Invasive alien species are plants and animals that have been introduced outside of their native range by humans and have a negative effect on the new ecosystem. Many invasive alien species cause harm to native flora and fauna. Others have a direct economic cost. Invasive non-native species pose a significant risk to Yorkshire’s waters. Find out about the major invaders.

Himalayan Balsam

Annual flowering plant

Himalayan Balsam quickly establishes dense growths along riversides and ditches.It out-competes native vegetation leaving riparian areas with low habitat diversity. Himalayan Balsam dies back in the winter leaving bare banks which are easily eroded.

Currently, pulling Himalayan Balsam is the only effective way of eliminating it. Efforts need to begin at the top of the catchment because seeds spread downstream. Pulling is very time consuming and needs to be repeated for several years until all plants have gone. Scientists are currently investigating using rust, a natural pathogen of the plant, to control Himalayan Balsam. Find out more here

Japanese Knotweed

Perennial plant

Japanese knotweed was introduced to Europe from Asia. It colonises quickly and is common on road verges and river banks. It outcompetes native plants and its strong root system can damage concrete foundations, buildings, flood defences and roads.
Japanese Knotweed reproduces only from fragments of stem and root, not from seed. It is therefore essential that fragments of Japanese knotweed are not spread between areas. It is an offence under the Wildlife and Natural Environment Act 2011 to plant or otherwise cause Japanese knotweed to grow in the wild. The most effective control method is herbicide.

Signal Crayfish

Aquatic Invertebrate

American Signal Crayfish were introduced to the UK to be farmed for food. They are larger and more agressive than our native white clawed crayfish and and carry a fungal plague to which the native White Clawed Crayfish is susceptible. This means that the native species is becoming less common as Signal Crayfish spread north. Signal crayfish are also thought to cause bank erosion and to predate fish eggs. They are a particular problem in the River Ure and parts of the River Swale.

Giant Hogweed

Flowering plant

Giant Hogweed was introduced to the UK in the 19th century as an ornamental plant. It is common on riverbanks where it forms dense stands and can displace native plants and reduce wildlife habitats. The sap of giant hogweed causes our skin to be phototoxic. When the skin is exposed to sunlight it blisters and can cause burns that last several years.

American Mink

Semi aquatic mammal

The American mink was introduced to Europe to be farmed for its fur but escaped into the wild. It is a carnivore which feeds on rodents, fish, crustaceans, frogs, and birds and has been linked to the decline of European mink and water vole populations.
Mink have been spotted on the River Ouse near York.

How can you help?

Check your clothes and boots before moving between rivers.


Clean your boots and equipment. Submerging them in hot water for 10 minutes is the most effective way of killing seeds, pathogens and pests.

Dry your boots and equipment for 48 hours before using them in a different river.

Record Invasive Non-Native Species sightings using the INNS mapper website.