Invasive non-native species

American Mink were brought to the UK in the 1920s to be farmed for their fur. They were first recorded breeding in the wild in Devon in 1956 and by the 1960s were widespread on Devon’s rivers. Mink will feed on whatever they can including birds, mammals, crustaceans, fish and amphibians. Water Voles were once common in Devon but became extinct in the county by 2002 due to habitat loss and predation by mink. The population of American Mink in Devon has dramatically declined due to a range of reasons, possibly including the increased otter population. However, they remain a threat to the reintroduction of Water Voles. 

American Signal Crayfish were introduced to the UK in the 1970s as a farmed species. The crayfish escaped into the wild, rapidly spreading across the country.

Signal Crayfish out-compete native White-clawed Crayfish (a Devon Special SpeciesDevon Species of Conservation Concern which have been 'shortlisted' as needing particular action or attention.) for food and habitat. Females can carry up to 300 eggs (known as berries) – three times as many as White-clawed Crayfish. They also carry a fungal disease called crayfish plague which is fatal to native crayfish and can be easily spread via unwashed waders, wellies, watersports equipment and nets.  

Signal Crayfish, pollution and habitat degradation all led to a huge decline in the White-clawed Crayfish population during the 1980s and 90s, with up to 95% of populations lost in southern England. The only two populations of White-clawed Crayfish in Devon are now on the Creedy and Culm rivers. See White-clawed Crayfish for more information.

Signal Crayfish also have significant negative affects on freshwater ecosystems due to their voracious consumption of native invertebrates, vegetation and fish. Their burrowing activities also cause siltation of rivers and damage bankside habitats. 

There are currently no reliable methods of Signal Crayfish control.  

Formerly widely sold for ornamental planting in bog gardens and around ponds, this invasive species has escaped and is now found locally in wet marshy areas and wet woodland.

The plants form dense colonies, out-competing and eventually smothering native vegetation. It spreads along watercourses and ditches through seed and rhizomes. On Dartmoor it’s threatening flower-rich marshy grassland of national importance for rare butterflies and other insects.  

American Skunk Cabbage was first recorded in the wild in Devon at Lydford Gorge in 1980. Most of Devon’s records are from the fringes of Dartmoor and the South Hams, with a few from eastern Devon, and recently Exmoor.

Britain’s deer population has grown significantly for many reasons, including lack of predators, less hunting and its size. Red and Roe Deer are native species while Fallow Deer, Muntjac and Sika Deer have all been introduced, mainly for hunting.

All deer species (native and non-native) graze on young trees and shrubs, which can prevent them from developing and is a significant issue for existing woodlands and woodland creation.

Other issues include rising tick populations, damage to agricultural crops and the risk of road accidents.

Floating Pennywort was introduced to the UK in 1990 as a garden pond plant. Because it spreads rapidly from tiny fragments it can invade rivers and canals.   

In summer, Floating Pennywort can grow up to 20 cm every day, blocking sunlight, displacing native plants, raising water temperatures and creating mosquito breeding areas. Its dense mats slow water flow and reduce oxygen levels, harming aquatic wildlife and disrupting food webs. In winter, the plant dies back and releases excess nutrients, further degrading ecosystems.   

Complete removal is difficult and costly. CABI is trialling biological control with South American weevils which has been successful so far. Due to the threat it poses to wetlands, there is a national Floating Pennywort strategy group led by the Environment Agency.

Floating Pennywort isn’t well-recorded in Devon. It was recorded at Exminster Marshes where an eradication project took place between 2000 and 2013. It’s also been recorded near to Slapton Ley in south Devon.

Listed under Schedule 9 of the Wildlife and Countryside Act 1981, it’s illegal to cause Floating Pennywort to grow in the wild. It was banned from sale in 2014. 

Giant Hogweed grows in habitats such as riverbanks, woodland edges, gardens and common land. It can grow to more than five metres tall, shading out native plants and creating bare patches in winter that increase the risk of erosion, especially along watercourses.

Giant Hogweed is a serious public health risk. Even slight contact with the plant can cause severe skin burns and long-term sensitivity to sunlight. 

It spreads through seed and a single plant can produce up to 50,000 paper-thin seeds, each about the size of a penny. Hogweed can self-fertilise, forming dense stands near parent plants. Seeds also disperse via waterways. 

Giant Hogweed can be easily confused with the very widespread Common Hogweed and Cow Parsley. The Tamar valley has the longest running catchment control programme for Giant Hogweed in Britain.

Listed under Schedule 9 of the Wildlife and Countryside Act 1981, it’s illegal to allow Giant Hogweed to grow in the wild. 

Grey Squirrels were introduced to the UK from North America in the late 1800s as part of the Victorian trend of collecting plants and animals. They soon spread across the country and are now a common species in rural and urban areas. Their three major impacts are:

Decline in Red Squirrel populations. There’s evidence that Red Squirrel populations were already declining due to loss of habitat. However, Grey Squirrels are larger and better at competing for food. Grey Squirrels also carry the squirrel pox virus which doesn’t harm them but is fatal to Red Squirrels. Red Squirrels have been considered extinct in Devon for at least 50 years.

