The Big Question: Are so-called 'extinct' species really extinct, and will we rediscover any?

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Why are we asking this now?

Because ornithologists have just launched an international quest to rediscover a large group of "lost" bird species – believing that some may not be lost after all.

Why does that matter?

Because the human pressure on the natural world is increasing to such an extent that more and more creatures face being wiped out. According to the Red List of the International Union for the Conservation of Nature, 12 per cent of the world's birds, 21 per cent of mammals, 30 per cent of amphibians, 31 per cent of reptiles, 37 per cent of fishes and 70 per cent of the world's plant species are now threatened with extinction. Extinction is one of the great blights of our times. Any creatures that are labelled extinct, but which can be rediscovered, offer enormous hope for conservation.

Are there likely to be many?

More than you might think. In the past few years there has been a whole series of rediscoveries of birds, mammals, fish, insects and other creatures that were supposed to have died out.

For example, the mahogany glider, an Australian possum, was rediscovered in 1989 after an absence of more than 100 years, while the New Zealand storm petrel, a seabird, was thought to have vanished a century and a half ago – it was only known from museum specimens – until it was rediscovered in 2003, and India's large-billed reed warbler was thought to have died out a similar time ago, until it was rediscovered in 2006.

Why have we labelled so many things extinct when they are not?

Perhaps because of the old sin of the pride of knowledge (which was Adam's sin, if you remember); we really enjoy saying I know. It is much more tempting for humans to express certainty about a point than to express doubt, and because we are generally anthropocentric – that is, we put ourselves at the centre of the universe – we tend to think that if we can't see something, then it's not there.

We forget that absence of evidence is not evidence of absence – it's harder to prove something isn't, than it is to prove something is. And we also forget that while life in the natural world can be fragile, it can also be astonishingly resilient, and capable of clinging on against all expectations.

There have been so many recent rediscoveries of supposedly vanished birds – five in New Zealand alone – that, last month, BirdLife International, the Cambridge-based global partnership of bird protection organisations, was prompted to launch a campaign to try to confirm the continued existence of no fewer than 47 bird species, which have not been seen for up to 200 years.

Such as the dodo?

No, not the dodo, although that would be the supreme example of what some biologists now call a "lazarus species" – one that comes back from the dead – as the dodo is our most familiar icon of extinction (greatly helped by its appearance in Alice in Wonderland). But the list included birds from every continent, ranging forward from the hooded seedeater of Brazil, not recorded since 1823. Marco Lambertini, BirdLife's chief executive, said that some of these species had not been seen by any living person, but birdwatchers around the world still dream of rediscovering "these long-lost ghosts".

Why not the dodo?

The dodo was a big-beaked, fat, flightless pigeon confined to the island of Mauritius in the Indian Ocean. It was discovered by Dutch sailors in 1571; by 1700, it had disappeared. The cause of its extinction was probably a combination of hunting and the destruction of its habitat following the introduction of European domestic animals, such as pigs. As its range was so small and restricted, and it could not fly anywhere else, this is one case where we can be certain that extinct really means extinct.

We can also say that for the passenger pigeon, which is the great symbol of extinction in the United States. This bird once flocked in countless millions across the Great Plains but, in the late 19th century, it was hunted with such systematic ferocity that by 1914 there was only one bird left – named Martha – and she died that year in Cincinnati Zoo. No one thinks we'll find the passenger pigeon again.

Then what sorts of supposedly 'extinct' creatures might we rediscover?

Ones that live in places largely inaccessible to humans, such as the open ocean, remote islands or deep forests. In terms of birds, at the top of the list is America's ivory-billed woodpecker, a wonderfully charismatic creature that has not been reliably recorded in the US since 1944, although there have been countless unverified sightings; in 2005, a group of senior US ornithologists sensationally claimed to have rediscovered it in the forests of Arkansas, but their claim has been strongly disputed and not since backed up. However, to find the ivory bill remains the holy grail of birdwatching in the US, and perhaps in the world.

What sort of 'extinct' mammals might we rediscover?

Some of the rhino species, perhaps, in dense south-east Asian forests; perhaps the tiger in regions of central Asia, such as Iran; a range of smaller creatures. The most beguiling possibilities were suggested by a Belgian zoologist, Bernard Heuvelmans, in a riveting book published in 1955 entitled On The Track Of Unknown Animals. This was a calm look at all the legends of mysterious beasts – from the Yeti, or Abominable Snowman, down – that did not necessarily offer conclusions, but made interesting suggestions.

The most fascinating of all concerned the woolly mammoth, which died out about 8,000 years ago, although a population is known to have clung on in Wrangel Island, off the coast of Siberia, until about 3,500 years ago (when the pyramids of Egypt were already well established). Quoting accounts from hunters, Heuvelmans suggested that a few mammoths might still survive in the taiga, the vast evergreen forests that stretch, largely unpenetrated, for thousands of miles across Siberia's whole breadth. It has to be said that Heuvelmans's book was the foundation for what is known today as cryptozoology, an interest which at its wackier, New Age end, concerns itself largely with legendary creatures such as the Loch Ness Monster and the Bigfoot of the American north-west. But Heuvelmans himself was a sober investigator.

How great would it be if we could rediscover the mammoth?

It would be even greater if we could not lose the tiger. Although the possibility of "lazarus species" is intriguing, it is not half as important as holding on to the species we already have – some of which are daily ever-more threatened.

And as the human pressure on the natural world grows more and more intense, for the ones that we lose in future, extinction really is likely to mean extinction.

Might an 'extinct' animal like the woolly mammoth still survive somewhere?

Yes

*Even today there are some vast areas, such as the taiga forest in Siberia, which are still unpenetrated by humans

*Experience shows that sometimes small numbers of rare species can cling on unknown for a very long time

*The natural world has a habit of surprising us

No

*It was not just hunting but global climate change that drove the mammoth to extinction

*There would have been clear, recorded human contact had any mammoths still survived

*It is simply too long since the species died out to think that a few individuals might have carried on

m.mccarthy@independent.co.uk