Reliable population data is essential for conservation work

Conservationists say incomplete data makes it hard to assess which species and habitats face the greatest threats.

"We want eventually for the status of mammals to be used as an indicator of the quality of life," Dr Valerie Keeble, of the Mammals Trust UK, said.

"The government does this with birds, which have data going back to 1970; but we simply don't have it for mammals."

The trust has just released its State Of Britain's Mammals 2004 report.

Scottish cats

It is the third annual audit and has been compiled by Professor David Macdonald and Dr Fran Tattersall, of the Wildlife Conservation Research Unit, Oxford University.

It traces some familiar themes, such as the continuing retreat northwards of the red squirrel in the face of competition from greys and outbreaks of parapox virus.

Scottish wildcat "on the brink"

And it details particular concerns about the Scottish wildcat, which it says has interbred so extensively with domestic felines that it has now become critically endangered and is likely to go extinct.

But the trust recognises its audit gives only a partial picture and a more comprehensive system of monitoring is now required.

"What we need to do now is keep tabs on mammals from year to year because in many instances, we really don't know how many of them are doing," said Dr Keeble.

"For example, many years ago the otters declined to the point where they nearly disappeared, simply because no one was keeping an eye on them.

"If they had, the decline would have been picked up much more quickly." Otters are now making a concerted return as waterways are cleaned up.

Hedgehog puzzle

Dr Keeble praised the Royal Society for the Protection of Birds (RSPB) for the way it had gathered population data over several decades, and cited studies of polecats and cetaceans as proof of how mammal numbers could be assessed.

Otter numbers are rising again

"Consider hedgehogs," she said. "In some areas, our work has shown they have declined by 50% in 10 years. Why is this happening? At the moment we simply don't know.

"We're now looking at hedgehogs on different types of farms, including organic, collecting the animals' urine to see if they are accumulating poisons.

"We want to see if they're not breeding well or the food is not there for them."