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...
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