Wings of change: How butterflies give vital clues to the state of the ecosystem

Butterflies herald the return of spring sunshine and long summer days. There can be few more welcome visitors to the garden than a flamboyant red admiral or peacock, clinging sated to a buddleia bush or fluttering energetically over the lawn. A cloud of blues almost underfoot on a downland walk is a quintessential mid-summer sight. And these beautiful insects are also key indicators of a healthy ecosystem.

Butterflies are particularly sensitive to climate change, pollution and habitat loss, explains Dr Martin Warren, chief executive of Butterfly Conservation, the UK charity dedicated to butterflies, moths and their habitats. "Butterflies have annual lifecycles, they make the transformation from egg to caterpillar to chrysalis to adult every year. Their populations can build up or decline very quickly, which makes them far more responsive than plants or birds."

Butterflies also need the right conditions every year to thrive; if a bird fails to breed one year, it will have another opportunity the next, but for butterflies there is no second chance.

Butterflies can be loosely classified into two types: habitat specialists that rely on one very specific set of conditions, such as chalk grasslands, and generalists that are far more mobile. The two types are faring very differently as the climate warms up. "The generalists are doing very well and the specialists are doing very badly," says Dr Warren. "The generalists by and large have benefited from climate change and their food plants, such as nettles, are thriving. These species are at least maintaining their profiles, and even increasing; the peacock was rare in Scotland 50 years ago and is now common."

The specialist species, however, are struggling. While opportunistic generalists are steadily spreading north, chasing warmer weather, specialists can't follow because there are no suitable habitats. "You would expect the specialists to do well as the climate gets warmer, but they don't. Their habitat problems are so big that they override any benefit. My hypothesis is that the current decline would be even worse without climate change," says Dr Warren. Habitat losses include 60 per cent of chalk grasslands, 97 per cent of unimproved hay meadows and 50 per cent of ancient woodlands since World War II.

Another reason why butterflies are ecologically important is the large amount of accurate data that exists about them, collected by Butterfly Conservation. "Insects make up three-quarters of all [animal] species, but most of them we know very little about," says Dr Warren. "Most conservation strategies are based around birds and plants, but butterflies are a window on the world of insects."

Butterfly Conservation's database is founded on five million reports from volunteers. "We have hard data to back up what we say and it is used by researchers all over the world, particularly in relation to modelling climate change," says Dr Warren.

Butterflies, he says, are declining more rapidly than plants or birds and any attempt to stem this must rely on more sensitive agricultural practices and land management. "Farmers and landowners are key and many are embracing schemes such as environmental stewardship. The choice we are facing is to maintain practices that are sympathetic or to lose whole species: there is no middle way."

The National Trust is a major player in conservation and recognises butterflies' role as canary in the coalmine. "They are hypersensitive to changes in the weather and can speak massively to us about how these are affecting nature generally," says Matthew Oates, the National Trust's butterfly expert. "They are brilliant thermometers and an early warning system, not least because the science is there in the Butterfly Conservation databases."

One of the most important strategies, he explains, is battling against fragmentation of butterfly colonies. "You can't conserve much on pocket-handkerchief-sized nature reserves. Small populations are vulnerable. What's needed are whole-landscape projects and the National Trust owns huge tracts of land with connectivity, so it is able to deliver these."

The trust, he adds, is also able to influence land use and develop butterfly-friendly practices: appropriate grazing patterns, tree cutting and the control of invasive species such as bracken. "Butterflies are one of our top priorities and there is an awful lot we can do. Our focus to date has been on saving rare species and trying to arrest declines. Sadly, though, we sometimes have to recognise lost causes; we are also committed to sustainability."

A successful collaboration between the National Trust and Butterfly Conservation has resulted in the saving of Exmoor's heath fritillaries, which were only discovered 20 years ago. A thriving population crashed in the 1990s, due to the collapse of traditional grazing patterns and the encroachment of gorse and bracken. "Butterfly Conservation provided the science and we took care of the practical side," recalls Oates. "We had to burn vegetation on the steep coombs and use herbicide on the bracken, although we didn't want to do that. But we turned that population around."

Despite success stories such as these, many of our native butterfly species remain susceptible to changing conditions. Beatrix Potter's butterfly, the mountain ringlet, lives high on the Lake District fells and flies for a short period in midsummer. "It may be very vulnerable to climate change," warns Oates. "As the climate warms up it will move up the mountains - until it reaches the top and has nowhere else to go."

