ALIEN invaders are threatening the Lake District, according to the National Trust and English Nature.

Alarm bells rang when David Preston, National Trust forest warden for Coniston and Little Langdale, recently discovered an unwelcome aquatic weed in the water at Cathedral Cavern in the Little Langdale valley.

The offending plant was a Water Lettuce, Pistia Stratiotes, a non-native floating weed resembling an open-leaf lettuce which colonises lakes, rivers and ponds worldwide. Unchecked it can form dense mats, out-competing native plants and changing the entire eco-system – it is a major problem in parts of Australia and the USA. In one case in Australia Water Lettuce carpeted a 25-kilometre stretch of river before the authorities had a chance to deal with it.

The plant would not normally survive winter frosts in this area, but Mr Preston pointed out that, underground in Cathedral Cavern where frost may not reach, it might have survived the winter, or had it been introduced in spring, it could have flourished and spread into the nearby Little Langdale Tarn and beyond with potentially devastating effects on the environment.

Mr Preston said that the plant must have been put into the water at Cathedral Cavern on purpose. But, rather than being an act of environmental hooliganism, he suspected it had been introduced in ignorance, possibly by someone releasing goldfish into the water.

The Water Lettuce was removed and further scraps have since been taken from the site.

“We will be going back almost every day to make sure there is no spread,” said Mr Preston.

He appealed to people to take more care and think carefully before introducing anything into the environment, warning that some species could be carried on the bottom of boats, on angling equipment and even on footwear.

“I think people just need to be aware that we have these problems and take a bit of responsibility themselves – really it’s just about taking care and thinking about the possible consequences of their actions,” said Mr Preston.

Ian Taylor, a botanist in English Nature’s Kendal office, explained that the problems of invasive non-native species reverberated through the whole ecosystem because foreign plants tended to be free from predation and competition which keep natural flora in balance.

Other invaders - Water Lettuce is just one of several non-native plants threatening the sensitive local ecology on land and water.
Pygmy Weed, Latin name Crassula Helmsii, is a pond plant with a grip on Bassenthwaite and Derwentwater and, according to Mr Preston, is gaining a hold in Coniston.
Himalayan, or Indian Balsam (Impatiens glandulifera) is another garden escapee to colonising the countryside and costs £150 to £300 million a year in the UK.
Giant Hogweed, Heracleum mantegazzianum, was introduced to Britain in the 19th century as an ornamental plant. It is poisonous and is now widespread especially along riverbanks.
Japanese Knotweed, Fallopia japonic, the most invasive weed in Britain, is already a familiar threat. This fast growing perennial it is capable of cracking concrete and undermining buildings.