Invasive Non-Native Species

Species which have been introduced into areas outside their natural range through human actions and that pose a threat to native wildlife are known as invasive non-native species. Although originally introduced as attractive garden plants or as escapees from farms, they can seriously damage biodiversity and often cause substantial economic or health problems. 

In the UK there are several invasive flora and fauna species that threaten our river ecosystems and cause millions of pounds worth of damage every year.

Himalayan Balsam

What is Himalayan Balsam?

Himalayan balsam (Impatiens glandulifera) is an annual herb, native to the Himalayan region of Asia. This plant has covered much of Britain spreading particularly rapidly along riverbanks. The attractive flowers appear in July with seeds that start to scatter by October not only around the plant, but also onto water.

Why is Himalayan Balsam a problem?

Every year new plants grow from seed, but they soon reach and often exceed head height. One plant can produce up to 800 seeds, which are viable for around 18 months. Seeds can even germinate underwater. Once established, Himalayan Balsam competes effectively against native plants, suffocating vegetation.

How can we help?

Currently, pulling Himalayan balsam is the only effective way of eliminating it. Efforts need to begin at the top of catchment because seeds spread downstream. Pulling is very time consuming and needs to be repeated for several years until all plants have gone.

Himalayan Balsam

Japanese Knotweed


What is Japanese Knotweed?

Japanese knotweed (Fallopia japonica) originates from Japan’s volcanic mountain ranges. In the 19th century it was regarded by many as a decorative and ornamental plant but quickly escaped and is now commonly found alongside roads, wasteland, rivers and railway lines.

Why is Japanese Knotweed a problem?

It can cause damage to property including drains and pipework, patios, paths and drives, boundary and retaining walls, outbuilding, conservatories and gardens. Treatment programmes are both long and expensive and can last up to three years and there is potential for regrowth after laying dormant for up to 20 years.

How can we help?

Its spread is controlled in the UK under the Wildlife and Countryside Act 1981, and waste containing Japanese knotweed is covered by the Environmental Protection Act 1990 Part II and must be treated and disposed of by a competent accredited contractor. The most effective control method is herbicide.

Giant Hogweed

What is Giant Hogweed?

Giant hogweed (Heracleum mantegazzianum) is a close relative of cow parsley originally from Southern Russia and Georgia. It can reach over 3m (10ft) in height. It is most commonly found on riverbanks where it forms dense stands and can displace native plants and degrade wildlife habitat.

Why is Giant Hogweed a problem?

The sap of giant hogweed contains toxic chemicals known as furanocoumarins.  When these come into contact with the skin in the presence of sunlight, they cause a condition called phyto-photodermatitis: a reddening of the skin, often followed by severe burns and blistering.  The burns can last for several months and even once they have died down the skin can remain sensitive to light for many years.

How can we help?

Under the Wildlife and Countryside Act 1981, it is an offence to cause Giant hogweed to grow in the wild. It can also be the subject of Anti-Social Behaviour Orders (ASBOs) whereby landowners of infested areas can be required to remove it. Local authorities will often remove it from publicly accessible areas but if you spot it, be sure to raise attention to it by contacting your local council.
Giant Hogweed

American Mink

American Mink

What are American Mink?

The American mink (Neovision vision) is a semi-aquatic mammal that escaped from fur farms in the 1950s and 1960s and now breeds across most of the country except the far north of Scotland and some islands.

Why are American Mink a problem?

They are active predators, feeding on anything they are big enough to catch, including our native water voles, which are now under threat of extinction. They hunt on the riverbanks and are good swimmers, enabling them to enter the water-line burrows of water voles. American mink are much more likely to be seen than the shy and secretive otter. They can be distinguished from otters by their smaller size, darker, almost black fur, and small white chin and coat. 

Signal Crayfish

What are Signal Crayfish?

The Signal crayfish (Pacifastacus leniusculus) is a freshwater crustacean native to the Pacific northwest. It was introduced to Europe in the 1960s to supplement native fisheries in Scandinavia that were suffering due to crayfish blight. Ironically, the signal crayfish turned out to be a carrier of the disease. They rapidly escaped and are now widespread across 25 European countries. They inhabit small streams to large rivers and lakes from the coastal to the sub-alpine regions. In the UK, they occupy a similar ecological niche to the white-clawed crayfish, and so compete with them for habitat.

Why are Signal Crayfish a problem?

They are known to burrow into banks up to 1.2m which causes damage and affects the habitat of bank-dwelling species such as the threatened water vole. It also speeds up erosion, causing changes in the bank-side vegetation and increases the silt load in the water. 

Signal crayfish predate on fish eggs laid below the gravel and they will attack small fish such as bullhead and salmon fry. They are also capable of outcompeting fish through their consumption of plants and insects. The loss of plants and insects has further knock on effects for both aquatic and terrestrial species.

As well as preying on insects, signals will also eat juvenile crayfish (thus trapping must be done in an appropriate manner as the removal of larger specimens who self thin can increase the population).