On a bright spring day, the chalky slopes of the Chilterns smell of warm thyme. Tiny purple violets bloom underfoot. For miles beyond, the Vale of Aylesbury unfolds in a tapestry of newly minted trees, yellow fields and the spires of village churches. This great vista of the English countryside seems gloriously immutable, unchanged since Victorian times, when Walter Rothschild would set out from Tring Park, his country house in the valley below, to throw his net at our summer butterflies and place them in his extraordinary zoological museum.

Not everything, however, would please the eye of Victorian lovers of nature. An easyJet plane casts a shadow across the downland. The air is filled with the complaint of two diggers, quarrying chalk from the bottom of the hill. But what would really make Rothschild weep is what is missing: the sky and the steep meadows dotted with the white flowers of wild strawberry are almost bereft of butterflies.

Peacock butterfly. Photograph: Getty Images/Getty Images

A casual eye might not notice it. Butterflies are still a conspicuous symbol of our summers, much celebrated by everyone from Wordsworth to Nabokov. On the Chilterns, a male orange tip patrols a hedgerow, two peacocks spiral into the air in a territorial dogfight and a speckled wood jinks its way through the trees. This scattering of a few common species is pitiful, however, compared with the riches that once adorned our countryside in summer. Near contemporaries of Rothschild wrote of skimming hundreds of purple hairstreaks from the trees or catching 100 Lulworth skippers in an hour. In 1892, SG Castle Russell took a walk through the New Forest: "Butterflies alarmed by my approach arose in immense numbers to take refuge in the trees above. They were so thick that I could hardly see ahead and indeed resembled a fall of brown leaves." A few centuries earlier, Richard Turpyn recorded a probable mass migration to or from Britain in his Chronicles of Calais during the reigns of Henry VII and VIII: "an innumerable swarme of whit buttarflyes ... so thicke as flakes of snowe" that they blotted out views of Calais for workers in fields beyond the town.

Swarms of butterflies have long disappeared. And a relentless decline may now become terminal for some of our best-loved species. Following the wet summer of 2007, last year was a disaster for butterflies: the lowest number was recorded for 27 years. Of Britain's precious 59 resident species, 12 experienced their worst ever year since the scientific monitoring of butterfly numbers began in 1976.

A common blue butterfly. Photograph: Laura Sivell/Papilio/Corbis

I began a less than scientific monitoring of butterflies in a little notepad when I was eight, helping my dad count the tiny brown argus on the Norfolk coast where we spent our summer holidays. Finding this darting, chocolate-brown gem ignited an awkward passion for butterflies that I kept well hidden during my teenage years. Dad and I would go on expeditions to discover, and photograph, rare species: we would sit in a wet meadow in Cumbria waiting for the marsh fritillary to emerge, or hover by piles of horse manure in the woodlands of Surrey, hoping the majestic, haughty (and turd-loving) purple emperor would descend from the treetops for us. Twenty years on, some of the nature reserves we visited have lost their precious rarities. If trends continue, another couple of bad summers could kill off some species for ever.

Numbers of the delicate wood white were down by 66% last year on dismal 2007; its population has slumped by 90% over the long-term recording period. The duke of burgundy and the high brown fritillary are most at risk of extinction. The high brown survives in just 50 small sites: at one spot in Dartmoor, there were 7,200 in 1995; last year, there were just 87. Nationwide, numbers have fallen by 85% over 10 years. "This run of bad weather has really pushed those species to the brink in many areas," says Martin Warren, the chief executive of Butterfly Conservation.

Wood white butterfly. Photograph: Peter Eeles

Butterflies find it difficult to fly, feed and mate in bad weather but these figures are not just a seasonal blip caused by freakishly soggy summers. The collecting of British butterflies has ceased to be acceptable and yet butterfly populations have still plummeted. Far more devastating than unscrupulous collectors of old has been industrial agriculture and the loss of 97% of England's natural grassland and wildflower meadows; planting conifers or letting our broadleaved woodlands become too overgrown for woodland flowers; and the sprawl of motorways and urban development.

