CORFE Castle is home to some of the country's rarest species of lichen including a variety found at only one other UK location.
A conservation survey carried out this autumn revealed 102 different lichen species have colonised the castle, including four nationally rare and 11 nationally scarce species.
The castle also boasts the second ever British site for Caloplaca granulosa, which, when it was discovered at Corfe in 1982, was the first recording of the species for over 107 years.
Populations, which go to the very top of the keep on south facing walls, are easily identifiable by the mass of small warts, the consistency of granulated sugar which lie in the centre of every orange circular patch.
Lichen expert Vince Giavarni, who conducted the survey, concluded that "no other man-made structure in mainland Britain has such a distinguished group of Mediterranean Atlantic/southern European lichen species".
He said: "Many old churches and other historic buildings have been overly restored for wildlife. Corfe is a fantastic example of what happens when nature is allowed to take over."
He added: "It's all about location, location, location. Being so close to the south coast, the lichen has been enjoying a sunny Mediterranean aspect, free from pollution, for centuries.
"In essence, they have been sunbathing since the castle was ruined during the English Civil War and they are probably the only continuous inhabitants of the castle since that time."
Angela Peters, National Trust ecologist, who commissioned the survey, said wildlife at Corfe Castle was treated on a par with the historic environment, and restoration work had included maintaining its unique habitats.
She added: "The lichen survey shows just how important our historic buildings can be as sites for wildlife. Our continued sensitive management of the castle will ensure that these species continue to thrive here for centuries to come."
A major conservation project began at Corfe Castle in spring 2006.The keep stonework is scheduled to be completed by summer this year.
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