They were utterly dependent for their survival on people continuing to look after the countryside in traditional ways, Dr James Mallet of University College, London, said.
But, unfortunately for the butterflies, those traditional practices had ceased across much of the country, to be replaced by intensive modern agricultural practices. That had caused a few species to become extinct in Britain and left some others on the brink of disappearing.
Heavily grazed chalk downlands were crucial for several species, such as the large blue-and-silver spotted skipper, Dr Mallet told the festival. But that habitat had dwindled drastically this century.
Grazing had stopped on many of the steeper hillsides, with farmers preferring to graze their stock on flatter, more easily managed pastures where chemical fertilisers were used.
The rabbit, also introduced to the British Isles by man about 1,000 years ago, had had its population severely hit by myxomatosis, Dr Mallet said.
That, and the abandonment of sheep- and cattle-grazing, had allowed the grass to grow tall, scrub to invade and the butterflies to disappear.
Traditional coppicing of woodlands, in which the wood is cut down to near the base of the trunk every dozen years, had also ended across much of the country.
As a result the trees had grown taller, the canopy had closed and the woods were shadier and cooler through spring to autumn, Dr Mallet added. Several now rare species, including some of the fritillary butterflies, depended on coppicing.
The changes had benefited a few species, such as the Lulworth skipper, which prefers longer grass and has spread out from a small part of Dorset, and the speckled wood, which prefers closed-canopy woodlands to coppice.
But five species are known to have become extinct in Britain since the middle of the last century, and others that were once common are now very rare. There are now believed to be 56 resident butterfly species.
Dr Mallet has been looking for evidence of a decline in the genetic diversity of rare butterfly populations caused by them becoming isolated and increasingly inbred. But that appeared to have happened only to a small extent so far.
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