To this deadly cocktail has been added a new poison: climate change. In theory, a gentle global warming should benefit almost all of Britain's butterflies. Creatures of sunshine, most of our butterflies are found in southern England where many are at the limit of their natural range; as our summers become hotter, these butterflies should thrive and spread further north. There are a few winners already: the beautiful comma is moving north and the rare silver-spotted skipper has done well thanks to hotter summers. Britain may also be visited more regularly by exotic species that were once rare migrants.

A painted lady butterfly. Photograph: The Linnean Society of London

The fate of one much-loved native shows that this happy outcome, however, will not come to pass for most species. The small tortoiseshell is the labrador of the butterfly world: cheerful and content to live close to humans. Its caterpillars devour ubiquitous nettles. As an adult butterfly, it feasts on suburban flowers and hibernates in garden sheds, pitter-pattering against our windows when spring comes round again. Thanks to climate change, it is spreading north and is now seen for the first time in remote parts of Scotland. Unfortunately, so too is Sturmia bella (how the person who named this ugly brute could call it beautiful is beyond me), a species of parasitic fly.

This nasty fly was recorded for the first time in Britain in Hampshire 11 years ago. By last summer, it had reached Merseyside thanks to a modus operandi every bit as gory as the Alien films. It lays its microscopic eggs on patches of nettles where small tortoiseshell caterpillars feed. These unwittingly eat the fly's eggs which become tiny worms inside the caterpillar, bursting out of their bodies just when the small tortoiseshell is beginning its miraculous transformation into a butterfly inside its chrysalis.

Last year was the worst ever year for small tortoiseshells, its population slumping by 45% compared with 2007, despite thousands of migrant small tortoiseshells arriving from Europe in September. In southern and central England, it appears to have been virtually wiped out: during my afternoon roaming the Chilterns last week, I saw 10 peacocks and 12 yellow brimstones and the odd rather more elusive species, such as the grizzled skipper, but not a single small tortoiseshell.

Is Sturmia bella wiping it out? Where the fly finds small tortoiseshell caterpillars, their mortality rate is 61%, according to research by Dr 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.

"Whichever 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. "If you had an intact countryside, butterflies should be going through the roof, but the species can't move through the countryside like they once would have done," says Brereton. "Habitats are too fragmented. There are vacant suitable habitats in parts of the countryside but the butterflies won't necessarily find them."

Swallowtail butterfly. Photograph: Alamy

Our largest and most charismatic native butterfly, the swallowtail, was once found across the fens of East Anglia 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 by humans 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 playing God like this has already happened. The last species to become extinct in Britain was the large blue in 1979. Despite heroic scientific endeavour, 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 tantalising 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.

With this kind of ingenuity, could we turn the whole country into a giant butterfly farm? Could we save every species by reintroducing them to tailor-made nature reserves or boosting populations with specimens from abroad? "We might do it for a few species, but it's not the basis for a conservation strategy," says Warren. "What about all the other insects? We want to get the habitats right and butterflies will tell us if we are getting it right, and then we'll be getting it right for biodiversity as a whole."

Amazingly, despite all our knowledge, we still get it wrong. The pearl-bordered fritillary was known as "the woodman's friend" because it would faithfully follow foresters around broadleaved woods as they coppiced or cut down patches of trees, attracted to the flowers that blossomed in the freshly cut glades in subsequent years. Like many butterflies, it became inextricably linked to the way we managed our landscape, but has undergone a dramatic decline in numbers since this traditional way of "harvesting" our wood died out.

While conservation management has reintroduced coppicing - which is rarely economically viable because of the falling demand for wood fuel and is now often carried out by volunteers on nature reserves - pearl-bordered fritillaries have continued to die out, often because the work has not been carried out on a big enough scale. Even Monks Wood, a national nature reserve and the site of a celebrated government research station that has been the source of much of our scientific wisdom about butterflies, has lost 12 of its 40 species of butterfly since 1954, including the pearl-bordered fritillary.

The decline of butterflies is "not all farmers and climate change", as Brereton puts it. Some of our rarest butterflies have been inadvertently decimated by conservation efforts. Matthew Oates, the National Trust's advisor on nature, takes me to the beautiful Rodborough Common in the Cotswolds, to see the first duke of burgundy butterflies of the year. The delicate beauty of this small, fritillary-like butterfly belies its pugnacious urge to scrap with every other insect that comes near as it suns itself on the steep sides of the common. "The Oates motto is 'never underestimate a butterfly'," says Oates, a jovial polymath who brings his scholarly training in poetry to bear on butterfly conservation. If climate change brings better summers, he points out that some species will become more capable of travelling across our decimated landscape to look for new sites. "But I am seriously worried for burgundies. The figures are very alarming. What's messed it up in the last 20 years is conservation management."

