Why did the little snail cross the road?

September finds nature adrift: wind-sift of thistledown over the dunes, solitary gannets fishing southwards, whimbrels whistling their way from headland to headland. For birds, it's migration; for plants, propagation; for mammals and insects, dispersal from nesthole and crevice.

Wildlife must have room to travel, to meet and mate, swap genes within species, and escape from change and disaster. But as landscape is developed, new barriers appear and habitats are fragmented.

Some tracts of countryside are preserved for nature, but these are fast becoming islands in an inhospitable, humanised sea of intensive farming, roads and rural suburbia. The smaller the reserve, the fewer species it can sustain, and interchange between colonies gets harder all the time.

This is why the idea of ecological corridors and "stepping-stones" has become so important in European conservation strategies. Not only have the EU states had to create SACs - Special Areas of Conservation - under the Habitats Directive but must also link them into a network by managing linear features such as rivers, with their banks, and continuous hedgerows, and also the ponds and small woods which would help to bridge gaps between refuges. The idea has been accepted as important for Ireland. In the highly-regarded Alias of the Irish Rural Landscape, Professor Fred Aalcnsees the future countryside webbed by an "ecological framework of protected areas with buffer zones and connecting natural corridors."

Development of a Pan-European Ecological Network was endorsed by all EU ministers of environment four years ago, and a tentative network for Ireland was sketched Out in a Dutch study of Europe's rural areas. As part of it, an ingenious if sprawling long-distance corridor would link Connemara and south-west Mayo with the Mourne Mountains by way of the Burren, Shannon Callows and midland raised bogs. Another long corridor stretches from Achill to Malin Head.

In the Netherlands' own network, corridors are meant to help migration by wide- ranging mammals such as otter, badger and red deer, and fish such as trout and salmon. Other EU countries have wolves, brown bear, lynx and wild boar to consider, or deep-forest owls and woodpeckers. A good many of the "priority" species threatened in Europe are simply not relevant to Ireland, but a study of the potential of corridors for Ireland has just been made available by Duchas in its new series of Irish Wildlife Manuals*. Written by Dr Jervis Good, it gives some intriguing insights into the ways and means of "dispersal ecology". In the windswept Burren, for example, the main means of seed dispersal might seem obvious. But, except for a few well-designed aerial species, most seeds are blown no further than a few metres from the parent plant. After the wind, "epizoochory" - transport by adhesion to animals or man - is the most important means of wide dispersal, and about half of the 460 flowering plants of the Burren are well suited to it.

Sheep, with their fine-meshed fleeces, are well known for carrying seeds in calcareous grasslands. The Burren has no sheep, but it does have roaming flocks of feral goats with shaggy, often matted coats, and hares, which travel long distances in a day and groom their coats in sheltered places. Cattle must also play a part, but are probably better at spreading seeds by the other means, known as "endozoochory": in one experiment. 662 seedlings of 24 common species were germinated from a single cow-pat.

So, it may be quite important to protect the roaming rights of feral goats throughout the Burren, and to keep them long-haired, culling the short-haired domestic goats, sometimes abandoned in the hills, which might interbreed with them. In simple cost-benefit terms, Dr Good is sceptical of the value of setting up big ecological Corridors with planning protection. Peatland corridors, for example, are unlikely to help dispersal of most of the more characteristic bog mosses. Better to spend money on saving precious bog habitats from being cut or drained.

RARE and often tiny snails, such as Ireland's precious Ice Age fenland relicts in the Vertigo family, arc not, unsurprisingly, all that good at travelling. Dr Good quotes a German finding that only four Out of 53 species of snails were capable of crossing a two-lane asphalt road. Rather than try to keep a long regional corridor marshy enough for snail journeys, it might make more sense, he says, to resettle some of the threatened species at a new site every 10 or 20 years.

For other species, short local corridors may be much more effective. Bats, for example, often need particular hedgerows to link their roosting-place with their feeding-ground at a wood (this is true of lesser horseshoe-bats at a protected site in Co Clare). Wide local corridors of heathland along the verges of roads can help the dispersal and breeding of non-flying beetles with special tastes in habitat. Ireland's most threatened butterfly is the lovely marsh fritillary, tied to damp meadows and hillsides where its food-plant, the devil's-hit scabious, grows abundantly. Most Irish colonies are isolated, and conservation needs a lot of colonies relatively close to each other. It may be more important to identify the barriers to the butterflies' dispersal than to try to manage corridors for them. Rivers and their banks are ready-made regional corridors, certainly vital to otters, if pollution and habitat destruction start to threaten their dispersal. But on corridors in general, says Dr Good, it's better to spend the money on making small reserves bigger and protecting the connections between small, isolated populations at risk of extinction.

His arguments must carry special weight among Irish ecologists just now, as the flood of new development bears in upon every wild and vacant corner of the Republic. Never mind, for the moment, grand corridors for nature -just man the local stockades.

* Contact Dr Ferdia Marnell at Duchas 7 Ely Place, Dublin 2