Getting the measure of extinction

Posted: 6 May 2008

Trying to measure just how fast species are becoming extinct is a tricky business. For a start, we have a very incomplete knowledge of the earth's biota.

Estimates for the total number of species range from 10 to 30 million, with some convergence around the figure of 14 million. Of these, only some 1.7 million have been identified and categorized, and only a small percentage of these have actually been studied.

Among animals we know the mammals and birds best. There are about 14,000 species and we know most of them. But these numbers are swamped by the one million described insects - which probably make up fewer than 5 per cent of the total.

There are around a quarter of a million described plant species, making up about half the total - and 80,000 described fungi, or about 5 per cent of the total. The remaining species - bacteria, viruses and protists - are so poorly known that nobody can credibly estimate their number. So most of what can be said about extinction relates best to larger plants, mammals and birds.

Species extinction

Like all species, these have been subject to extinction as a fundamental part of evolution. Indeed, of all the species that have ever lived during the 600 million years of the fossil record, only about 2 to 4 per cent survive today.

Over the past 500 years, 785 species have become extinct in the wild, while a further 65 are found only in captivity or under cultivation. Over 200 extinctions are known to have occurred since 1800, 50 times greater than the natural background rate for extinctions. Based on the fossil record, the background rate is one extinction per one million species per year.

But how do extinction rates compare with those in the past? Looking at the fossil record it appears that invertebrate species have had an average life span of 5 to 10 million years. The poorer data for mammals suggests a lifetime of perhaps 1 to 2 million years.

Of recent extinctions the best, but still very incomplete record, is for birds and mammals lost over the last century. This indicates that one species out of 14,000 becomes extinct every year. - giving an average species lifetime of 10,000 years. This may sound a long time, but is 100 to 1,000 times shorter than the lifetime of species in the fossil record.

The Partula snail (Partula tristis) is one of the rarest species on earth. The total known global population amounts to six adults and four babies, living in a clear plastic box at The Zoological Society in London.© TVEEstimating the lifetimes of extant species involves projections based on loss of wild habitat and the extinction processes of well known species.

Drawing on the IUCN Red List of threatened animals, published in 1996, species lifetimes for well-studied groups of birds, mammals and reptiles are estimated at approximately 300-500 years. Across broader groups lifetimes are between 100 and 1,000 years. These values are similar to the estimates made using habitat projections.

So our estimates show that species extinction rates are 1,000 to 10,000 times higher than in the past. This makes current rates of species loss at least equivalent to the mass extinctions of the past - and in as short a time.

We do seem to be on the brink of a large-scale extinction spasm. But a major difference now is that almost all extinctions are due to the impact of human activities. People now so dominate the earth that very few species are completed unaffected by our existence.

Note: The IUCN Red List threat categories are the following, in descending order of threat:

  • Extinct or Extinct in the Wild;

  • Critically Endangered, Endangered and Vulnerable: species threatened with global extinction;

  • Near Threatened: species close to the threatened thresholds or that would be threatened without ongoing specific conservation measures;

  • Least Concern: species evaluated with a low risk of extinction;

  • Data Deficient: no evaluation because of insufficient data.

This is section was provided by Georgina Mace, a research fellow at the Institute of Zoology (London) and member of the steering committee of IUCN's Species Survival Commission.