Invasive Species in Rivers: Check, Clean, Dry

Emily Cooper


As Invasive Species Week draws to a close, we want to talk the impact invaders can have on rivers. In this blog, we're going to give you the low-down on some of the most destructive river invaders. These invasive species have already caused massive damage to river habitats and wildlife across the country, and it's important that we deal with them before it's too late.

Floating Pennywort

Floating Pennywort

Introduction: First used as an ornamental plant in garden ponds, Floating Pennywort quickly found its way into wild waterways and became an invasive species.

Why it's a problem: Floating pennywort forms dense mats on the surface of the water, preventing sunlight from reaching other aquatic plants. This causes a drop in oxygen levels, which can lead to the death of fish in the water. As a result, the ecosystem can begin to collapse. Floating pennywort can also prevent water from draining properly, leading to flooding and other problems.

Removal: Regular cutting of the plant can help to prevent it from achieving dominance, giving native plants and animals a fighting chance. Chemical treatment can also be used at the end of the growing season, when other native plants have died off.

Fun Fact: Floating pennywort can grow by up to 20cm per day! Fun fact for us, not so fun for the ecosystems that it dominates.

Himalayan Balsam

Himalayan Balsam

Introduction: Prized for its delicate pink flowers, Himalayan Balsam was first imported to the UK during the mid-19th century as an ornamental garden plant. By 1855, it was spotted growing in the wild. Since then, this innocent looking shrub has taken over river banks and lake shores across the country, quickly reaching invasive status.

Why it's a problem: Himalayan Balsam grows in dense thickets and projects its seeds up to four meters away, meaning it can quickly dominate any ecosystems it's introduced to. These thickets can impede water flow, as well as leaving river banks vulnerable to erosion when dieback occurs over winter. Finally, Himalayan Balsam pollen proves particularly enticing for visiting insects, which researchers believe may decrease the pollination of native plants.

Removal: Removing Himalayan Balsam is a time-sensitive matter; it must be dealt with before the seeds set. Due to their shallow roots, these plants can be pulled out and disposed of appropriately, which means composting them or even burning them if seeds are present. Repeating this process on a regular basis should be enough to achieve control, as long as seeds are not allowed to set.

Fun Fact: Himalayan Balsam is also referred to as Gnome's Hatstand, which would be really cute if it weren't so destructive.

Signal Crayfish

Signal Crayfish

Introduction: Signal crayfish are a North American crayfish species. First imported to England during the 1970s, these crustaceans were to be cultivated for export as a delicacy to satisfy growing demand in the Scandinavian market. However, they quickly escaped from commercial fisheries, established themselves in the wild, and became invasive.

Why it's a problem: Signal crayfish make their burrows in river banks, which can cause them to become unstable and potentially collapse. This can result in the displacement of native species, including endangered water voles. One of the greatest threats posed by these invaders is to our native crayfish: the American species are much bigger and grow much faster, which enables them to achieve competitive dominance. They also carry a disease known as crayfish plague which has ravaged native crayfish populations.

Removal: Otters can play a significant role in controlling crayfish populations through predation. Trapping has also been operated on a trial basis, but currently doesn't seem to have a long term impact on signal crayfish populations. Any signal crayfish caught must be humanely destroyed.

Fun Fact: Native crayfish declines have been estimated at 50-80% since the introduction of signal crayfish. Not fun.


Japanese Knotweed:

Introduction: This plant was introduced during the Victorian era, when it was grown in gardens as an ornamental plant.

Why it's a problem: Japanese Knotweed grows almost everywhere, from cities to rural land and river banks to railway lines. Even the smallest fragment of knotweed can form a fully grown plant, which means that it's incredibly difficult to properly remove. Disturbing the plant can actually lead to more knotweed growing if fragments are spread. This plant is so metal that it can penetrate walls and even road surfaces. It quickly dominates the ecosystems to which it is introduced, resulting in the death and displacement of native species.

Removal: It's incredibly difficult to totally eradicate Japanese Knotweed. The best practice for removal entails cutting/spraying the plant in early summer, then repeating the process in winter. Even under perfect conditions, it can still take 3 years to achieve total dieback using this method.

Fun Fact: The UK government estimated it would cost £1.56 billion to control Japanese Knotweed. Imagine all the rivers we could clean with that!

American Mink:

Introduction: The American Mink was imported to the UK in order to create fur farms. Unsurprisingly, the tiny mammals soon escaped from confinement and established themselves in the wild.

Why it's a problem: Mink are voracious predators, with a tendency to eat whatever they can get their paws on. This includes the critically endangered water vole and various seabirds. Due to their size, mink are able to enter water vole's dens and eat their young. Partially as a result of predation by mink, water voles are now classed as seriously threatened.

Removal: In areas where conservation in prioritised, mink-proof fences can be used to prevent mink from gaining access to nesting sites and dens. Control can also be achieved through shooting and trapping.

Fun Fact: Otters may be a helpful ally in achieving mink control; they react to mink with hostility, which could help to stabilise vole and bird populations.

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