction rate is about 100 to 1,000 times the natural, or background, extinction rate.
Declaring a species extinct too soon has two downsides, says Jonathan Baillie, who was part of the Zoological Society London team that made the echidna breakthrough.
"One is that if it gets rediscovered then people find it hard to understand. We want to be pretty certain when declaring an extinction and if we declare it too soon then it doesn't receive the conservation attention and may be ignored just when conservation efforts are needed."
The 844 figure is a gross underestimate, he says, because we are only aware of about 10% of the planet's life, so many species are dying out that we're not aware of.
Furthermore, out of the 1.8 million species we are aware of, only 41,000 have so far been classified in conservation terms, says Mr Hilton-Taylor.
The vast bulk of the unclassified are invertebrates, which suffer from a bit of an image problem that makes getting funding for research difficult.
"To try to get donors excited about insects is hard and persuading the public of the need to conserve ants, flies and cockroaches - well, I can imagine their reaction.
"But if you conserve the charismatic species, we should at the same time be capturing the less charismatic species, the ones most important for keeping the ecosystem going.
"For large mammals, you need large areas to conserve them, so there's a knock-on effect for other species."