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THE world’s mammals are in crisis, with about 1 in 5 species threatened with extinction. That is the bleak headline news from the latest revision of the IUCN Red List of Threatened Species, released on 6 October.

But one bright spot in this gloomy picture is that biologists have acquired a powerful new tool for discovering whether other groups of plants and animals are faring any better. Called the Sampled Red List Index, it provides a snapshot of the conservation status of groups of animals or plants – much as financial indices such as the Dow Jones allow investors to monitor the health of the stock market by tracking a selection of publicly traded companies.

The grim news that 1141 of 5487 mammals could disappear from the planet comes from an assessment of the conservation status of all known species (Science, ). Similar assessments exist for birds and amphibians. The threats to mammals are well known: marine mammals are at risk from pollution and fishing, and land mammals from habitat destruction and hunting, especially in south and south-east Asia, where 80 per cent of primates are threatened.

Conservation needs to be about more than fur, feathers and frogs, however. The vast majority of Earth’s biodiversity lies in other groups, and for many of these the threats are poorly understood.

The (IUCN) estimates that the status of fewer than 2.5 per cent of all known species of plants and animals has been assessed so far. As a result, conservationists are being forced to set their priorities without having all the facts. “People take educated guesses, but there’s no way to say whether insects are doing better or worse than birds,” says , chief scientist with the US-based conservation organisation The Nature Conservancy.

This is where the Sampled Red List Index comes in. A team led by of the Zoological Society of London, working on an IUCN project, has crunched the numbers for birds and amphibians: this shows that it is possible to get a reliable picture of the conservation status of a group of species as a whole by assessing the status of 900 selected at random (). As good information may be unobtainable in practice for up to 40 per cent of species, the team recommends a sample of 1500 to obtain enough data.

Samples of this size can be assessed more quickly and cheaply than the comprehensive surveys done for mammals, birds and amphibians – which can take five or more years to complete and cost millions of dollars. Baillie says that it would in any case be virtually impossible to make a comprehensive assessment of some groups of invertebrates and plants because they contain so many species.

“Some groups of invertebrates and plants contain so many species that a comprehensive survey is virtually impossible”

The first sampled indices were released at the in Barcelona, Spain, this week. They reveal that at least 16 per cent of freshwater crabs, and a minimum of 9 per cent of dragonflies and damselflies, are threatened with extinction. Indices for several more groups of invertebrates – including butterflies, dung beetles and freshwater molluscs – should follow by 2010. So too should an index for the monocotyledons, a major group of flowering plants that includes grasses and orchids. “That will take us out of the conservation dark ages,” says Baillie.

One result will be to overcome the geographical patchiness of information on some important groups of plants and animals, which has led to biased views of the global threats facing them. For instance, freshwater bivalve molluscs have been extensively studied in North America, where 77 per cent of species are threatened with extinction or have already been lost. Elsewhere in the world, they are thought to be in better shape, says Mary Seddon of the in Cardiff, UK, who chairs the Mollusc Specialist Group of the IUCN’s Species Survival Commission.

The indices’ main value will emerge as they are recalculated every few years, revealing trends in species survival. But as with the stock market indices, on which his assessment technique was loosely based, Baillie fears that the news won’t be good. “It will be very similar to what we’ve seen on the financial markets in the past few weeks,” he predicts.

Endangered species – Learn more about the conservation battle in our comprehensive special report.