IT IS already unique - but is it in decline? This is the question being studied by scientists about one of Britain's rarest butterflies, the green hairstreak, and they are asking for help from people in the Yorkshire Dales to ensure that it survives.

The fact that it is the only green butterfly in the British Isles makes it a one-off. But its picky diet makes it extremely vulnerable at a time when the populations of all butterflies is in a period of uncertainty: several native species are said to be on the verge of extinction whilst others are moving in from mainland Europe, thanks to global warming.

The green hairstreak is rare because its caterpillars feed on bilberry bushes, which grow mainly on rough moorland at often quite high altitudes, and are therefore seen by few people except sheep farmers and ramblers.

And an appeal is going out for people who spot this unique creature to note its exact location by map reference and report it to scientists running a joint survey for the Yorkshire Dales National park and the charity Butterfly Conservation.

This is part of on on-going census which began last year but revealed mixed results: cold wet weather in April 2006 may have reduced the number of hairstreaks on the wing. The appeal is being repeated this spring so that researchers can plot as many breeding sites as possible to draw up a survival action plan should the species be at risk.

"We know relatively little about the distribution of the green hairstreak in the park, which makes it difficult to keep track of numbers and to ensure they are surviving here," said Ian Court species officer for the national park.

"It is unmistakable, the only butterfly in this country with green wings and when it is perched it closes them, showing the green underside and white streaks. We are asking people who see them while they are out and about to make a note of the date, the place, including a grid reference, and the number seen."

Last year's survey did produce some concrete results, however, bringing in sightings from 27 people and showing that the butterfly is widely spread out, not just in the national park but in other areas of high moorland neighbouring Craven.

There were 359 butterflies recorded in total, with 196 from within the Yorkshire Dales National Park and 163 from outside the national park. Most of those came from known sites but, even more importantly, previously unknown locations were recorded.

These included Lower Wharfedale, Barden and Beamsley, Rombalds Moor and Kildwick/Skipton Moor. Despite the poor spring last year, these findings delighted scientists by increasing the known sites for this rare creature by 10 per cent.