The UK's invasive species

Some of Britain's best loved wildlife, including bluebells, red squirrels and water voles, are threatened by invasive species. The red squirrel has suffered since the introduction in the 19th century of the stronger, more adaptable, grey squirrel Photograph: Martin Bydalek/Getty

The Department for Environment, Food and Rural Affairs (Defra) says that non-native species, such as this floating pennywort, damage Britain's wildlife and cost the economy some £2bn a year Photograph: Jeff Foott/Getty

Red-eared terrapins, originally from the US, are foot-long former pets that can terrorise ducklings Photograph: Michele Lamontagne/Getty

British bluebells are threatened because they hybridise with Spanish bluebells Photograph: Ian Cartright/Getty

An American mink. Farmed until the 1980s, hungry escaped minks are blamed for the collapse in water vole numbers. Water voles have declined by 90% since 1990 because of habitat loss and the spread of American mink which prey on them Photograph: Lorri L. Franz/Corbis

Some invaders pose a risk to human health, like this giant hogweed, whose sap causes blistering Photograph: Amanda Darcy/Getty

The Azolla fern, a fast-growing floating aquatic plant, is capable of spreading completely over lake surfaces in a matter of months Photograph: Tony Wharton/Corbis

Japanese knotweed is known in its native language as Itadori, which means simply "strong plant". Experts have put the cost of removing the species from Britain at £1.6bn. The plant has no natural enemies here and causes problems through rapid invasion of habitats, exclusion of other plants, and damage to property Photograph: Yasuhiko Akaishizawa / Sebun Pho/Getty