Country Matters: Urgent: please send more rooks

WHERE else would land crabs threaten to undermine the foundations of a space tracking station? Where else would frigate birds, hanging like kites in the wind above the coast, be a menace to aircraft on their way in to land?

Only on Ascension Island, in the South Atlantic, does nature persist in throwing up such extravagant obstructions to progress - and in few corners of the earth have men been more thoroughly frustrated in their efforts to create a balance of fauna and flora. Having first seen this extraordinary place 25 years ago, I was delighted to discover, on a recent return visit, that nature is still rampant.

When the British took possession of the island in 1815, they found it uninhabited except by sea birds, land crabs, rats that had come ashore from wrecked ships, and goats landed by Portuguese mariners to provide meat for anyone stranded there. For more than a century the Royal Marine garrison struggled to establish an equilibrium by importing animals, birds and plants, but time and again their efforts were frustrated.

The marines were astonished, and not a little dismayed, by the physical nature of their new possession. The top of an extinct volcano, it consisted mostly of black lava, with brick-red cinder cones rising from it. Only round the summit of the highest hill did they find any cultivable soil, and from the sparse vegetation growing there they called the peak Green Mountain.

Much of the low ground was then colonised by sea birds - mainly sooty terns, which bred among the lava in hundreds of thousands, if not millions. Imitating their shrill cries, the marines called them 'wideawakes'. But, finding that rats had multiplied horribly by feasting on the sea birds, they imported cats to prey on them - and that was the start of their ecological troubles.

The cats escaped into the lava and went wild, eating birds in preference to rats, and evolving extra long legs to cope with the savage terrain. To thin out the cats, the marines imported dogs; a pack of bull terriers was assembled, and cat-hunting became the islanders' principal sport.

Bounties were offered in the form of money or spirits, and Admiralty records show that a great many cats were dispatched. 'So numerous are they,' reported the commandant in 1824, 'that I have killed 16 in a day and should have killed more but that the dogs were knocked up for intense heat and lack of water.'

In spite of such persecution, the feral cats held out, and their descendants, skulking about the lava to this day, have almost cleared the sea birds from the main island. Local regulations now prescribe that all domestic cats must be spayed or neutered, and feral cats are culled regularly; nevertheless, they have driven the wideawakes to a few last resorts in the south-west corner, and even there they prey heavily on them as soon as the birds come ashore to nest.

When I was there, the breeding season had not quite begun, but the wideawakes had started the preliminary activity known as 'nightclubbing', in which they assemble overhead and touch down briefly in the dark; and every morning the ground was dotted with freshly eaten corpses. Other sea birds, such as frigates, boatswains (or tropics) and boobies (various forms of gannet), have withdrawn to inaccessible cliffs, or to the cat- free stronghold of Boatswain Bird Island, off the east coast.

Less vicious yet equally persistent survivors of the marines' occupation are the wild donkeys - large, light-coloured animals, apparently able to live on a diet of cinders and thorn bushes - which make off with loud hisses when you approach them. To prevent them wandering on to the runway, the whole of the 3,000-metre (10,000ft) airstrip has had to be fenced - no easy undertaking, when the ground is of solid rock - and, as with the cats, discreet culling is needed to keep numbers at an acceptable level.

It was on Green Mountain, where the marines established a small farm, that they met their most serious ecological reverses. Some vegetables grew well, but all too often crops were devoured by plagues of insects or caterpillars. The obvious answer to insects was birds, so many species were imported: guinea fowl, partridges, pheasants, starlings, mynas. A cri de coeur to the Admiralty sent cables headed ROOKS FOR ASCENSION flashing back and forth across London. A plague of mice brought urgent appeals for owls; another call went out for hedgehogs, and several for bees to help with pollination.

Of all these creatures, very few survived. The guinea fowl lasted for some years before dying out, and even now a few red- legged partridges still cling to the upper slopes of the mountain; but the bees were blown away by the ceaseless trade wind, which hustles across the island from the south-east, and the owls and hedgehogs succumbed to the equatorial climate. Only the mynas, which had been procured with great difficulty in Mauritius, settled in happily: today flocks of them inhabit not only the mountain, but the human settlements below.

Attempts at horticulture were further undermined by the land crabs - strange, sidling creatures, some bright purple, some yellow, which live in burrows and come out mainly at night. During the 1870s vermin bounties increased to 1s/6d a head for wild cats, a halfpenny a head for rats, and 1s/6d a hundred for mouse tails and pairs of land- crab claws. These incentives produced phenomenal bags - in 1879, for instance, 66 cats, 7,683 rats, 4,013 mice and 80,414 land crabs - and the commandant felt able to report:

'But for this destruction, I believe not a bird would be left on the island. Rats would have devoured all crops, and landcrabs would have honeycombed the ground to such an extent that much vegetation would have been destroyed.'

No wonder that one gardener, Joseph Spearing, had to be taken away in 'a condition of quiet dementia', suffering from a malady officially described as 'formication', which caused him to believe that not only his bed but his whole house was swarming with ants.

Alas for such Herculean labours] The marines never won their war with nature, and today much of their good work has been undone. It is true that the upper slopes of Green Mountain are cloaked in trees and shrubs - ginger and bamboo especially - but the farm has largely been let go: sheep run wild, and a small band of cattle, descendants of the dairy herd, roam the high pastures at will.

As for the land crabs - they survive in immense numbers, and they are still capable of causing havoc when they migrate down to the sea to lay their eggs.

In 1990, on the coast at North- East Bay, the European Space Agency built a brand-new, multi-million pound earth station for tracking the Ariane rockets that launch commercial satellites from French Guiana. Hardly was the installation finished before staff found to their dismay that the foundations of the main building were being undermined by land crabs.

Extra concrete solved that problem, but still the crabs come down from the mountain in such strength that sometimes when the technicians drive away after tracking night-launches, their cars have to crunch their way out of the park over a carpet of exploding bodies.