Butterflies are the 'miner's canary' of the natural world. 'They're early warning indicators,' says Dr Warren. 'They'll disappear before other species.' Butterflies are highly sensitive to any disturbance to their habitat and will start to decline in numbers within five to 10 years after a particular site is disturbed. And by the time butterflies die out, other plants and animals will already be treading the same path to extinction. The next species to go may be even more sorely missed: the British dormouse, for instance, may survive for a few years where the resident butterfly species have disappeared, but it, too, will eventually be wiped out.
One of the major reasons butterflies are dying out is mismanagement of their natural habitat. What many butterflies need are sunny woodland clearings. Over half the woodland sites in which butterflies are found are completely unmanaged or only partially managed, yet almost half are designated as sites of special scientific interest, supposedly protected and maintained by law. However, it's not just bad management that is to blame but also a change in our use of wood.
In the past, small woodlands of hazel were grown and trimmed for firewood but these have now given way to woods composed of larger, slower- growing trees. As a consequence, active coppicing of wood has been reduced by 94 per cent since the turn of the century. And the dense woodland which has replaced the coppiced hazel has few sunny clearings.
After the Second World War, wood was desperately needed and whole tracts of ancient woodlands were replaced with fast-growing conifers. As these trees are all of the same age, they create even more shade than they would have done had the trees been planted at different times. Because they prevent light from filtering through to the forest floor, plants that are vital for butterflies and other insects cannot grow.
It is not just woodland butterflies that have suffered, but also those associated with more open habitats. Development and intensive agriculture have reduced grasslands by 92 per cent in the past 52 years.
In a few cases, changing land use has actually benefited butterflies. Bracken is normally the scourge of conservationists, because it spreads so quickly and little animal and plant life is associated with it. But the high brown fritillary butterfly is actually encouraged by bracken as it depends on violets found near the edges of bracken stands. In addition, the dead litter retains warmth which allows the caterpillars to grow better.
Young conifers that are widely spaced and still small also provide suitable sunny, open habitat, but Dr Warren stresses that this is only temporary. Living on the margins of areas that have resulted from human intervention is not enough to outweigh the destruction of suitable, more traditional habitat; despite the increase in bracken-infested areas, the number of high brown fritillary has still decreased by 97 per cent.
One solution would be to open tracts in the forest, known as rides, and to resume coppicing. However Dr Warren says that opening rides in coniferous forests is difficult: conifers do not put forth fast-growing shoots in the way that trees such as hazel do, and so the rides become dominated by grasses rather than the plants butterflies and other animals need. Moreover, a few rides in a few forests does not provide an extensive enough area to maintain a viable population of butterflies. Coppicing is also expensive in terms of staff and because it rarely serves any useful purpose to local people who no longer need firewood.
But the situation is not completely hopeless. 'Many sites where butterflies are left are small, but if they were well managed, butterflies could be saved,' Dr Warren says. There have been some successes: at Gait Barrows in Cumbria, butterflies that are extinct in most other areas have started to return following an increase in coppicing. The Forestry Commission is changing its policy and has agreed to provide grants to enable coppicing to be carried out.
'Up to now the emphasis has been on protecting sites from the bulldozer - simply emergency measures - but even if you put a fence up, most butterflies will still disappear,' he says.
We may not want to go back to heating our homes with hazel twigs, but if we want to protect British butterflies as well as the rest of our woodland wildlife, we need to conserve and manage our woods properly.
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