(CNN) -- The word "extinct" sounds pretty final -- and as a concept, it certainly should be. But the normal harbinger of bad news - the "Red List" issued by the World Conservation Union (IUCN) - will actually reveal some good news when it is published in October, for one species at least.
The Asian gharial is one of the species on the IUCN's Red List that is threatened with extinction.
The La Palma Giant Lizard that had been believed extinct for 2,000 years, turns out has been alive and well and living in the Canary Islands.
CNN talked to Simon Stuart, chair of the IUCN Species Survival Commission's Biodiversity Assessment Subcommittee on why sometimes the final curtain isn't so final after all.
CNN: How do you decide when a species is officially extinct?
Simon Stuart: Generally we are very, very cautious about declaring a species extinct. To declare a species extinct it needs obviously to have not been seen for a long time.
There needs to have been very extensive surveys done within the geographic range of where it was known to have occurred before; and in similar places where it wasn't known before but where it might be expected to occur.
And that needs to be done in appropriate habitats at appropriate times of the year -- if it's a nocturnal species it needs to be done at night, all this sort of stuff. So only after we are satisfied that they have surveyed adequately enough to be sure that there are no individuals left in the wild will we declare a species extinct.
CNN: How long does a species have to be missing for until you declare it extinct?
Simon Stuart: It is done on a case by case basis. It is usually years and of course some species which are very easy to see, one can be more certain that they are extinct than others. It really depends on how easy the animal is to detect.
CNN: Can you ever be absolutely sure that a species is extinct?
Simon Stuart: You are obviously more sure, and you can be absolutely sure. You can get an extreme example like the Dodo which was found on the island of Mauritius -- it's not a big island. And the Moas, the huge birds that lived in New Zealand, there is no chance that the Moa survived today. But there are a whole load of other species where it is obviously much, much harder.
CNN: How often do you reverse an extinct status?
Simon Stuart: Actually we are going to do it in the 2008 IUCN Red List when it is launched at the beginning of October. There is a species of giant lizard, the La Palma Giant Lizard, from one of the Canary Islands that went extinct, we thought, about 2,000 years ago and it was rediscovered last year on a very steep rock face, almost inaccessible.
This species had been displaced by introduced rats and cats on the island and it was long thought it had gone.
But as it turned out these cats and rats couldn't get on the steepest rock faces and the lizard hung on for the last 2,000 years without anyone noticing until someone went down there on a rope and found it.
So in fact, with that rediscovery, all the giant lizards that were supposed to have been extinct in the Canary Islands have now been rediscovered in very, very difficult terrain.
CNN: So are you hopeful for other officially extinct species turning up alive?
Simon Stuart: It is getting less frequent because IUCN standards for declaring a species extinct have got more rigorous over the years. In the past things got declared extinct fairly easily and now it's much, much harder to declare something extinct.
The whole business of not just declaring a species extinct but declaring it threatened as well have become much more rigorous. The standards are much, much higher than they used to be.
CNN: So do you believe the Tasmanian Tiger could still be with us?
Simon Stuart: That one we do have as extinct. The survey intensity for that species has been very great, it is an extremely distinct species; it would leave signs, footprints, stuff like that experts would notice.
There have been various hoaxes and famous ones in years gone by, photographs taken but turned out to be stuffed animals and stuff like that. It's a shame as it was a fantastic animal. But I am afraid we have given up hope on that one now.
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