Is wiping it out? Where the fly finds small tortoiseshell caterpillars, their mortality rate is 61 percent, according to research by Owen Lewis, an ecologist at Oxford University who is studying the impact of the fly. As with many declining species, there is seldom just one cause and the case against Sturmia bella is not yet conclusive.
In most instances where new predators arrive, the attacked species eventually adapt to elude them. Other research suggests that, before the last two wet summers, the dry summers of a warming world also hit small tortoiseshell caterpillars: Low moisture reduces the nutritional quality of nettles.
?hichever way you look at it, it's linked back to the climate,?says Tom Brereton, head of butterfly monitoring at Butterfly Conservation.
Climate change, he says, is a particular problem for our butterflies because our countryside is so fragmented. Decades of ploughing up grassland and ripping out hedgerows means that more than half our butterfly species are now confined to small islands of land. When the climate makes the current sites unsuitable, butterflies will no longer be able to fly elsewhere and find new sites.
?f you had an intact countryside, butterflies should be going through the roof, but the species can? move through the countryside like they once would have done,?Brereton said. ?abitats are too fragmented. There are vacant suitable habitats in parts of the countryside, but the butterflies won't necessarily find them.?br />
Our largest and most charismatic native butterfly, the swallowtail, was once found across the fens of eastern England and beyond until the draining of these wetlands for arable agriculture caused its extinction. It is now confined to the Norfolk Broads. When global warming causes the Broads to be inundated with sea water ?widely expected within 100 years ?the swallowtail will die unless it is relocated to suitable inland sites. These new sites will have to be meticulously created to cultivate a single, rather neurotic wetland plant used by this notoriously picky species.
Conservationists are playing God like this has already happened. The last species to become extinct in Britain was the large blue in 1979. Despite heroic scientific endeavor, the full complexity of this butterfly's weird lifecycle was not understood until it was too late. When tiny, the large blue caterpillar throws itself on to the ground and secretes a tantalizing scent which tricks ants into carefully taking it into their underground nests, whereupon the nasty caterpillar devours ant grubs until it is fully grown. Its dependence on ants was known, but not that it relied on a very particular species, which in turn needed a very specific kind of rough grassland to survive. So, in the 1980s, conservationists brought stock from Sweden and successfully re-established the butterfly on a small field on the edge of Dartmoor. Dad and I were ticked off by a warden when we found this secret meadow, still known only as Site X. The large blue has since been successfully reintroduced into other areas.