What's that lurking under your stones?

ANY TIME I lift up something big and flat on the acre these days (a concrete block, a fishbox, an old log - anything thats just been sitting there for ages) I brace myself for an encounter with Artioposthia triangulata, the dreaded New Zealand flatworm, coiled on the earth in a stocky, dark, purple-brown ribbon.

This predatory pest is so widespread in Ireland now that some naturalists feel there's scarcely any point in adding to the map of new occurrences. And anyone who buys plants from garden centres has to reckon with the chances that, sooner or later, they will bring in a pot that's sown with the flatworm's egg capsules (shiny, black, up to 8mm long, seen from June to September).

It's now almost 30 years since the flatworm was first found in Northern Ireland, and a decade since the alarm was raised among agricultural scientists. The flat- worm eats earthworms rapaciously and it appears to have met no natural enemies of its own. The implications of its indefinite spread for the drainage and fertility of our soil would seem to be horrendous.

There are other more immediate, threats to Ireland's earthworms from the techniques and chemicals of intensive farming. And the flatworm's impact may settle eventually into the cyclic alternation of numbers that balances many predators with their prey. But Ireland's cool moist climate offers the flatworm a free run of open farmland - of fields that, in New Zealand, would be too dry and hot - and only time will show how much that matters.

Gardeners, meanwhile, might like to take heart from the experience of a friend who has lived with the flatworm in his plant nursery for well over a decade. He regularly traps the sticky beast by providing moist, dark places in his tunnels and squashing all he finds on a regular patrol. At the same time, he boosts the earthworm population of his soil by making and using plenty of organic garden compost.

Old compost, stacked in contact with the land, is usually teeming with earthworms. The flatworms have never got the upper hand on my friend's holding and seem, indeed, to be fewer than they were - but the hot, dry summer last year, followed by such a cold winter, may have helped to keep their numbers in check.

Arrroposthia is not the only alien from Don Under to find a niche in the Irish countryside. There is another little Antipodean animal, making small but steady advances through the woods, which we also need to keep an eye on.

Arcitalitrus dorrienii ("Archae" to its familiars as for something strikingly old) is a tiny, flattened, shrimp-like creature - an amphipod crustacean - that has been around since the early days of life on Earth. You are familiar with sandhoppers, leaping up in swarm from rotting seaweed on the tideline. Archae is of their terrestrial branch - a land hopper.

THE northern hemisphere doesn't naturally have landhoppers. They evolved, very early, in Gondwanaland - the huge supercontinent that broke up to make Australasia, Africa and South America. Their natural home is the forests of New South Wales, southern Queensland and the north island of New Zealand, where they feed at night in the leaf litter on the ground.

When Arcitalitrus was first found in these islands - on the Scilly Isles in 1924 - it was thought to be a creature new to science. Then, when the hopper was discovered in its natural Australian habitat, it was obvious that Arcitalitrus had been introduced to Britain, probably with exotic plants.

Today it is fund( at about 90 places in the warm south-west of England, and also on the island of Colonsay, off the west coast of Scotland. There are particularly high densities in the litter beneath Australian tree ferns, Dicksonia anarctica, which were imported intact to "sub-tropical" gardens like those on Tresco.

In Ireland, the biggest colonies of the land hopper centre on Kylemore, the warm, moist mountain valley that holds the Connemara National Park. The creature was spotted first near Kylemore Lough in 1936

However it arrived, the hopper has thrived especially on the litter under Rhododendron ponticum, the lovely but invasive alien shrub which has done its best to choke so many western woodlands. The past few weeks have seen the roadsides brilliant with. rhododendron blossom right through the Kylemore Valley to Killary Harbour and beyond, and the hopper proliferates beneath. An ecological study begun in 1989 has found it at Aasleagh Falls, at the head of Killary Harbour, and round into the Delphi Valley, in Mayo.

Its colonies occur over an area of 50 square kilometres and show, in fact, a wide range of habitats, from mixed woodland and lodgepole pine to bracken and grassland - even the litter under wayside fuchsia hedges. The hopper is apparently capable of moving up to 40 metres in a night, but wet blanket bog and open moorland are not to the animal's taste, and a ring of mountains has helped to stop it spreading further.

The hopper sometimes reaches quite dramatic densities - up to 12,000 in a square metre. And it can process up to a quarter of the annual carbon put into the soil by the breakdown of litter. Such industry could sound like a good thing - helping, in particular, to recycle the tough leaves of rhododendron. But in the wider ecological view, the hoppers are competing with native soil invertebrates, such as earthworms, woodlice and millipedes. Together with soil microbes, these have been handling the carbon system satisfactorily for thousands of years. A takeover by the hoppers could change the local system, and perhaps not for the better.

Hence the importance of the on going study by UCD zoologists Thomas Bolger and R. P. O'Hanlon. Their objectives: "To examine the population biology of this species, the responses of the local fauna to its presence, and its role in the nutrient dynamics of the forest.