Invasive species management

Working together to control non-native species and restore balance.

what are invasive non native species?

Invasive non‑native species (sometimes called invasive alien species) are organisms, such as plants, animals, fungi, or microbes, that humans have introduced outside their natural native ranges, either deliberately or unintentionally.

Once these species establish themselves, some can spread rapidly, becoming invasive. They then often pose serious threats to biodiversity, ecosystem function, local economies, and even human health.

In our precious freshwater environments, especially rivers, streams, and wetlands these invasive species can:

  • Outcompete indigenous species for critical resources like light, space, food, and nutrients

  • Disrupt food webs and ecological relationships

  • Alter water quality and flow dynamics

  • Cause erosion of riverbanks and affect flood resilience

(above) American Signal Crayfish

Tackling INNS in the Clwyd catchment

In partnership with the North Wales Wildlife Trust, North Wales Rivers Trust are delivering a series of hands-on events across the Clwyd river catchment to tackle invasive non-native species. Activities will include Himalayan balsam bashing days, and community outreach at local events to raise awareness about how we can all help to Stop the Spread of invasive species and protect our rivers.

These events are a great opportunity to learn more about local wildlife, support river health, and take practical action for nature. Whether you're a seasoned volunteer or completely new to conservation, everyone is welcome to get involved.

To take part, keep an eye on our events calendar or get in touch with Angharad at angharad@northwalesriverstrust.org for more information.

This project is funded by Dwr Cymru.

Examples of non-native invasive species….

Grey squirrel

The grey squirrel (Sciurus carolinensis) is a non-native species introduced to the UK from North America in the late 19th and early 20th centuries. While common across much of the country today, it poses a serious threat to our native red squirrel through competition for food and habitat, and by carrying the squirrelpox virus, which grey squirrels are immune to but which is often fatal to reds. Grey squirrels can also damage trees by stripping bark, impacting woodland health and regeneration. Their spread has significantly reduced red squirrel populations across the UK, including parts of North Wales.

Himalayan balsam

Himalayan balsam (Impatiens glandulifera) is an invasive plant introduced from the Himalayas in the 19th century as a garden ornamental. It thrives along riverbanks and in damp habitats, where it spreads quickly by releasing thousands of seeds that travel downstream. Although it is attractive to pollinators, it outcompetes native plants for light and space, reducing biodiversity. In winter, when it dies back, it leaves riverbanks bare and vulnerable to erosion. Its spread is a major concern for river health and habitat stability.

American mink

The American mink (Neogale vison) is a non-native predator introduced to the UK through fur farming in the early 20th century. Escaped or released individuals established wild populations and have had a severe impact on native wildlife, particularly water voles, ground-nesting birds, and fish. Agile and semi-aquatic, mink are well adapted to river and wetland habitats, where they hunt along the banks and through reedbeds. Their presence has contributed to the drastic decline of water voles across much of the UK, including parts of North Wales, making them a key target for conservation-led control efforts.

japanese knotweed

Japanese knotweed (Reynoutria japonica) is an invasive plant species originally introduced from East Asia as an ornamental garden plant. It spreads rapidly through underground rhizomes, forming dense stands that outcompete native vegetation and reduce biodiversity. In riverbanks and riparian zones, its aggressive growth can destabilise soils and increase erosion, especially when the plant dies back in winter. Japanese knotweed is difficult to control and expensive to manage, posing challenges for landowners, infrastructure, and conservation efforts across North Wales.

Can’t attend an event but still want to help?

Report sightings on INNS mapper

Reporting sightings on INNS Mapper helps conservation teams track the spread of invasive species and respond more effectively with targeted management. It also builds a clearer national picture, supporting long-term strategies to protect native wildlife and habitats.

Following the Check, Clean, Dry routine helps prevent the spread of invasive species by removing hitchhiking plants, animals, and pathogens from equipment and clothing. It’s a simple but effective way to protect our rivers, lakes, and wetlands from harm.

Follow CHECK CLEAN DRY guidance

Dispose of waste responsibly

Litter and dumped garden or pond waste can introduce invasive species into the wild, harming local ecosystems. By disposing of waste responsibly, we protect our rivers and wildlife from pollution and unwanted invaders.