Damage to broadleaved trees. Grey Squirrels strip bark from trees to access the nutrient-rich sap underneath. This can damage or kill young trees. Squirrel (and deer) damage is a significant deterrent for landowners who want to plant new broadleaved woodland.

Predation on birds, eggs and chicks.

Himalayan Balsam is a tall, annual plant with pink flowers that can form dense stands. Each plant can produce up to 2,500 seeds. When they’re ripe the green seed pods explode and can shower seeds as far as 7m. If the seeds land in a waterway they can then travel much further.

Himalayan Balsam can be found across Devon in moist soils, riverbanks, ditches, woodlands, waste ground and, increasingly, in hedges. They form dense stands that can out-compete native species. When the stands die back in winter, the bare soil is vulnerable to erosion, increasing sediments in rivers.  

CABI is trialling biocontrol for Himalayan Balsam across the country.

It’s listed on Schedule 9 of the Wildlife & Countryside Act 1981 (as amended) which means that it’s an offence to let it grow in the wild (or extend beyond your land). 

Japanese Knotweed (Fallopia jappnica) was originally introduced as an ornamental plant and is now widespread across the UK and Devon.

It’s capable of penetrating tarmac and concrete and can cause significant structural damage. It can grow quickly, forming dense stands and outcompeting native vegetation.  

Japanese Knotweed can regenerate from tiny fragments of the stem or underground rhizomes and must not be controlled by cutting or strimming. Specialist advice must always be obtained.  

Listed under Schedule 9 of the Wildlife and Countryside Act 1981, it’s illegal to allow Japanese Knotweed to grow in the wild. 

Pacific Oysters (Magallana gigas) are now widespread along the south coast of England and can form large reefs that change the structure of coastal habitats. These reefs can affect sensitive areas, including Marine Protected Areas, and are considered a key management concern. Other pressures, such as disease and natural predators, also play a major role in limiting the recovery of native oysters. 

Removing Pacific Oysters is hard. Manual removal can help in small, localised areas but does not offer a realistic way to eradicate them, especially where new larvae arrive from other populations. Research shows that even intensive efforts cannot fully eliminate wild stocks and must be repeated to keep numbers down. Oyster farmers can help by harvesting wild oysters, which could support both management and local businesses. 

Because Pacific Oysters are well established, management is shifting towards reducing risks while recognising that they can provide habitat in some degraded areas. Policies focus on biosecurity for aquacultureThe farming of aquatic organisms like fish, shellfish, and seaweed in controlled water environments. and careful site assessments near protected areas. Most new settlement comes from wild populations rather than farms, so controlling spread requires a strategic, evidence-based approach. 

Pacific Oysters are a particular problem on the south Devon coast. Less is known about records in north Devon.

Topmouth Gudgeon is a small, highly territorial freshwater fish native to Asia and introduced to the UK as an ornamental species. It prefers still or slow-flowing freshwater. The fish can breed rapidly and out-compete native fish for habitat, spawning areas and food. They eat invertebrates and the eggs and larvae of other fish. They can also carry fish diseases and parasites.

Topmouth Gudgeon is resident in the UK. It’s been recorded in Devon but has been eradicated. The Environment Agency has a national invasives fisheries team which will carry out eradication when the fish is reported in ponds.

Water Primrose is from South America and was introduced to the UK as an ornamental pond plant.

It grows aggressively, forming dense mats on freshwater surfaces. These dense mats impede water flow, exacerbate flooding and create anoxicCompletely lacking oxygen. conditions detrimental to fish and other aquatic life. It can obstruct irrigation channels and drainage systems, posing challenges for agriculture and flood management. 

Water Primrose is still rare in the wild in the UK but it is increasing. In Devon it occurs on the Grand Western Canal near Tiverton where it’s controlled through spraying and being pulled up.

Yellow-legged Asian Hornets were first recorded in France in 2004 and are thought to have arrived there on a pottery shipment from China to Bordeaux. They rapidly spread across Europe, with the first record in Britain in 2016 in southern England. They aren’t currently considered resident in Britain and The National Bee Unit (part of the government’s Animal and Plant Health Agency) is responsible for monitoring, tracking and destroying any nests. However, the unit relies on reports from the public to identify new nests.

Most nests have currently been recorded in the south east (especially Kent and in east and west Sussex). The only nests reported in Devon to date have been in Plymouth and north Devon (Woolacombe) and these have been destroyed.

The hornets are voracious insect eaters, especially bees. Research in France found a single nest can consume over 11 kilograms of insects including bees, wasps, other pollinators and spiders. Honeybees make up around 40% of their diet. They have no natural predators and can rapidly increase in numbers, with each nest producing up to 350 queens in the autumn.

Yellow-legged Asian Hornets can be easily distinguished from the larger native European Hornet. Look out for their bright yellow leg tips.

Zebra Mussels are found in slow-flowing water, including reservoirs and lakes. They were first introduced to Britain via ships from the Baltic sea. They can rapidly form large colonies that attach to almost any submerged hard surface including pipes, stones and boats, where they affect native wildlife and reduce water flow by clogging pipes. 

Zebra Mussels can be spread in a number of ways, including through water sports and angling as they attach themselves to equipment.

There’s a record of a small population of Zebra Mussels in the Exeter Canal where it’s presumed that the sub-optimal conditions are limiting population growth.