By Mark Kinver
Science and nature reporter, BBC News
UK conservationists are celebrating a successful summer for a rare butterfly, which went locally extinct in 1979.
An estimated 10,000 large blue butterflies have been recorded at sites across southern England - the largest number for at least 60 years.
Efforts to rebuild the population have been underway since 1983, when Swedish caterpillars were introduced to the UK.
Experts hope the project will show that such programmes can help other species threatened with extinction.
More than 150 scientists, conservationists and volunteers have been involved in the Large Blue Project, a partnership of 11 organisations, co-ordinated by the Centre for Ecology and Hydrology.
Dave Simcox, manager of the project, said there was still more work to be done.
"Whilst one landscape in Somerset is reasonable secure, the real challenge is to replicate this success throughout the Cotswolds, South Devon, and the Atlantic coasts of Devon and Cornwall," he explained.
Dr Nigel Bourn, director of species conservation at Butterfly Conservation, said it offered hope for other threatened species.
"Given the proper resources, we can restore a countryside full of butterflies and other wildlife," he said.
In the 1970s, scientists discovered that the reason why the large blue (Maculinea arion) became locally extinct was a result of changes to the way the rural landscape was managed.
Jeremy Thomas, from the Centre for Ecology and Hydrology, Dorset, found that the survival of the butterflies was closely linked to a particular species of ant.
Professor Thomas observed that up to five species of red ants would "adopt" a large blue caterpillar, but the butterfly would only survive in the nest of one - the Myrmica sabuleti red ant.
The butterflies are dependent on the Myrmica sabuleti red ant (Image: Jeremy Thomas)
He said changes to farming practices were behind the demise of the ants, which in turn caused the large blue butterfly to disappear from the UK.
"The old-fashioned pastoral grazing of unfertilised downlands and valleys was abandoned by farmers over a period of many years," Professor Thomas said.
"The butterflies need this tight grazing because the shorter the grass is, the warmer the soil. And the particular ant needed by the large blue thrives in very warm soil."
He added: "It all came to a head in the 1950s onwards because for many years wild rabbits had filled the void left by the removal of the sheep and cattle from the land.
"But then there was an outbreak of myxomatosis in the mid-50s and there was nothing left to graze these sites.
"All the sites became overgrown and the soil became too cool, and the one species of ant the butterfly needed declined," he added.
As part of the reintroduction programme by the conservationists, grazing was re-established on the sites chosen for the butterflies.
"Very quickly, the ants spread from the little pockets where they had been surviving and became widespread," Professor Thomas said.
Thyme, the herb on which the large blue laid its eggs, was also planted at the sites.
The locations were then constantly monitored to ensure conditions remained suitable for both species.
Conservationists are hopeful numbers will increase (Image: Dave Simcox)
The only site in the UK that actively encourages the public to come and see the butterflies is on Collard Hill, Somerset, which is managed by the National Trust.
Matthew Oates, a butterfly expert for the Trust, said the location has had about 4,000 visitors since it opened to the public in 2002.
"We worry a little bit about trampling damage, and disturbance to egg-laying females," Mr Oates said.
"But the really good thing we have found is there is a wide diversity of people coming to visit, including families, locals and people who are not naturalists but are interested in seeing something rare and special."
The partnership's reintroduction programmes are funded by the wildlife watchdog English Nature, which is to be merged with a new organisation, Natural England, in October.
Despite the successful summer, the butterfly remains on the international list of endangered species, and is part of the UK's Biodiversity Action Plan.