Recently, creeping water primrose was identified in a garden pond, near Sneem, Co Kerry, and prompt action is being taken to eradicate it. A native of South America and some US states, this is the first time it has been reported here.
Water primrose is a water-based herb with deep roots and tough stems. It forms dense, tangled mats strong enough to impede the passage of boats and its presence also prevents angling, swimming and other water-based activities.
Ominously, Dr Joe Caffrey, senior research scientist with the Central Fisheries Board (CFB), warned: “If this invasive species were to spread throughout Ireland, the economic and ecological cost to the country would be too great to quantify.”
It had been a cause of concern for some time that many of these non-native invasive species can be purchased from retail outlets, pointed out Dr Caffrey who also welcomed legislation, shortly to be enacted, to make it illegal to import or sell such species.
The number of non-native freshwater species recorded in Irish watercourses increased significantly in the 20th century and the trend is continuing. However, not all non-native species are invasive and current problems are caused by only a small percentage.
Oonce they take root, though, it’s very difficult to get them out. Indeed, it could be said the only way to control them is not to allow them in the first place.
Some of these species are better known than others. We have had the zebra mussel for many years, while we also have the American grey squirrel and the Chinese mitten crab. The more obvious, though, are plants which are easily observed in water.
Invasive species represent one of the greatest threats to the natural world, second only to that caused by direct habitat destruction. They do this by out-competing our less robust native species, by preying on native species or by altering the natural habitat.
The effects are only too obvious, including restrictions on boating, angling, swimming and other water-based leisure pursuits. Plants can also impact on industry by clogging engines, turbines and water intake pipes with significant costs to the economy.
Lough Lein, often described as Killarney’s most valuable tourism asset, has been affected by such plants. So has Lough Corrib, in Co Galway, Ireland’s second largest lake and a wild brown trout fishery that attracts anglers from many parts of the world.
Curly-leaved waterweed was first noticed in Lough Corrib in 2005. Investigations failed to trace the source, but the seed material is suspected of having come from an ornamental garden pond. A task force of representatives from various government agencies was set up and a detailed case study carried out.
A native of southern Africa, the weed was seen as a serious threat to Lough Corrib, a lake of major conservation importance under the EU Habitats Directive. The lake is one of the top five sites in the country for wintering waterfowl and a vital source of food for these birds.
The weed creates a thick, green canopy through which virtually no light can penetrate. It spreads quickly across the water and creates the sort of conditions that do not suit brown trout and are much more preferred by coarse fish, such as perch and pike. The newly-hatched fry of such fish can be protected by the vegetation and pike can avail of the cover provided by dense vegetation to stalk prey.
Various trials have been carried out into ways of controlling the spread of the plant in Lough Corrib and even removing it altogether. A weed-cutting boat has been getting good results, removing over 95% of the weed in some cases.
According to Dr Joe Caffrey, new control methods will continue to be trialed and, if adequate funding is provided, he believes they will be able to bring the weed under control and eradicate it from large areas of the lake.
Meanwhile, the South Western Regional Fisheries Board, which covers much of the Cork/Kerry region, is notifying garden centre owners, gardeners and people in regular contact with ponds and waterways about the threats of such species and how they should be dealt with.
The board is also renewing its requests to anglers, boat owners and other water users to be vigilant and not to transport invasive species from infested waters to unaffected waters.
Other well-known invasive plants include New Zealand pigmyweed, Himalayan balsam, giant hogweed, water fern, Nuttall’s pondweed, Japanese knotweed, parrot’s feather and fringed water lily.