New threat to grasslands

Easter in the midlands was definitely worth chancing. Those huge skies over the Shannon and the bogs were a perfect theatre for the weather: Arctic showers looming in great, black curtains, hailstones swirling and bouncing round one's ears; squalls on the river, raising waves and rocking boats; magical clearances to a deep, polar blue, and the sudden, golden blaze of furze and dandelions.

"Glory be to God, what furze!" wrote Stephen Rynne, my favourite artist-farmer of the midlands. "May I marry a girl with furze-coloured hair! Give me bouquets of it, sheaves and posies; stick it on wands for me; nail it to my mast; and hang it out of my windows should I come riding home in triumph."

Rynne was in ecstasy at the view (from a seat in a friend's new yellow-and-white sports car) just 60 years ago, while the midlands' raised bogs were still largely intact, each with its thick fringe of furze. The fields were then much smaller and often fenced with furze. Most midland roads were furze-hedged lanes, and the hillsides were "gaudy with blazing furze in a scented spate", to give Rynne his head.

All in all, a lot more gorse bushes than there are today. Yet the shrub could now have embarked on a new wave of weediness, as dire in its own way as the spread of rhododendron in the western woods.

A companion at Easter was the ecologist John Feehan (author, with Grace O'Donovan, of the wonderfully encyclopaedic The Bogs Of Ireland). Monitoring the midland landscape from his home in Birr, he finds furze creeping further every year along the eskers, those sinuous ridges, shaped beneath glaciers, that are such a distinctive landform.

The steeper slopes of the eskers hold some of the last of Ireland's semi-natural grassland; like the nearby Shannon callows, they have never been ploughed. In summer, the plants form a brilliant community, the native grasses starred with dog-daisies, bird's foot trefoil, devil's bit scabious, meadow clover, valerian, even the green flowers of lady's mantle. It is a rare and special vegetation, echoed in Britain on the high chalk-hills of Sussex, or in the meadows of some Alpine valleys.

The grassland is "semi" natural because farmers originally created it by clearing the eskers of trees. During thousands of years, it was fertilised only by cow-dung, and mown for hay by hand, and the cattle themselves were too small and light to do much damage to the slopes. Today, along the gentler eskers, ploughing and reseeding with ryegrass has wiped out most of this ancient vegetation, and today's heavy cattle are pushing the gravelly soil downhill. The continued existence of the old esker grassland depended, in fact, on an active farming management. When mowing stops, the slopes are invaded by spiny, cattle-proof shrubs such as furze, part of the natural succession that ends up in woodland. So those wonderfully vivid yellow ribbons winding along above the fields of Offaly are anything but cheerful ecologically. Just one species of shrub is smothering slopes that, until yesterday, grew almost 100 different grasses and wildflowers; ryegrass or furze, the effect is a huge loss of biodiversity.

Ironically, that other brazen weed of spring, the dandelion, has never fared better than in the ryegrass fields below the eskers, its myriad flowers as intensely golden as the grass is intensely green. It is the one wildflower to have survived the monocultural onslaught, squirrelling the wash of fertilizer down into its tap-root.

At the Banagher marina over Easter, the holiday cruisers jostling at the jetties were being hoovered and fitted with fridges. All over Ireland, no doubt, anglers were addressing more modest craft, fixing and scraping, and thinking, perhaps, of trying a new lake or water-system this year.

Anyone who cares what happens to the balance of life in our waterways, not to mention mundane matters such as piped water supplies to homes and industries, will not think of moving a boat from the Shannon without cleaning it very well. Just one dinghy lifted heedlessly from Lough Derg and launched, say, for the mayfly fishing in the Corrib or Sheelin, could introduce several thousand zebra mussels, a most appalling pest.

Dreissena polymorpha, with dark and pale zig-zag bands, has been building up in Lough Derg and the lower Shannon for two years or more, arriving, probably, on the hulls of private boats and barges imported from the UK. It is the trailered boat that is most likely to spread them beyond the Shannon catchment.

Dreissena is notorious as a fouling organism, smothering piers, buoys and anchor chains as well as clogging intake pipes, but, paradoxically, the effect of its feeding is to filter water of its plankton and thus produce lakes as clear as Ballygowan. The huge population in Lough Derg already seems to have improved its water quality, giving rise to the suspicion that the mussels may even have been introduced deliberately.

Unfortunately, the ecological cost of clarity could be great, because millions of zebra mussels, cloaking every hard surface in a lake, are bound to compete with other species, both physically and for their food. In Lough Derg, for example, the mussels are colonising gravel used for spawning by pollan, one of Ireland's endangered species of fish: this could cause the pollan's local extinction. Dreissena will also wipe out existing populations of the native swan mussel by smothering them with their offspring. The Marine Institute has already issued 10,000 pamphlets to warn Shannon boat-owners about the dangers involved in moving their craft. Small mussels can be almost invisible on the hull and can fix themselves to ropes and nets, even the filter grid of an outboard engine, so cleaning a boat and its gear for a transfer will need to be really thorough.

Sightings of the mussels in new areas should be reported to Dr Dan Minchin, Fisheries Research Centre, Abbotstown, Dublin 15; 01-8210111; email: dminchin@frc.ie or Dr Kieran McCarthy, Zoology Department, NUI, Galway; 091-750379; email: tk.mccarthy@ucg.ie.