Flight of the tiger

When a rare species disappears, you may not have to look far for the cause. Most of our more exotic plants and animals are tied to particular habitats, where their survival depends on maintaining their particular patch in its pristine state. Quite small and subtle changes, such as increased shade or the sinking of a borehole, may be enough to tilt the scales towards extinction. But when a familiar and seemingly unfussy species starts spiraling downwards, something more serious and widespread is indicated. We have seen it before in the mysterious decline of the house sparrow, which recently joined the Red Data List of birds, among the likes of dotterels and honey buzzards. Now, it seems, another household name is heading for the rocks - the woolly bear or garden tiger moth.

When a rare species disappears, you may not have to look far for the cause. Most of our more exotic plants and animals are tied to particular habitats, where their survival depends on maintaining their particular patch in its pristine state. Quite small and subtle changes, such as increased shade or the sinking of a borehole, may be enough to tilt the scales towards extinction. But when a familiar and seemingly unfussy species starts spiraling downwards, something more serious and widespread is indicated. We have seen it before in the mysterious decline of the house sparrow, which recently joined the Red Data List of birds, among the likes of dotterels and honey buzzards. Now, it seems, another household name is heading for the rocks - the woolly bear or garden tiger moth.

The evidence is to be found in insect-trap records monitored by Rothamsted Research in Hertfordshire, the world's first agricultural research station. The Rothamsted Insect Survey operates about 450 light traps across the country, although only 80 traps are in use at a time. The original purpose of the survey was to monitor fluctuating numbers of potential crop pests, but it also created a detailed record of common insects, especially moths.

The survey was conceived as a kind of early-warning sign of potential crop pests. However, this unique record is also of interest to entomologists and conservationists, as it records the fluctuating fortunes of several hundred species of moths. These trends can be compared with climate data to find out why a moth may be more common in one year than the next. Only recently has funding become available for the work. A grant from the Esmée Fairbairn Foundation has enabled Kelvin Conrad to analyse the survey's records in detail for the first time. And the early results on six target species are disturbing.

Once-common moths such as the lackey and the figure-of-eight have declined by 72 per cent and 64 per cent respectively. As for the garden tiger moth, its range has contracted by 60 per cent and its numbers are down from an average of 140 trapped moths in a year to about 60. In some parts of southern England, including Rothamsted itself, it seems there are no tigers at all. It has quietly quit the scene - and, but for the chance of this series of records, perhaps no one would have noticed.

Why does this matter? Well, in the first place, the garden tiger is not just any old moth. Its hairy, fast-moving caterpillar was familiar as long ago as the Middle Ages, when it was known as the "walbote" or, in modern English, the "woolly bear". The moth itself is as pretty as any butterfly, with its chocolate and cream forewings and spotted scarlet hindwings, which it flashes when disturbed by a potential predator. The bright colours carry a warning - this moth is not for eating.

Nor is the woolly bear caterpillar, whose dense covering of irritant fur is too much for any bird except the cuckoo. Being poisonous, the moth doesn't bother to hide, and you could find them sitting about by day on fences and tree trunks, or even in the garage or garden shed. It is the sort of moth you can rear in a jam jar on dandelion leaves - the woolly bears eat almost any plant that's going - and it was a popular favourite on the nature-study table at school.

It would not be so bad if this were a hypothetical heathland tiger or sea-cliff tiger, confined to marginal land on the fringes of Britain. Their decline would be a fact of modern life. But this is the garden tiger, the familiar of humankind, chewing the dandelions as we dig the vegetables, preparing to fly as we take in the washing. It is the least fussy of insects, as much at home in town as in the country, and perhaps as common in gardens as anywhere. What could possibly have gone wrong? (It can't be the cuckoos - they are in decline, too.)

A convincing answer must explain why the decline is general, and why it affects numbers as well as the moth's range. It must show why the moths are vanishing not only from gardens, but from nature reserves, where many of the Rothamsted traps are sited. And it should explain why this moth has become rarer as the climate has grown warmer: moths, like butterflies, generally appreciate warm summers.

In fact, climate change may offer a clue. Climate monitoring stations across Britain are revealing a shifting pattern in the weather. Analysis has shown that our winters are becoming more frost-free, and wetter relative to summer. Cold snaps in winter are fewer, and sogginess instead of hard frost is the rule. The nights are getting warmer, and the growing season has lengthened by a month since 1900. These changes are not always benign.

My own window on the world is a regular walk through the fields and woods near my home in the Kennet Valley of Wiltshire. During the past 10 years, I have seen many changes. Stinging nettles are replacing flower-rich marshes where the river floods. And something terrible is happening to our crack willows. They are dying slowly of a blight that begins with the tips of the branches and works its way inwards to the core, reducing a healthy tree to a skeleton within three or four years. The cause is fungal infection, or rather a combination of wilt, canker and leaf-spot operating together, sometimes on the same tree.

Although these diseases are always present, they have never before threatened to kill every large willow tree in the valley. Experts blame our mild, damp winters and rain-sodden autumns. In the past, freezing winds off the Marlborough Downs ensured lengthy spells of air frost, which held fungal infection in check. In the Kennet Valley, at least, climate change means fewer trees and longer views. The knock-on effects on the wildlife will be incalculable.

As for the garden tiger moth, there may be a chink in its environmental armour. The moth is single-brooded, and its caterpillar is obliged to hibernate when only one-third grown. Little is known about this stage, but they are believed to pass the winter communally in loose silk cocoons on the ground, among leaf litter. Their cocoons were once found among discarded chip-wrappers.

Are milder winters causing the caterpillars to wake prematurely, and to waste energy crawling off in search of food? And are they encouraging the same kind of fungal or viral infections that have devasted our willows? It is impossible to prove - as Kelvin Conrad points out, "correlation doesn't prove causation". But there is a strong tie-in between wet winters and warm springs and the observed decline of the garden tiger moth. For whatever reason, it doesn't like climate change.

Nor is the tiger likely to be alone in this. Of the six moths targeted by the Rothamsted Insect Survey, all but one has declined significantly. This is little surprise to moth enthusiasts. The charity Butterfly Conservation has logged many comments and data-sets of declining catches of common moths. As its director, Martin Warren, points out, what affects a moth will also affect the bird that eats that moth. "They are a vital part of the food chain, and our results will help us to understand how serious the knock-on effects of their declines will be for other species," he says.

The good news for the garden tiger is that it seems to be hanging on in its stronger sites, especially in the North, and is unlikely to become extinct. But it is functioning as a kind of ecologist's canary, warning us that warmer doesn't necessarily mean better. The fate of the ancient walbote, like so many others, depends on our own ability to live and prosper without wrecking the natural systems of the earth.