Of all Earth's surfaces, a glistening estuary mud-flat bared at low tide can seem the least inviting to the human foot: how deep would you sink? A city backdrop and a whiff of sulphides adds to our apprehension of the ooze.
For vast flocks of wading birds and wildfowl, however, these margins of fine, well-nourished sediment are valued feeding grounds, rich in marine worms, water snails and amphipods. A shelduck taken from the mud-flats beside the North Bull salt marsh in Dublin had 3,000 tiny Hydrobia snails in its stomach.
A new wave of development, dredging and construction is now reaching out into the tidal zone at Galway, Cork, Waterford, Wexford and Drogheda; even Dublin Bay may not be immune from change. In future patterns of sediment in our major estuaries and inner bays, a steady rise in sea level seems the only certainty.
All this brings fresh significance to a reappraisal of common cordgrass, Spartina anglica, an alien species that has already brought great change to Irish estuaries over the past 75 years. Long regarded as a rampant menace, robbing waders of their feeding areas, it may now come to be accepted for its ecological benefits, and even welcomed as a natural defence against the erosion of "soft" estuarial coasts.
S. anglica, a species scarcely a century old, has a remarkable evolutionary background. It stems from the accidental introduction of an American coastal species, Spartina alterniflora, to the south of England, where it crossed with S. maritima, a native European species, to form a sterile hybrid. But this grass then went on to double its chromosomes in a new fertile species, dubbed Spartina anglica.
This "mutant" cordgrass spread with incredible vigour. The dense mat its roots make in salty mud (like couch grass) trapped tide-borne sediment to firm up the seaward fringe of salt marshes along the Channel coast - an effect that soon caught the eye of harbour authorities with land reclamation in view.
During the 1920s, the new grass was planted deliberately around the coasts of Britain and north Europe. In Ireland, the first settings were made in Cork Harbour in 1925 and subsequently in estuaries such as Baldoyle, Co Dublin; Fergus estuary, Co Clare; Westport, Co Mayo; and Belfast Lough. Its seeding was prolific, so that clumps of Spartina in salt marshes and almost pure stands of the grass on mudflats are now commonplace along the coast of Ireland.
The cordgrass first appeared on the mud-flats at the Bull Island nature reserve in Dublin Bay, soon after a causeway was built to it in 1965. Almost at once there was worry it would spread, closing out the flocks of migrant waders from this vital wintering ground.
Dublin Corporation tried to dig it all up, then to eradicate it with herbicide sprays - a control that has continued, with limited success, for some 30 years.
There have been similar and sporadic efforts elsewhere, notably at Strangford Lough, Co Down, where Spartina was thought to threaten beds of Zostera, the soft eel-grass that feeds wintering flocks of brent-geese. But the image of Spartina as an alien threat to nature conservation may have to be revised in the light of a new study by wetland ecologists in the botany department of University College Dublin, funded by Dublin Corporation.
MARK McCORRY, researching with Dr Rinus Otte, has come to take a much more positive view of the impact of Spartina, to the point of advising Dublin Corporation against any further spraying. With fingers crossed, he predicts the cordgrass in the bay won't spread much more, and that the initial, invasive impetus of a new species, while still apt to surge here and there, is beginning to reach equilibrium in the Irish environment. Indeed, in a paper written with Dr Tom Curtis, the Duchas botanist, McCorry argues that the new stands of the cordgrass add to diversity of intertidal habitats. Like the long-established swards of Spartina alterniflora along the east coast of North America, they could come to support "a rich and varied fauna".
Already, his research at Bull Island has found that clumps of S. anglica had higher densities of invertebrates such as Hydrobia snails than other areas. It suggests that cordgrass can support a flora as rich and diverse in species as the marsh samphire, Salicornia, that it often replaces.
Ornithologists will need some persuading that waders and wildfowl used to probing bare mud for their food are not put off by dense stands of cordgrass. But McCorry finds that evidence of this is rare, and no data exist to suggest that waders and wildfowl have declined in Irish estuaries due to the spread of Spartina. Indeed, the organic material it puts into the mud-flats should boost the numbers of marine invertebrates to the benefit of birds and fish.
Where does this leave Spartina in the scenario of rising sea levels? Its root system is outstanding at binding the seaward edge of a salt marsh and holding it together through tidal storm surges. Salt marsh will be vital as a first, "soft edge" defence against the sea in estuaries with harbours and cities.
While planting of spartina is "not recommended" in Ireland's new Ecopro coastal-protection manual because of its reputed ecological damage, some authorities may prefer to reflect on the conclusion of McCorry and Curtis: "Had the invasion and spread of S. anglica occurred 500 years ago, control would not have been an issue . . . is just another example of a species colonising a vacant niche in Ireland's flora." The wider issue of marine invaders and their effects on native Irish species will be discussed during a two-day conference on marine biodiversity in Ireland and adjacent waters, to be held at the Ulster Museum in Belfast on Thursday and Friday. It is organised by CEDAR, the North's Centre for Environmental Data and Recording, with the Marine Institute among its supporters, and topics range from whales, dolphins and deep-water coral reefs to the life of sandy and muddy shores. The conference fee is £50 sterling.
