WHY ARE WE ASKING THIS QUESTION NOW?

AS2006 drew to a close, the polar bear was about to be classified as a threatened species by the US government. Melting Arctic sea ice could significantly reduce numbers of the world's largest terrestrial carnivore over the next 50 years. And, just before Christmas, a 38-day search for the Yangtze River dolphin ended without finding a single member of the species. It is feared that the aquatic mammal may be the latest in a long line of extinct animals.

Extinction is as old as life on earth . . .

about 3.5 billion years . . . but scientists calculate that we are losing species at a rate somewhere between 1,000 and 10,000 times higher than the natural 'background' rate of extinction. Technically, we are going through a period of "mass extinction", the sixth that we know about over the hundreds of millions of years of the fossil record. But unlike the previous five mass extinctions, this one is largely caused by the actions of a single species, homo sapiens.

HOW MANY SPECIES HAVE BECOME EXTINCT IN THE PAST 100 YEARS?

This is a notoriously difficult question to answer, due to the difficulty in recording the declining populations of a particular animal or plant, and due to the technical definition of extinction. Experts estimate that 15,589 species are threatened with extinction.

But, a species is only accepted as extinct if exhaustive surveys in its known habitat fail to find any record of it.

So even if scientists strongly suspect that an animal is extinct it cannot be defined as such until some time has elapsed since an individual was last observed . . . which can mean 30 to 50 years for some species. The decline and eventual extinction of an animal or plant may take many decades or even centuries and the final stages are seldom observed. This make it difficult to decide when something has completely died out.

Conservationists calculate that since 1500 there have been more than 800 extinctions. However, the true number is likely to be much larger because of what is known about the rate at which habitats are being lost or broken up.

WHAT GROUPS OF ANIMALS OR PLANTS ARE AT THE HIGHEST RISK?

The more we know about a particular group of species, the more we realise that they are at risk. One in four mammals and one in every eight birds is threatened. Half of all tortoises and freshwater turtles are endangered. Amphibians . . . frogs, toads, newts and salamanders . . . are perhaps the largest group of animals at serious risk.

One in three species of amphibians is seriously endangered. More than 120 species may have died out over the past 25 years. Amphibian specialists point to a several factors such as habitat loss and the spread of a deadly fungus. The latter is aided by the human trade in an African toad, a known carrier of the disease.

Up to 2,000 species of amphibians . . . the first vertebrates to conquer the land . . . are classified as endangered. The group is thought to be particularly vulnerable because their life cycles generally depend on two habitats, terrestrial and aquatic, for survival.

HOW MANY SPECIES ARE STILL ALIVE?

This is another difficult question. About 1.5m animals and plants have been identified and formally or informally named. The number of species alive today is likely to fall between five and 15 million, although some scientists suggest even higher numbers, perhaps 30 million.

According to studies of the fossil record, which gives a good indication of the diversity of life on earth over the past 600 million years, only between 2% and 4% of the species that have ever lived are believed to survive today . . . the disappearances of the earlier species occurred long before the arrival of the first modern humans some 160,000 years ago.

DO SPECIES HAVE A NATURAL 'LIFETIME'?

Some species are better than others at surviving for long periods of time. Some adapt to changing environmental conditions and evolve into new species. Most species eventually become extinct. Animals without backbones, the invertebrates, have an average evolutionary 'lifetime' of five to 10 million years. Mammals are thought to have a lifetime of one to two million years.

Scientists say the current lifetimes of some birds and mammals are much shorter than natural lifetimes based on the fossil record. One scientist calculated that the lifetime of a typical bird or mammal species is now about 10,000 years . . . significantly shorter than the natural average lifetime.

WHAT CAUSED THE PREVIOUS MASS EXTINCTIONS?

We know about the previous five mass extinctions from the fossil record. Studies of the number of marine families over the past 600 million years show five points in time when large numbers of species disappeared abruptly. The last was 65 million years ago, when the dinosaurs became extinct. This is thought to have resulted from an asteroid hitting the earth and causing a dramatic change in the climate. In fact, all previous mass extinctions are thought to have been caused by some large-scale geophysical process, such as supervolcanic eruptions or the sudden release of vast quantities of greenhouse gases from the seabed.

The biggest mass extinction of all occurred some 251 million years ago, when about 90% of marine life and 70% of land species vanished. One theory is that something happened to the earth's atmosphere at the end of the Permian period, robbing it of vital oxygen.

IT HAS HAPPENED BEFORE, SO WHY WORRY?

Firstly, in the past it has taken life on earth between 10 and 100 million years to recover from a mass extinction. Next, despite all our technology, we still rely on the delicate ecological balance of the natural world for survival. The earth's biodiversity provides us with clean air, drinking water, food and even new drugs . . . upsetting it could cause the collapse of this vital life-support system.

Finally, there's the philosophical argument.

If we conserve works of art, why should we not also conserve nature? The extinction of a humble beetle is no less important than the wilful destruction of a Rembrandt or a Picasso.