Aquatic invaders threat to biodiversity

| 19th June 2009
Ecologist: News Analysis
Non-native invasive water plants threaten biodiversity and ecosystems. You’d expect to find them on a banned list, not on sale at UK garden centres.

Invasive water plants that out-compete native species, reducing biodiversity and causing problems such as
flooding, are still on sale as ornamental pond plants in garden and aquarium centres around the country, warns
the charity Plantlife.

In May, Plantlife launched a survey, ‘Against the Flow’, appealing for the public’s help in researching the extent to which these plants have escaped into Britain’s rivers, canals and lakes. It lists five of the most invasive species: parrot’s feather, New Zealand pigmyweed, creeping water primrose, floating pennywort and water fern.

‘These plants cause a variety of problems,’ says Sophie Thomas of Plantlife. ‘They form dense carpets that block
light, warmth and oxygen from the water, affecting other plants and animals, including rare species such as great crested newts and water voles. Through the survey, the public can be our eyes and ears to report back the extent of their spread.’

Parrot’s feather ( Myriophyllum aquaticum) chokes waterways and can lead to flooding. Originally from Central America, ‘it is now surviving the UK winter, which is a worry for the future as this will allow it to spread more widely,’ says Thomas. In ponds on Kingston North Common, near Ringwood, Hampshire, parrot’s feather is
overtaking the habitat of brown galingale, a threatened native plant.

Yet parrot’s feather is still on sale around the country. At World of Water in Essex, for instance, it costs £2.99
for a 9cm diameter pot. ‘Yes, it spreads very easily,’ responded a sales assistant to a phone enquiry from the

World of Water stores also sell curly waterweed (Lagarosiphon major), which ‘forms dense masses in standing water, reducing other plant life and harming invertebrates,’ according to Thomas. It is one of many invasive ‘waterweeds’ (sometimes called ‘pondweeds’), but is not yet widespread in the wild across the UK. Originally from southern Africa, it was first recorded in the wild in Britain in 1944. ‘Its spread is probably from new introductions, when dumped in the wild by people clearing out their ponds or aquaria,’ says Thomas. World of Water aquatic centres, of which there are 20 around the UK, sell curly waterweed for 75p a bunch, wrongly
advertising it on its website as Elodea crispa.

It can be dificcult for the public to know what they are buying at garden centres, as plants are often mislabelled. ‘Many genera are dificult to tell apart; some arrive incorrectly labelled or with their old scientific names,’ says Thomas. ‘We hope it’s not deliberate. Some garden centres are even selling plants with names that, taxonomically, don’t exist.’

One species the Salisbury-based charity is asking people to look out for is floating pennywort ( Hydrocotyle ranunculoides), which can grow 20cm a day, clogging waterways. It has been discovered on sale labelled as ‘water pennywort’ or ‘pennywort’, and has also been mistakenly labelled native British pennywort. Several species, such as New Zealand pigmyweed (Crassula helmsii), introduced in 1911, are often labelled as generic ‘oxygenator’ plants. New Zealand pigmyweed
is also sold as Australian swamp stonecrop, Crassula recurva, Tillaea helmsii and Tillaea recurva.

Sometimes consumers buy a plant without realising it, when a smaller one ‘hitch-hikes’ in the pot of another.

A spokesperson from the Garden Centre Association says it works closely with Plantlife to educate its 200 UK
members about invasive species, but ultimately, ‘it’s up to individual centres what they stock’. There is no law
to prevent the sale of invasive species, only a voluntary code, the Horticultural Code of Practice. Plantlife says
this should change. ‘There’s a very small handful of plants, about 20, that should be banned by law,’ says

In 2005, an audit by the GB Non-Native Species Secretariat reported 2,721 non-native species of plants and animals in England. The government body says on its website: ‘Invasive non-native plant and animal species are the second-greatest threat to biodiversity worldwide (after habitat destruction). They can negatively impact on native species, can transform habitats and threaten whole ecosystems, causing serious problems to the environment and the economy’.

The Government has estimated that invasive species cost the UK economy £2 billion a year.

There are those who argue that some of our most quintessentially ‘British’ flora and fauna – such as poppies and hares – have been introduced over the centuries, and we should welcome invasives. However, most biologists agree that recent human-brought arrivals that significantly out-compete native plants and animals
pose a serious threat to healthy ecosystems and biodiversity. As climate change takes hold, niches will open up for invasive species to exploit even further.
Managing weeds can be a Sisyphean task, as many can reproduce from tiny fragments. ‘The best way is to tackle
infestations at the top of a water catchment area and work downstream,’ says Thomas.

In the Lake District, some invasive plant species are being effectively managed thanks to multi-stakeholder
community groups begun by volunteers. Windermere Invasive Species Group began as a small band of
concerned individuals. Their dedication and commitment led to the involvement of the Environment Agency
and Lake District National Park.
‘Community involvement is vital to the success of tackling invasive species,’ says Bev Dennison, one of the group’s initiators.


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Paul Miles is a freelance writer and photographer


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