More than a fifth of the world's reptiles are threatened with extinction, a new method of monitoring the fortunes of groups of species revealed today.

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Instead of assessing each individual species to see if it is at risk of dying out, the Sampled Red List Index (SRLI) examines a sample of 1,500 species from a group such as reptiles, and uses it to model how groups are doing overall.

The method, which revealed 22 per cent of reptiles are at risk of extinction, can be used to track large groups such as insects, where it is not feasible to monitor every individual species.

Complete assessments have been done on all known bird, mammal and amphibian species - but they only make up around 2 per cent of the world's wildlife, according to Dr Jonathan Baillie, director of conservation programmes at the Zoological Society of London (ZSL).

Other plants and animals are covered on the IUCN Red List of Endangered Species, but the picture is far from complete - which means conservation decisions are being made on the basis of knowledge of less than 4 per cent of the Earth's biodiversity.

The new index, which uses the IUCN's criteria for threatened species, is "an amazing tool for communicating the status of the world's biodiversity", he said.

It will enable conservationists to identify families of species which are under threat - for example the results showed 43 per cent of crocodiles were at risk of extinction - and also which ecosystems or parts of the world have high levels of at-risk wildlife.

Dr Baillie said: "This is a quantum leap forward in our understanding of biodiversity. The disadvantage is you can't look at all individual species, but to address the bigger problems we have to understand things at an ecosystem, or habitat, level.

"The index enables us to identify a family of species or region that is particularly threatened."

Adding the new data on reptiles to the assessments on birds, mammals and amphibians has revealed that a quarter (24 per cent) of the world's land-based vertebrate species are threatened with extinction.

The scheme, developed by ZSL and IUCN, will go on to sample other groups such as crayfish and lobsters, dung beetles, butterflies, freshwater molluscs and squid and octopuses.

The findings will feed into work toward halting the loss of global biodiversity by 2010.