On the threat of an alien mussel invasion

Another Life: The boreen's clumps of purple loosestrife are reaching the last flowers on their spikes, whereupon the singing magenta of their blossom will follow the froth of meadowsweet into the end of summer.

How odd to think of one of Ireland's most brilliant wildflowers ever becoming a troublesome weed, "invasive and competitive and unavailing to native wildlife" - yet that has been loosestrife's fate in North America.

Almost two centuries after its arrival in the New World, dumped out with ships' ballast from an English estuary, it has been sweeping across wetlands and damp grassland on a scale - and with a fierce, monocultural intensity - unimaginable in its home landscapes. In the hills of Washington State alone, where it arrived as late as the 1970s, it has blanketed 23,000 wetland acres, displacing native plants and birds.

A mature plant of loosestrife produces 2.7 million seeds, the size of ground pepper. Both in North America and Ireland they are blown on the wind and swept along by traffic. Chunks of loosestrife roots get spread in roadworks. Yet Lythrum salicaria is a welcome decoration of Irish ditches - and a purple plague in Canada and the US.

The explosive waywardness that can overtake species displaced far from their home ecosystems - usually by human activity - has become a worldwide problem. Threats from invasive terrestrial aliens (grey squirrels, rhododendron, and so on) have been aired at several Irish wildlife conferences this autumn. But the reality of global concern is really brought home by next week's 13th International Conference on Aquatic Invasive Species in Ennis, Co Clare. Its 100 or so scientific papers will deal with invaders as far apart as the golden apple snail in the rice fields of the Phillippines and the red swamp crayfish of Italian marshes.

The conference location in Ennis, however, is no random choice. It brings delegates close to the "frontier" of the most recent national example of an invasion by the most notorious aquatic alien - the zebra mussel, Dreissena polymorpha. This small freshwater bivalve, with a stripy shell now all too familiar to many Irish boat-owners and anglers, has so far been unstoppable in its smothering advance from eastern Europe.

Carried across the Atlantic a couple of decades ago to the Great Lakes of the US on the anchor chains of cargo ships, it spread in a mere seven years to 18 American states, two Canadian provinces, and down the Mississippi to the Gulf of Mexico.

It reached the Shannon a decade ago, stuck to the hulls of second-hand pleasure craft from the UK. Since then, despite an intensive education campaign among boat-owners meant to bring home the need to clean boats, engines and trailers, it has travelled throughout the Shannon catchment to Loughs Derg, Ree and Key, and onwards through the new northern canal to the whole of Lough Erne. Boats taken into the Grand Canal for the winter have carried the mussel with them, and, according to a national review led by Dr Dan Minchin, the numbers in the canal's Shannon Harbour had reached more than 120 million by 1998.

The mussel's most urgent impact on human affairs is in its physical clogging of underwater pipes and filters, from the intake pipes of power stations and industries to those of boat engines and local water supplies.

Carpeting the stones of lake beds, its underwater masses cut the feet of bathers, who may then risk infection with water-borne Weil's disease. A Lough Derg Science Group has been formed by Dr Minchin to advise county councils on these and other heritage management problems.

The mussel's impact on freshwater life is already evident. Its dense clusters have smothered and destroyed the lakes' native swan and duck mussels. They have climbed the stems of water lilies and dragged them down with their weight. They envelop vegetation where fish and insects choose to lay their eggs.

The filter-feeding mussels clear lake water marvellously and can even seem an answer to eutrophication, the big problem of pollution. In Lough Erne, some anglers have thanked the clearer water for better trout fishing. Dr Robert Rosell of the Northern Ireland Department of Agriculture grants that it may have improved the catchability of trout - but stresses that it hasn't increased their numbers.

The conference, in the Lynch West Country Hotel from Monday to Friday, is hosted by the Sligo Institute of Technology, whose chair, Frances Lucy, has herself been studying the zebra mussel populations of Lough Key.