INTERNATIONALLY renowned ecologists visited Stonehenge last week to conduct the first-ever comprehensive survey on the rare lichens that grow there.

A survey was carried out in 1994, which revealed 79 species of lichen, but ecologists were keen to re-examine the site and assess what major changes in flora had taken place during that time.

Peter James, a distinguished lichen expert and ex-curator of lichens at the Natural History Museum, in London, said: "We have noticed very little change, which is extremely gratifying. Diversity is about 80 to 90 species and we've found some very interesting species new to the area."

But it is not just the richness of species that makes the site unique.

"Another reason it is so interesting is the types of community you find here - the combination of species," said Mr James.

"The quality of preservation is also very high."

Mr James said there were several factors that could explain the high lichen count and unique combination of species - "firstly, because Stonehenge has been here so long, secondly because there are several geological rock types, and thirdly the fact that you've got exposed and sheltered areas - different rock faces will allow different types of community to flourish."

Stonehenge is also one of the more important sites for lichens in lowland Britain because of the high proportion of maritime species present, a feature that is still largely unexplained.

Dr Oliver Gilbert, one of the country's most eminent lichenologists, said: "About ten of the species found here are maritime, species which normally only occur on the coast.

"It's not something yet fully explained, but I do have a theory. Although we are 60 miles inland, under certain weather conditions, thermals from the coast reach Stonehenge carrying salt with them.

"These are deposited on the stones, particularly on the sheltered parts of the stones, and that is where most of the maritime species occur."

English Heritage said they would continue to preserve the lichen at Stonehenge as part of their conservation and management of the stones.