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Invasive species on our rivers

Invasive species on our rivers

RiverCare groups in East Anglia are no strangers to a variety of invasive species which they come across in their activities caring for their local watercourses.


Last month was Invasive Species Week, where we sought to learn, inform and take action against invasive species in our environment. To follow up, here are our top 3 most common invasive species that our RiverCare groups see on their rivers, and what can be done about them!


Himalayan Balsam


This bankside plant spreads very quickly and effectively via its ‘explosive’ seed heads. It grows in dense stands where it out-competes other native species. When it dies back completely in winter, it leaves the river banks bare and vulnerable to erosion. 

‘Balsam Bashing’ is simply uprooting the plant before it flowers. The short roots mean the plant can be pulled up very easily, making it a very satisfying task, although one which needs lots of volunteers to keep on top of the problem.






             Floating  Pennywort


This aquatic species of plant grows very rapidly, up to 10cm a day, and can quickly blanket the surface of a river, brook or stream. It causes a reduction in light, temperature and oxygen levels in watercourses, and prevents flow of water and amenity use. It can reduce the biodiversity of a river if left unchecked.

Due to the weight and density of Floating Pennywort ‘rafts’, it is most often treated by using heavy machinery to lift it out of the water. Any remainders then need to be hand-picked, as it can regrow from any small fragments left behind.  Some places even use Water buffalo to remove it, as it makes a rather tasty treat for them!






Signal Crayfish


An escapee from commercial fisheries serving the food industry, Signal Crayfish are a big threat to our native and endangered White-clawed Crayfish which they transmit disease to and are also known to predate. These larger non-native crayfish also cause problems by burrowing into riverbanks, causing bank erosion and in some areas displacing Water Voles.

Eradicating Signal Crayfish completely is not yet possible. Their current density and ability to reproduce and  recolonise by travelling along water courses and over land make eradication difficult. However, it has been shown that populations can be reduced and controlled using consistent trap and remove schemes.

Please note that setting traps for Signal Crayfish requires a licence. If one is caught on purpose or by accident, it is illegal to reintroduce it to the wild.







All photos from GBNNSS.