Country: Plain tales from ant heaven

Which single property can boast that it has 46 species of butterflies, 22 different orchids, more than 700 fungi, 18 nesting pairs of rare stone curlews, more than 200 ancient monuments, and the largest set of prehistoric flint mines in Britain? Answer: Porton Down, the secret defence establishment on the southern edge of Salisbury Plain.

Now officially known as Dera - the Defence Evaluation and Research Agency - Porton Down abandoned its offensive programmes of developing chemical and biological weapons 40 years ago, and now many of its 700 scientists concentrate on producing protection against such horrors. The place still has a sinister air, being surrounded by high mesh fences, with red flags permanently flying and prominent notices warning "Road liable to closure without notice", "Prohibited place", "Photography not allowed", "DO NOT ENTER", and so on.

Yet, for a non-scientific outsider, by far its greatest interest lies in the state of the land within its boundaries. For the best part of a century the chalk downland has gone un-ploughed and unfarmed, except for occasional light grazing, and the result is that it looks much as large parts of England must have looked centuries ago, whenever an agricultural depression temporarily depopulated the countryside. Juniper, dogwood and scattered trees rise from seas of waving grass.

The land was hastily requisitioned by the Government in 1916, when a cri de coeur came from the British Army on the Western Front in France. The memorandum - which is still exhibited in the entrance hall of the headquarters - complained that the Germans were using noxious gases, and urgently requested not only protective measures, but means of hitting back.

The area chosen for experimental work was a strip of 7,000 acres running east and west, 11 miles long and four-and-a-half miles wide. Two major advantages commended it to the War Office: first, in its centre lay a wide depression, known for obvious reasons as "The Bowl", which could not be observed from the perimeter; and second, the prevailing south-west wind would carry any dangerous emissions harmlessly away over the lightly inhabited wastes of Hampshire.

Over the next half-century thousands of shells and bombs rained down on the range. According to the present director, Paul Taylor, "only a few tens of shells were ever fired containing nerve agents, and every one of them has been recovered". Most of the missiles contained pitch or water, used as simulants for chemical warfare agents, but the result is that today, three feet below the surface, there are 200,000 "metallic traces". These alone, in Mr Taylor's view, make it "highly unlikely that the land will ever be sold off to build supermarkets on"; but in fact the work that his team does is of such importance that the site will certainly remain in its present state for the foreseeable future.

This is good news for conservationists, because the flora and fauna at Porton Down are astonishingly rich. On a tour of the estate with Terry Jeanes, the deputy site manager, and Clive Bealey, an ecologist on secondment from English Nature, we were constantly stopping to inspect some unusual plant or insect.

One of the most fascinating stretches is that known as the Antscape, where yellow meadow ants have raised extraordinary numbers of mounds. After a survey, Clive reckons the count of anthills at 3 million, and the number of ants at 35 billion. Because there are no heavy farm animals to tread the surface down, the carpet of grass and herbs undulating over the mounds is soft and springy enough to turn your ankle.

Whole hillsides are ablaze with yellow rock-roses, and the air is heavy with the scent of wild thyme and lady's bedstraw. Experts can point out any number of unusual plants: sainfoin, dropwort, meadow clary. Where military vehicles have churned up strips of earth along roadsides, there are amazing explosions of purple viper's bugloss.

The butterflies send lepidopterists into ecstasies. Silver-spotted skippers, marbled whites, meadow browns and many blues thrive in this primitive environment. Dark green fritillaries feed on the hairy violet, which flourishes here because rabbits do not like it. Clive, who is doing research into butterflies, roughly calculates the dark green fritillary population at 377,000, and that of meadow browns at 1.5 million.

The number of rabbits is such that the place might well be called Watership, rather than Porton, Down. To some extent they are useful, in that they keep grass short, nibble away at the shrubs that constantly threaten to invade the open ground, and provide food for foxes, thus taking pressure off the stone curlews. But they also destroy the earth bunds surrounding magazines by burrowing into them, and they have to be heavily culled; one good night's shoot can easily produce a bag of 200.

Roe are another potential menace; a reasonable population is acceptable, but if numbers become excessive, the deer can cause unacceptable damage in farm crops on the periphery. On top of her official job, Terry Jeanes is chairman of the local conservation group, which has 170 volunteers, and a member of the deer control team. Out with her .243 Mannlicher in the evenings, she contributes substantially to the annual cull (the target this year is 71 bucks and 68 does), and knocks off as many foxes as she can to give the stone curlews a better chance.

The said curlews must be among the most pampered birds in Britain. Because they like to nest in open areas, where all-round visibility gives them a defence against predators, conservation workers scrape five-acre sites clear for them every spring, two sites for each pair, so that they can rear their first brood on one, and their second on another.

The accent, then, is very much on active management, the aim being to retain the grassland and its riches indefinitely. But all the time, in the middle of it, Dera's work goes on. As we passed a group of buildings with a tall chimney from which steam was issuing, Terry casually remarked that it was the incinerator.

"Of course," she said, "there's a complex filter system on the chimney. They're getting rid of mustard - and I don't mean Colman's."

Meanwhile, to the north on Salisbury Plain, the conservation story is much the same. A century of occupation by the military has fossilised the landscape and retained textures long since lost elsewhere. At 94,000 acres, the training area is by far the largest in Britain, and over the past five years management has been tightened to minimise damage to the environment, in particular to the 2,400 archaeological sites that the plain contains.

According to Ian Barnes, the resident archaeologist, the policy now is to avoid disturbing known monuments, many of which have survived far better than those on the Marlborough Downs to the north, where burial mounds and barrows have been ploughed out of existence. "On the other hand," he says, "if we're putting in a track and come on something, we excavate. Last year we found a large Iron Age settlement ahead of a road we were building."

In the central impact area, which is bombarded by shells, rockets and missiles for seven days and two or three nights a week, wildlife flourishes to an extraordinary degree. "The place is full of deer, badgers and foxes," says Paul Toynton (who, like Clive Bealey, is on secondment from English Nature). "You often see badgers and foxes in broad daylight, and the roe are so unused to people that you can walk right up to them. In spite of the noise, loads of birds would rather be there than in a barley field, because the plants are covered in insects."

Paul is immensely enthusiastic about the fecundity of his enormous patch. "Toads!" he exclaims, of an area in the west. "That valley's full of toads. They breed like mad in the temporary pools created by tanks." He was delighted to find, in one pool, a fairy shrimp, whose eggs are distributed about the countryside on tank tracks.

Yet his pride and joy is a small, grassy bank near the abandoned village of Imber. There, within two minutes, he identified 11 species of butterfly and more than a dozen wild flowers. "Autumn gentian, hedge bedstraw, hoary plantain, self-heal, fairy flax - I bet you could find 40 different plants in one square metre."

Even on experts, though, this fossilised landscape can spring surprises. The other day people came in with blisters on their hands. Someone raised a scare that they had been contaminated by ancient relics of mustard gas, but the villain turned out to be wild parsnip, whose juice reacts with sunlight to cause nasty swellings.