Aliens in the undergrowth

© Faye Davies

Our rivers play host to a number of unwelcome invaders that have a negative effect on these delicate ecosystems.

Invasive non native species (INNS) are animals and plants that have been introduced intentionally or accidentally to our countryside and the damage they can do to our environment is only superseded by habitat destruction. They can alter the local ecology, spread disease, clog waterways, cause a decline in other species and, in the case of giant hogweed, even pose a danger to humans. INNS also cost the UK approximately £2bn per year!

There are several common INNS found in and around our rivers and wetlands. These include plants such as himalayan balsam, japanese knotweed and floating pennywort, as well as animals such as the signal crayfish and american mink.

Floating pennywort

© Simon Mortimer - Floating pennywort in Foudry Brook / CC BY-SA 2.0

Floating pennywort

Native to North and South America and parts of Africa, floating pennywort was introduced to the UK  in the 1800s as an ornamental pond plant, but quickly spread to the wild. It was only banned from sale as recently as 2014 in the UK in a bid to combat the spread of invasive ornamental pond plants. 

Forming dense mats on the surface of water and growing up to 20cm a day, floating pennywort starves submerged plants of sunlight, reducing oxygen levels and leading to the death of fish and other aquatic organisms. This can result to the eventual collapse of the ecosystem and even increase the risk of flooding as it can prevent the flow of water.

Floating pennywort can be controlled through cutting, but is difficult to eradicate as it can regrow from very small fragments.

Himalayan balsam

© Amy Lewis

Himalayan balsam

Introduced as an ornamental garden plant in Victorian times from the Himalayas (as the name suggests), this pretty pink flowered plant was first recorded in the wild in 1855. 

Himalayan balsam has explosive seedpods and is able to fling its seeds up to 4 meters! Once these got in to our rivers, it didn't take long for the plant to establish itself along our waterways and it is now extremely widespread.

As well as outcompeting native plants, Himalayan balsam is an annual plant with very shallow roots. As it replaces perennial species whose roots bind the riverbank together, so erosion occurs as it dies back each winter. An added sting in the tail, its large pink flowers are also particularly attractive to insect pollinators, drawing them away from native species and reducing their populations further.

Himalayan balsam is fairly easy to control when removed before the seeds set and is easy to pull out, thanks to its shallow roots. However with many hundreds of miles of river and wetland affected, it is unlikely it will ever be fully eradicated. 

Japanese knotweed

© Philip Precey

Japanese knotweed

Famed for the structural damage it can cause to homes and other buildings, japanese knotweed was again introduced as an ornamental garden plant in the 1800s.

It can grow almost anywhere and is incredibly tough, able to push its way through brickwork, concrete and roads. When it enters a habitat it rapidly takes over and pushes out other species.

The control of japanese knotweed is incredibly difficult and its disturbance can even contribute to its spread. Chemical treatment is usually carried out by specialists and can take up to three years to be effective. The annual cost of controlling and eradicating japanese knotweed in the UK is between £160m and £200m!

Giant hogweed

Giant hogweed is an immensely tall umbellifer (a member of the carrot family) that displays large, white, umbrella-like clusters of flowers.

Giant hogweed was again introduced into the UK by the Victorians as an ornamental plant for lakesides and gardens. It escaped into the wider countryside and gained notoriety in the 1970s as an alien species that favours damp spots like riverbanks. At this time, many children started to display blisters as a result of touching the plant's sap while using the stems to make pea-shooters or telescopes: sunlight makes the skin sensitive to the irritants in the plant, causing the skin to redden.

Today, it is widely acknowledged that neither gardeners nor conservationists should attempt to cut the plant down (exposing its sap) as its toxins can cause serious, recurring skin damage.

American mink

© Tom Hibbert

American mink

Bread for the fur trade, by the 1950s there were some 400 mink farms in the UK. By 1967, wild mink were present in over half the counties of England and Wales and have become a significant threat to key UK species, most notably the water vole.

Mink are good swimmers and females are small enough to enter the water-line burrows of water voles and take their young. Mink are fiercely territorial; their dens are close to the water and the females have one litter of four to six kittens a year.

The control of mink is controversial, with trapping and euthanasia the most practical method of eradication. Many people mistake mink for otters, but mink are much more likely to be seen than the shy and secretive otter.

Signal crayfish

© Linda Pitkin/2020VISION

Signal crayfish

A North American crayfish species, the signal crayfish was first imported to England during the 1970s to be farmed as food. However, once individuals began to escape captivity, it was quick to spread in the wild and has now become an invasive species.

The signal crayfish is much larger than our native white clawed crayfish and grows much faster. This means it is quick to outcompete them and populations have crashed as a result. Signal crayfish carry a disease known as crayfish plague, which the white clawed crayfish have no immunity to.

Signal Crayfish also eat fish eggs and invertebrates and contribute to erosion and bank loss by creating large burrows at the bottom of riverbanks.

Trapping has little effect on signal crayfish population numbers, but the presence of otters goes some way to control their numbers. Any signal crayfish caught must be humanely destroyed by law.