Before climate change, another man-made event, the introduction of the rabbit-killing disease myxomatosis in the 1950s, caused the decline of many grassland butterflies which relied on large rabbit populations to keep the grass short and full of flowers. Conservation plans saw a widespread reintroduction of grazing to help rare plant species and butterflies such as the adonis blue. But the duke of burgundy requires longer, rougher grassland and a certain size of cowslip plants; overgrazing has caused its population to plummet. Now it exists in such tiny colonies it could easily disappear. "The track record of conservation management on this butterfly is bloody awful," says Oates. "I really think we could lose it."

We are belatedly getting better at conserving the right kind of land for fragile, complex and, frankly, contrary butterfly species. Butterfly Conservation had one conservation adviser a decade ago; today, 30 advisers help landowners manage 1,000 precious sites. Once the bete noire of conservationists, the Common Agricultural Policy now offers some funding - although not enough - to encourage farmers to manage their land for conservation. "It's by no means all doom and gloom but getting enough done in enough areas is the problem," says Warren. New demand for eco-friendly wood fuel from the sustainable harvesting of broadleaved woods would help too, recreating our traditional woodland system in which flowers and butterflies could thrive.

Climate change, however, makes it all much more complicated. As well as new predators, new diseases may destroy native trees, flowers and insects that butterflies depend on. Invasive weeds could crowd out butterfly food plants. Grass and bracken - with which many rare fritillaries have a delicate relationship - are already growing back more vigorously than in the past. Tangled woodland will need clearing more regularly. "A lot of conservation management won't necessarily work in the future," says Brereton. "With climate change, species are changing their habitats and their requirements are changing as well. It can be fatal to manage for what a butterfly needed 20 years ago. We need to keep on the ball with understanding what species need because their requirements are changing as the earth warms up."

Just as I kept my passion for butterflies hidden for fear of ridicule at school, so the butterfly hunters of old were often derided for such a whimsical, frivolous pursuit. Butterflies may be pretty but they seem inconsequential ornaments when compared with majestic eagles or pragmatically functional insects such as worms or bees. Every century, butterflies have become extinct in Britain. Why should we care if we lose a few more?

For a start, butterflies are an excellent indicator species: if butterflies are suffering then so too are thousands of less well monitored insects. (Thanks to the scientists who set up butterfly monitoring in the 1970s and the 1,500 volunteer butterfly recorders who count numbers every summer, we have excellent data showing their decline.) It is insects that pollinate many flowers, help matter decompose and protect other species by preying on pests. Plants, birds, rodents and big, greedy mammals - such as human beings - depend on them. "There is a good moral case for conservation but there is a pretty good selfish, economic case as well," says Warren. "With the economic downturn, people think saving butterflies is pretty low down on our list of priorities, but human beings and the natural world are linked very closely. If the natural world goes to pot, sooner or later we will go to pot. Butterflies' decline probably indicates a rapid decline in invertebrates in general. If the British situation is true across the world, we are heading for a sixth great extinction event. There have been five in the history of the planet and this one will be man-made."

Oates has another reason for saving our butterflies. Each species, in its own way, is part of the cultural identity of our landscape. Butterflies are a "conduit into natural beauty", he explains. "They take us on voyages of discovery to some of the most beautiful landscapes in this country." Many of our earliest memories of summer will involve a vivid image of a butterfly. If we seek out butterflies, they can lead us into a natural world from which we are increasingly estranged by our material, technological and suburban existences. "We underestimate the importance of beauty and wonder in our lives at our peril," says Oates. "As much as I love football, it's no substitute for the real thing."

