Invasive Species

Invasive Non-native Species

Species which have been introduced into areas outside their natural range through human actions and are posing a threat to native wildlife, are known as invasive non-native species.

In the UK, we give the term non-native species (NNS) to any species or race that does not occur naturally in an area. The term alien species is used in many other countries for non-native species. A  species that becomes so abundant after arrival that it damages biodiversity and often causes substantial economic or health problems is termed an invasive non native (INNS) or alien species. The term applies to invasive plant/animal diseases.
In the U.K. there are several invasive flora and fauna species that threaten our river ecosystems and cause £millions of damage each year. These INNS species include:

Himalayan Balsam

Annual flowering plant

Himalayan Balsam

Taken from:

What is Himalayan Balsam?

Himalayan Balsam 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.


Japanese Knotweed

Perennial plant

Japanese Knotweed

Taken from:

Where does it originate from?

It originates from Japan’s volcanic mountain ranges, in the 19th Century it was regarded by many as a decorative and ornamental plant.

Where is it found?

It is commonly found alongside roads, wasteland including development sites, rivers and railway lines.

Why is it such 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.

There is potential for regrowth and can lay dormant for up to 20 years.

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

What treatments are available?

The most effective control method is herbicide.


Signal Crayfish

Aquatic Invertebrate

Signal Crayfish
 Taken from:

Native to:

Pacific Northwest


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.

Environmental problems:

  •  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).

Giant Hogweed

Flowering plant

Giant Hogweed

Taken from:

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 common on riverbanks where it forms dense stands and can displace native plants and reduce wildlife habitats. It is widely distributed in the wild and poses a serious risk to people who are unaware of its potential for harm.

The sap of Giant Hogweed contains toxic chemicals known as furanocoumarins.  When these come into contact with the skin, and 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.


American Mink

Semi aquatic mammal

American Mink

Taken from:


Mink escaped from fur farms in the 1950s and 1960s, and now breed across most of the country. 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. Mink are much more likely to be seen than the shy and secretive Otter.

How to identify

Mink can be distinguished from Otters by their smaller size, darker, almost black fur, and small white chin and throat.

Where to find it

Widespread, now found throughout the country except the far north of Scotland and some islands.