Battle of the species

With the garden in shreds, and squelching underfoot, I take refuge in a greenhouse that smells like the bottom of the Royal Canal and I begin sowing seeds in yogurt cartons: early cabbage, sweet peas, a few, tentative tomatoes anything to hurry on the spring.

Shifting last year's pots and trays, I uncover drifts of woodlice that panic and flee from the light. They have multiplied hugely in a greenhouse full of rain drips, algae and rotten bits of wood - just their sort of habitat. In each separate cache of woodlice lives a centipede to eat them, but I never catch this going on.

I ought to know which kinds of woodlouse I have, but am lazy about it. There are three or four common species, spread widely through the island, and more than 20 others that belong to particular localities or special kinds of habitat.

There's Oritoniscus flavus, for example, a fast-moving, purple species quite common along river banks in the south-east. It has never been found in Britain, but it does occur in south-west France and Spain - one of our many "Lusitanian" puzzles.

The study of the places where insects, plants and animals live, and the reasons they live there, is called biogeography. The word has taken on a special currency in the past few decades, as science has realised the appalling rate at which Homo sapiens is wiping out other species. The biogeography of islands, in particular, may hold lessons for our hopes of protecting other creatures in isolated nature reserves.

Ireland's biogeography is often good for an argument among biologists and botanists, as they try to sort out how particular species arrived on the island after the last ice age (or survived it where they were). And while one might think that virtually all our species are well known at this stage, there are fresh finds all the time.

This is specially true among the insects - most of which, of course, have no problem reaching an island as close to a continent as we are. We have about 16,000 species - a "surprisingly rich" collection, says Dr Jim O'Connor, the entomologist in charge of the Natural History Museum. And the bulletin he edits for the Irish Biogeographical Society seems to add new names at a rapid rate.

The latest issue of the bulletin lists 19 sawflies new to Ireland, and a dozen new species of water beetle found in the cut-over bogs and drumlin fens of the north. An English entomologist, hunting beetles in the moss and reeds around lakes in the west, found five that have not been recorded and seven that Ireland has but Britain doesn't.

English names and addresses occur quite a lot in insect reports in the bulletin. This is partly because British entomologists enjoy bughunting holidays in Ireland, and partly because we have so few watchful entomologists of our own. In his lively contribution to the new book Nature In Ireland, Dr O'Connor deplores the dearth of jobs for entomologists in Ireland. A lot of Environmental Impact Assessments, so fashionable in planning, are meaningless, he says, unless accurate names can be put on the insects collected or recorded from the site and their significance assessed.

Ireland has not been cut off from Europe for long enough, and is not sufficiently remote, to have evolved any new species of its own. We do have some varieties and subspecies, in everything from butterflies and moths to stoats, hares and otters, but nothing to match the totally new forms of bird and mammal that evolved in isolation on, say, the far-flung islands of the Pacific.

Ireland's natural turnover of species - especially of insects and birds - is strongly influenced by our nearness to Europe. But the country does seem to follow the rules of island biogeography in that we have notably fewer species than the neighbouring, much larger, island, with its wider range of habitats and temperatures.

The effect of area on the number of species has been a controversial issue in studies sparked off by the now-famous Theory of Island Biogeography outlined in the 1960s by America's Robert MacArthur and Edward O. Wilson.

This proposed a set of ecological rules - mathematical ones - governing the equilibrium of species on islands of different sizes and ages and degrees of remoteness. And it highlighted the fact that "islands" aren't always surrounded by water. They can be mainland mountain-tops, or chunks of rainforest, or national parks set up to conserve wildlife.

The theory has been used to conclude that a big nature reserve can hold more species at equilibrium than a small one, and that a close cluster of reserves can support more species than the same number of scattered ones.

It sounds convincing, but applied across the board by scientific advisers can have some tragic consequences. In The Song Of The Dodo, his brilliantly readable book about evolution and island biogeography, science journalist David Quammen tells of the problems such ambitious design principles have caused in countries where even small reserves are hard to keep. Doubts about their value can give a weapon to predatory developers.

The alternative to theoretical shortcuts is fieldwork that weighs up the worth of a habitat on the ground, case by case. Even a small reserve can protect, for example, an especially good breeding ground for a threatened species. But a breeding-ground is not a whole habitat, and "small" may need to be much bigger than most people would expect.

For threatened insects and other invertebrates, for example, a few hectares are just not enough: it may take a whole marsh or woodland to secure their survival. A Council of Europe science team led by Dr Martin Speight, research entomologist with the National Parks and Wildlife Service, has proposed 100 hectares as the minimum for a site of international importance for protection of threatened European insects. But even 100 hectares is a very small, as islands go.