Butterfly Conservation 0870 774 4309 www.butterfly-conservation.org; the organisation runs more than 800 events every year and welcomes new volunteers. National Trust 0870 458 4000 www.nationaltrust.org.uk.

Where to spot them

The top 20 National Trust butterfly sites: countryside locations to watch butterflies feed, fly and breed.

* Afton, Compton and Brook Downs, Isle of Wight

* Arnside Knott, Cumbria/ Lancashire border

* Ashclyst Forest, Devon

* Ballard Down, Dorset

* Barrington Court Garden, Somerset

* Bookham Common, Surrey

* Calstone Combes and Cherhill Downs, Wiltshire

* Cissbury Ring, West Sussex

* Coleton Fishacre Garden, Devon

* Collard Hill, Somerset Compton Chine, Isle of Wight

* Denbies Hillside, Surrey

* Heddon Valley, Devon

* Horsey, Norfolk Broads

* Ibsley Common, New Forest

* The Langdale Pikes, Cumbria

* Murlough Nature Reserve, County Down

* Rodborough Common, Gloucestershire

* Watlington Hill, Chilterns

* Welshmoor Common, Gower Peninsula

For more information, see www.nationaltrust.org.uk/ butterflies

LARGE BLUE - LOSER

The large blue became extinct in Britain in 1979, despite conservation efforts dating back to the 1920s. However, it has since been successfully reintroduced to a handful of sites in the South-west and more large blues flew in England in 2006 than have been seen for a century or more. There are now four large blue colonies on National Trust land, in Devon, Cornwall and Somerset, and the Somerset site even encourages open access.

GLANVILLE FRITILLARY - LOSER

The Glanville fritillary is mainly restricted to the Isle of Wight, where it breeds on eroding cliffs. More frequent storms would erode the cliffs too rapidly to support the ribwort plantain this fritillary needs to survive. The butterfly needs to move and form new colonies and warmer summers may help it do this; a small colony was recently established in Hampshire.

DUKE OF BURGUNDY - LOSER

This woodland butterfly, in rapid decline, breeds on primroses and cowslips. There are fewer than 200 Duke of Burgundy colonies left, only five of which are of a significant size. "There are fewer individuals in Britain than of any other butterfly, but it is slipping through the conservation net, with no protection from the system," says Matthew Oates.

PURPLE EMPEROR - LOSER

Little is known about Britain's most spectacular butterfly, the elusive, indolent, forest-dwelling purple emperor. Matthew Oates is spearheading a project to determine the distribution of the purple emperor, which is badly under-recorded. "We seem to have worked out a method of surveying woods for it. It has immense appeal and I want to bring people to this butterfly - it's a cracker."

CLOUDED YELLOW - WINNER

A regular visitor to the south coast, it was formerly seldom seen inland, but is now breeding in several sites and is occurring in good numbers every summer, benefiting from climate change.

LONG-TAILED BLUE - WINNER

This small but distinctive butterfly breeds all year round in hotter climates and seems to be turning up more frequently in British gardens as summers become warmer.

HIGH BROWN FRITILLARY - LOSER

Britain's most rapidly declining butterfly breeds on violets growing under bracken; Butterfly Conservation's data suggests an 85 per cent decline since the mid-1990s. The National Trust has just lost one colony in Herefordshire, but there are thriving populations in North Devon.

MONARCH - WINNER

This giant butterfly is blown off course by the tail ends of hurricanes while on its autumn migration in North America. Increased climate instability will mean more specimens turning up in the South-west of England, particularly the Scilly Isles, though it won't become a colonist as its food plants don't grow in Britain.

RED ADMIRAL - WINNER

Many people will be surprised to know that the red admiral is in fact an immigrant from Europe and North Africa. Now, however, it is moving in permanently. "It is surviving our modern winters in viable numbers and deserves a UK passport and vote," says Matthew Oates. "The red admiral was abundant last autumn and may well have survived the winter better than ever."

HUMMINGBIRD HAWKMOTH - WINNER

A large, day-flying moth, the hummingbird hawkmoth hovers over flowers such as buddleia and valerian, hence its name. Formerly scarce, it is becoming an increasingly common summer visitor.