Australian invader threatens Scottish lochs 

JAMES REYNOLDS ENVIRONMENT CORRESPONDENT

AN INVASIVE aquatic plant responsible for localised extinctions of native plants in England and Wales is now spreading in Scotland, threatening to alter the fragile ecology of the country’s lochs and burns.

Australian swamp stonecrop (Crassula helmsii), which was brought to the UK from Tasmania in 1911, was first detected in the wild in 1970 in the English Home Counties, after being widely used by garden pond enthusiasts to oxygenate water.

By 1986 it had colonised 33 open water courses south of the Border, and in the following 13 years to 1999, it had spread aggressively to a further 574 lakes and freshwater systems. It is rapidly colonising Scottish lochs, where it is out-competing and displacing native plants.

Also known as New Zealand pigmyweed, the aquatic invader is found around damp margins of ponds and in water to a depth of three metres. It appears as small, light green tussocks, spreading rapidly to form dense, interwoven mats. Edinburgh’s Duddingston Loch, which is infested with the alien plant, is experiencing similar problems to the English lakes. Natural aquatic vegetation is being smothered by the invader, which grows throughout the year. Its progress kills the native plants and creates an oxygen-depleted habitat, which causes further problems for invertebrates and fish.

Dr Deborah Long, of the wild plant conservation charity Plantlife Scotland, said: "Crassula grows from tiny fragments and so is very difficult to control. Eradication methods so far down south have consisted of raking it out, but this needs doing every year and re-infestation is common. Widespread herbicide could also work, but obviously isn’t a great solution in ponds where rare species survive."

In a list compiled by Plantlife of the top 20 alien invasive species which threaten the UK’s flora, Australian swamp stonecrop comes in at number four. In particular it is threatening the survival of starfruit, one of Britain’s rarest plants, and pillwort, a rare aquatic fern present in Duddingston Loch.

In England and Wales alone the cost of adequate control is estimated to be about £3 million, with no prospect of complete eradication.

Dr Long added: "This plant is still sold as a pond oxygenator and is very vigorous, so outgrows garden ponds quickly. Any excess material preferably needs to be burnt, as any escaping into the wild, either deliberately or accidentally, by ducks for example, is likely to cause further problems."

It is most easily spread by fragments produced by cutting and tearing in the process of clearing; pieces as small as one node (5mm) can re-grow.

Dr Kevin Murphy, an expert on freshwater environmental biology at Glasgow University, said: "There is a strong chance that it is going to cause problems up here, especially as it has colonised large areas of northern England.

"In many ways this plant is more of a problem than those alien aquatic species which, for example, have proved unmanageable in other Scottish water courses such as Loch Lomond.

"Rather than being confined to the water, this thing is amphibious, meaning it also comes up the banks onto the edges of the water. It forms great swathes on to the margins, and this means that it doesn’t just compete with the aquatic plants but also with plants along the sides and is extremely aggressive."

Dr Murphy added: "It has caused lots of problems down south and I suspect it will do the same here. It will upset the current ecological balance of lochs and interfere with all sorts of things, including habitats that fish use for breeding, and the many types of invertebrates and bugs that can exist on the native aquatic plants."

The plant’s spread throughout the English Lake District presents a bleak picture of what could happen to Scottish lochs. Crassula was first discovered in Derwentwater in 1996. The following year it was found in Basenthwaite Lake, Coniston Water in 1998 and Grasmere in 1999. In one bay in Derwentwater, which is infested with the plant, eight native species have already been displaced.

Keith Davenport, the chief executive of the Ornamental Aquatic Trade Association said: "Some years ago, we made a recommendation that our members do not trade in this particular species because of these problems. To try to prevent the spread of this weed we also produced a point-of-sale poster which had the backing of both the charity Plantlife and the Consumers’ Association. This stresses that aquatic hobbyists should ensure that any plant material they remove from garden ponds is never disposed of by transferring to wild ponds, streams, canals, rivers or lakes.

A spokeswoman for Scottish Natural Heritage said: "Like most aquatic alien species, Australian swamp stonecrop is extremely difficult to control once it is released in the ecosystem, so our efforts are focused on prevention rather than trying to manage it. We are working with other agencies to tighten up the law against releasing non-natives like this in the future."

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