How the cows came home and saved the adonis blue

It is a remarkable story of how the survival of one species in the animal kingdom can be utterly dependent on another and apparently unrelated creature - no matter how great the disparity in size between them.

Four decades ago, the adonis blue, one of Britain's rarest and most beautiful butterflies and restricted to chalk down land in southern England, disappeared from large tracts of its former habitat, largely because the disease myxomatosis had reduced the rabbit population that kept the grass short, which the butterflies prefer.

Now, despite the steady decline in other butterfly species, the adonis blue, classified as a priority species, is making a remarkable comeback to many of its former habitats in parts of Gloucestershire, Kent, Sussex and Hampshire - helped by the grazing activities of cattle. For many years, it had been restricted to small areas of Dorset, Wiltshire and the Isle of Wight.

Possibly the most important resurgence has been on the 1,000 acres of the National Trust-owned Rodborough and Minchinhampton Commons in the Cotswold Hills near Stroud, where, over the past few years, breeds of cattle used to grazing on steep slopes have been introduced to help keep the grass down. And now, the hot weather of this summer has ensured that the adonis blue has returned in large numbers.

"For 40 years, since the species collapsed, these butterflies have been absent from the commons and now they have made a spectacular return. It is completely beyond our wildest dreams," said Matthew Oates, a conservation adviser to the National Trust and a butterfly expert. "It is one of out loveliest butterflies and we are delighted to have it back in the Cotswolds."

The return underlines the importance of the two commons, which are renowned both for their archaeology, which include prehistoric field systems and barrows, and their wildlife. Much of the grassland is unploughed and untouched by chemical fertilisers or pesticides and is said to be among the most species-rich in Europe; both commons are designated sites of special scientific interest.

The decline of the butterfly there was due in part to historic changes in agricultural practices, which meant the type of cattle grazed did not like the steep slopes. Then, in the 1950's myxomatosis killed the rabbits that had done their turn in keeping the grass mowed. In 1999, the Trust re-introduced two types of old breed of cattle, the Belted Galloway and the Welsh Black, both with short stocky legs suited to hillside grazing. Eventually, the butterfly returned.

But that is not the whole story. Since the adonis blue was believed to have been completely extinct in the area and traditionally it was believed that butterflies do not cover large distances, butterfly experts were puzzled over how the creature had managed to find its way back to the Cotswolds.

Originally, it was assumed that a collector or conservationist had re-introduced some captive stock into the wild and, finding the new environment to their liking, the butterflies had begun to breed.

However, Mr Oates said this theory had now been largely eliminated in favour of two possible solutions. "One is that a tiny residual colony, containing possibly less than a few dozen butterflies, had survived unnoticed for forty years at a spot near the M4.

The second theory is that during the very warm summer of 2003, some butterflies flew or were blown by the wind north from the Wiltshire Downs, where there had always been residual colonies."

Mr Oates said that he supported the latter theory, because recent research had shown that in warm weather, butterflies become "incredibly" mobile. "We think the adonis blue is benefiting from warmer summers and milder winters, so it is one of the short-term advantages of climate change."

Earlier this year Butterfly Conservation, a charity, named the adonis blue as one of six species which had begun to recover through intensive conservation efforts and despite the long term decline in butterfly numbers, which had affected around three-quarters of Britain's 60 species. The others are the heath fritillary, the high brown fritillary, the wood white, the silver-spotted skipper and the large blue.