After a day failing to see a single small tortoiseshell in the land where thousands once roamed - chased by the nets of obsessives such as Walter Rothschild - I head to a cool stone cupboard in Harrow School where a fraction of the 2.25m butterflies and moths gathered by Rothschild are stored in mahogany cabinets. This amazing collection of foreign butterflies with iridescent wings of purple, green and gold will be auctioned by Bonhams at the end of May. Beautifully preserved, they look as if they could have been flying last week. This glittering hoard is a melancholy reminder that we are only a hundred butterfly generations from summers of plenty. In time, these dried, dead beauties may be the only butterflies we can gaze upon in wonder.

• A Butterfly Year, Patrick Barkham's journey in search of British butterflies, will be published by Granta.

How you can help

Four steps to save Britain's butterflies

Gardening

Butterflies visit gardens to drink nectar from flowers. Many good nectar plants are hardy perennials and easy to grow. The best for butterflies are buddleia, ice-plant (sedum), lavender, michaelmas daisy and origanum (marjoram). But butterfly caterpillars need feeding too: plant holly and ivy in sunny positions where they can grow tall and flower for the holly blue. Don't pull up all your stinging nettles: leave a patch for comma, small tortoiseshell and red admiral butterflies.

Shopping

Buy produce from farmers who manage their land to support wildlife. Farmland is the main habitat for more than three-quarters of British butterflies but the flower-rich grassland and hedges where they breed have been lost to modern intensive farming methods. Some producers, and those with the Leaf marque, source food from farmers who take special care of wildlife.

Recording

Take a walk in the countryside and join Butterfly Conservation's army of volunteers, who record butterflies for the important scientific databases which track the health of populations. You can do it yourself and send in casual records, or join their surveying system: your sightings can help your countryside and butterfly-rich areas receive funding and better conservation management. Recording and monitoring details at butterfly-conservation.org

Volunteering

If you fancy getting fit, you can join working parties on nature reserves to help cut back weeds, trees and scrub and manage sites for butterflies. Your local wildlife trust will have details of how you can help. You can also support conservation efforts by becoming a member of Butterfly Conservation, which has more details of how you can help.

Patrick Barkham's favourite butterflies

Silver-washed fritillary

The most gracious of the fritillaries, which take their name from the delicate and subtly diﬀerent chequered patterns on their wings, this large woodland butterﬂy is one of the more common. Other fritillaries, particularly the high brown, pearl-bordered, heath and marsh fritillaries, are among our most endangered butterﬂy species. Many require speciﬁc management of woods and meadows to allow their caterpillars, and food plants, to ﬂourish.

Purple emperor

Our most majestic and mysterious butterﬂy, the purple emperor rules our ancient woodlands, gliding across the tops of trees in July. His Imperial Majesty, as he was known to entomologists of old, was a great prize for Victorian collectors. One even designed a 50ft butterﬂy net to try to entrap it. The female is rarely seen and even though the male ﬂashes iridescent purple, he is similarly elusive. I've searched for the emperor on numerous occasions and only spotted him once, ﬂeetingly, silhouetted against the sky.

Camberwell beauty

Named after the London suburb that was a leafy village when this large migrant roamed its lanes, the Camberwell beauty is the ultimate prize for butterﬂy fanatics. It was thought to be a British resident but it is actually a migrant. When the winds are favourable, only a handful of times every century, it reaches our shores from Scandinavia in its hundreds. The last great inﬂux was in 1995; I've never spotted one, so I'm praying 2009 will see another.

Dingy skipper

A dingy butterﬂy seems an oxymoron and this feisty but very grey little skipper certainly lives up to its name. It's not much to look at, and you can confuse it with a day-ﬂying moth. I spent an hour following what I hoped was a dingy skipper on Waterford heath near Hertford last week; it turned out to be a latticed heath moth. How can you tell the diﬀerence between a butterﬂy and a moth? The best way is to look at an insect's antennae or feelers: butterﬂies have distinctive clubbed antennae whereas moths have long

straight feelers.

Adonis blue

The most dashing of our blue butterﬂies as its showy name suggests, it is a creature of our ﬂowery chalk downlands in midsummer. Immediately after performing at Sadler's Wells, London, in the evening, one of the most famous clowns of the 19th century, Joseph Grimaldi, would walk all night to Dartford, Kent, so he could capture adonis blues, which he called the Dartford blue. When his butterﬂy collection was stolen, Grimaldi the clown was so devastated, he took up pigeon fancying instead.