The Big Question: How quickly are animals and plants disappearing, and does it matter?
Why are we asking this question now?
As 2006 drew to a close, the polar bear was about to be classified as a threatened species by the United States 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 of somewhere between 1,000 and 10,000 times higher than the natural "background" rate of extinction. This means that 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 gone extinct in the past 100 years?
This is a notoriously difficult question to answer, due in part to the difficulty in recording the declining populations of a particular animal or plant, and in part to the technical definition of an extinction. Experts estimate there are 15,589 species threatened with extinction. But a species is only accepted to have become extinct if exhaustive surveys in its known habitat range have failed to find any record of the individual.
So even if scientists strongly suspect that an animal has gone extinct it cannot be defined as extinct until some time has elapsed since an individual was last observed - which can mean 30 or 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 recorded extinctions. However, the true number of extinctions 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?
In general, 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 similarly endangered.
Amphibians - frogs, toads, newts and salamanders - are perhaps the largest group of animals at serious risk.
About one in three species of amphibians are seriously endangered in some way or other and more than 120 species are thought to have died out over the past 25 years.
Amphibian specialists believe that a combination of factors may be involved, such as habitat loss and the spread of a deadly fungus, 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?
Again, this is another notoriously difficult question. About 1.5 million animals and plants have been identified and formally or informally named. However, the true number of species alive today is likely to fall within the range of between five million and 15 million, although some scientists suggest even higher numbers - perhaps 30 million species in total.
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 about 2 and 4 per cent 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 humans some two million 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, on the other hand, are thought to have a lifetime of one or two million years.
However, scientists say that 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, in which the dinosaurs became extinct, and 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 geo-physical 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 year ago, when about 90 per cent of marine life and 70 per cent 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?
There are several reasons we should be concerned. The first is that in the past it has taken life on Earth between 10 million and 100 million years to recover from a mass extinction. The second is that for all our technology, we still rely on the delicate ecological balance of the natural world for our survival. The Earth's biodiversity provides us with clean air, drinking water, food and even new drugs - upsetting it too much could cause the collapse of this vital life-support system.
The third reason is philosophical. 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?
Should we be concerned about the rapid extinction of the Earth's species?
* Life is a delicate web of inter-related species: destroy too many and the entire fabric can come apart
* The natural world is the planet's only life-support system, and if we lose that we lose our own lives as well
* Protecting biodiversity is the moral equivalent to preserving human culture for future generations - it is not ours to destroy
* There are millions of species on Earth and the loss of a few won't matter too much
* Life has bounced back in the past when there have been similar mass extinctions
* We can live in a world of fewer species, provided we keep the ones that matter to us
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