Polden Hill is one of 20 "Butterfly Survival Zones" Pic:Emma Daniel
The large blue butterfly population on the East Polden Hills in Somerset has surprised experts by thriving despite poor weather and a drought in 2007.
The numbers were declining but Somerset Wildlife Trust (SWT) has reported more than 3000 large blues flying over the slopes during June 2008.
SWT manages Green Down nature reserve on the hills which is closed during the flight period to protect the species.
They were re-introduced in 1992 and the site has become one of Europe's best.
David Simcox, of the Centre for Ecology and Hydrology, said: " I have worked with the Somerset Wildlife Trust and their conservation staff for nearly 20 years on this project and have found them to be highly skilled at habitat management, excellent at working with the public and they have always delivered what they promised - in this field these are rare commodities."
The SWT is also grateful for the work of a local farmer, James Stratton, who grazes the site with his cattle to create the right environment required by the large blue butterfly.
Sir David Attenborough announced, last month, that the site on the Polden Hills would be one of 20 UK landscapes which have been identified as "Butterfly Survival Zones".