Science: Meanwhile, back at the bat cave: Bernardine Coverley looks at wide-ranging efforts to save one of Britain's threatened species from extinction

LAST WEEK, a discreet census was taken. Across the country as twilight began to fall, enthusiasts were busily counting the numbers of bats taking wing to feed. The results will be mixed. No one will have seen any mouse-eared bats this time, for the species was pronounced extinct a year ago - although the demise of the first mammal on British soil since the wolf in the 1740s has been barely noted.

But in Dorset, there is a reasonable hope of better things. This year should confirm the success of a project designed to halve the decline in another species: the greater horseshoe bat. After three winters in specially constructed hibernation quarters, numbers have risen from 90 to more than 200.

How long is it since the flutter of bats in the early evening was a common sight? As a creature only obvious at twilight, bats have been easily overlooked among the great changes in the countryside.

Bob Stebbings, Britain's foremost expert on bats, believes that since nature can no longer be considered entirely natural, at least in Britain, it follows that the survival of species will depend on tactful and well-

researched management. He is ensuring that the greater horseshoe bat has a better chance than its relative. As bats go, this one is medium sized, weighing less than 1oz, with brown silky fur. All 15 species are protected under the 1981 Wildlife and Countryside Act. The mouse-eared and greater horseshoe were the first to be covered by a 1975 act. But that did not prevent the former's disappearance and the latter's decline.

'It's such a difficult animal. As it moves between so many roosts, there's no way the statutory authorities can protect it,' Dr Stebbings says. There are about 4,000 greater horseshoe bats in total in Wales and south-west England. After many years with the Natural Environmental Research Council and independent consultancy work, Dr Stebbings does not quite know them all, but he must come close.

A local group offered help to protect the threatened species. This assistance made possible an ambitious plan to dig a tunnel and, later, excavate a concrete cave to provide a hibernation place within the existing breeding home, a disused farm building in Dorset.

'The concepts involved in protecting the bat are quite complex,' Dr Stebbings says. 'Needs vary with age. Older adult females need higher temperatures for hibernation, and social groupings vary according to season, while males are solitary and territorial in the breeding period.'

By creating the right conditions for year- round use, constant protection could be provided on one site. Although many places are used as roosts, the hibernating and breeding spots are the two essential points for the survival of a colony that may inhabit an area of up to 700 square miles.

The problems of conserving this key site were approached with practical ideas and considerable faith. Fortunately, the owners of the property were sympathetic, and English Nature and the Whitney Animal Protection Trust supportive. Nevertheless Dr Stebbings had to hope that the greater horseshoe bats' relatively late breeding age of five or six years and the infant mortality rate would not hinder the recovery of a local group depleted from several thousand.

The first step was to make an attractive winter venue to replace the many hibernation sites that had disappeared. 'The idea was to have a whole range of different temperatures based on measurements taken from various winter sites. We decided on a Y-shaped tunnel, and it had to be done when the bats were least likely to be in residence.' The tunnel was occupied that winter. After this success, the safe haven for hibernation could be reliably extended, with the expectation that more bats using the site as a nursery would also stay for the winter.

Again temperature was the important feature. Calculations resulted in a cave with a maximum height of 2.5 metres (8ft), built from concrete blocks with 1.5 metre-thick walls and insulated with polystyrene (recycled missile wrappings donated by the RAF). 'Essentially, we created a maze to reproduce ideal conditions,' Dr Stebbings says. 'It's a dynamic system and it's adaptable, so when I learn something new it can be altered.' A third of the colony, 36 greater horseshoe bats, as well as several other bat species, took to the cave in the first season.

The premise that making the site attractive all year round was the best approach has been confirmed by a rising birth rate. The roof of the building had been provided with electric blankets, set at 30C.

During the first summer 26 babies were born, with 39 in the second. Last year there were 38 births. After 31 years' involvement, Dr Stebbings confesses a personal as well as a scientific satisfaction. But it will be another few years before this Dorset colony can be described as securely re-established.

The greater horseshoe bat is regarded as an endangered species. Its population is believed to be only 1 per cent of that earlier this century - when Hampstead Heath and the British Museum were popular homes and colonies occupied Canterbury and Winchester cathedrals. Now there are only seven viable colonies with 14 breeding sites. A Welsh colony was almost destroyed when fire gutted the old stables it was using. Several thousand bats died, and now fewer than 400 remain.

The use of toxic chemicals in treatments for roof timber, accidents - the male of one breeding group was shut in a cellar by new owners - and a breeding pattern that averages one birth every two years add to the bats' vulnerability. Even in the wild, their private life is easily disturbed. 'People wander into caves and light fires or camp but aren't aware of the bats,' Dr Stebbings says.

Information from the Dorset habitat will be used to enhance protection at other, less private sites. And the next step there will be to look at the wider environment. 'We need to say to landowners in a positive way what kind of conditions the greater horseshoe bat needs, and make agreements,' he says.

Woodland and pasture are ideal. When milk quotas arrived and farmers disposed of dairy herds, the loss of permanent pasture deprived bats of their best food source. Without cowpats and consequent beetles and swarms of fat flies, bats must range further afield, using up precious energy.

Cockchafer beetles would make a perfect diet, but ploughing turns up the larvae for seagulls. Four or five adult beetles provide a good bat meal, whereas one of small, less nutritious craneflies means a bat must expend 20 times as much energy for foraging. Better food leads to better breeding.

One positive development that has taken place is a change in the public perception of these intelligent creatures. Bat conservation groups exist in every county under the Bat Conservation Trust, gathering information and encouraging people to be aware of this lesser-known wildlife. We may never meet them, and yet they still share our homes.

(Photograph omitted)