10 invasive species causing problems in the UK

These invasive non-native species are causing harm to environments and native wildlife.

10 invasive species causing problems in the UK

The Non-native Species Secretariat (NNSS) has the responsibility for helping to coordinate the approach to invasive non-native species in Great Britain.

These are examples invasive species which have become estabilished in Great Britain and are causing problems.


1. Japanese knotweed

Japanese knotweed breaking through concrete © GB NNSS

Originally from eastern Asia, the Japanese knotweed introduced to Great Britain by the Victorians in 1886 as an ornamental garden plant.

It is now widely established across most of Great Britain, and is notorious for forming dense stands which are extremely hard to control. It is estimated the Japanese knotweed costs the economy £166 million a year.


2. Rhododendron

Rhododendron covering a hillside in Wales © Natural Resources Wales

Rhododendron was first introduced in 1763 from the Iberian Peninsula for use in gardens, and there are now few areas of Great Britain are not affected by this plant.

It blocks light, preventing other species from growing beneath it and leaving only trees that are able to grow above the level of the rhododendron canopy. It also carries diseases which are fatal to some of our native trees.


3. Grey squirrel

Grey squirrel © GB NNSS

Originally a native of North America, grey squirrels were deliberately released into the wild in Great Britain in 1876, and carry a pox virus to which our native red squirrel is very susceptible.

Red squirrels have now been wiped out across most of Great Britain, mainly through disease transmission, and now only a few populations remain in England and Wales. Fortunately, red squirrels still have a stronghold in Scotland and dedicated programmes are helping to ensure their conservation.


4. American mink

American mink preying on a juvenile gannet © John W Anderson

Another American native, mink were introduced to Great Britain in 1929 for use in fur farming but individuals began to escape from farms and breed in the wild.

Since their introduction, the native water vole has experienced one of the most rapid and serious declines of any British wild mammal during the 20th century. There has been a decline in as much as 94 per cent in the number of water vole sites, and predation by the invasive American mink has been a key factor in this decline.


5. Himalayan balsam

Himalyan balsam © VWB photos / Getty

Like many other invasive plants, Himalayan balsam was introduced by the Victorians for use in ornamental gardens. It is still occasionally used today as a garden plant, but can also be found growing wild across much of Great Britain, where it crowds out native plants including tansy which is the only food plant of the endangered tansy beetle.


6. Floating pennywort

This North American aquatic plant was first recorded in the wild in Great Britain in 1990, having spread from garden ponds and aquaria into the wild.

Floating pennywort can grow up to 20cm in a day, quickly covering whole water bodies and harming aquatic wildlife by blocking out light and reducing the oxygen available in the water. The dense mats can also interfere with recreational activities like angling, and make it difficult for boats to move around.


7. Muntjac

Muntjac deer © GB NNSS

This small deer from China and Taiwan was brought to Great Britain in 1831 to be kept in collections, but is now common across most of England and parts of Wales.

Muntjac grazing can have serious impacts in woodlands where they can clear shrubs and prevent tree regeneration, affecting other wildlife including birds and butterflies.


8. American skunk cabbage

American skunk cabbage © GB NNSS

This North American plant was introduced in 1947 by ornamental plant collectors, who admired its striking flowers.

It has become established in some wet woodlands, where it crowds out native plant species and makes its presence known by emitting a strong skunk-like odour.


9. Carpet sea squirt

Carpet sea squirt © Natural Resources Wales

This marine hitchhiker from the North West Pacific was first discovered in Great Britain in 2008. While each individual organism is tiny (only 1mm long), carpet sea squirt grows in colonies which can cover several square kilometres, and any other species which get in the way. It is a nuisance for anglers and boat owners as it clogs up fishing equipment, covers boat hulls, and smothers reefs.


10. Signal crayfish

Signal crayfish © Trevor Renals

This lobster-like freshwater species was introduced from America in 1975 to be farmed for food, but quickly escaped and spread rapidly through Great Britain.

Since its arrival it has driven the white-clawed crayfish towards extinction through competition and transmission of a crayfish plague, which doesn’t harm signal crayfish but is fatal to white-clawed crayfish. It also burrows into riverbanks leading to erosion and increasing flood risk.


Non-native invasive species to look out for that could cause big problems if they become established.

Read more about the GB non-native species secretariat

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