Invasive non-native Species (INNS) – Arun & Rother Rivers Trust

Invasive non-native Species (INNS)

INNS – what are they?

Plant and animal species introduced to the UK from across the world are known as non-native species. The majority of these species are harmless, but some species are invasive and have a serious negative impact, causing harm to wildlife and the environment, others have an economic cost. The introduction of INNs is the second biggest threat to global biodiversity after habitat loss.

How do INNS cause problems?


Non-native species out compete native species for food, space, light and resources.
Example: Himalayan Balsam (INNS) grows prolifically along riverbanks and wet woodland, it grows extensively outcompeting our native flora for light and space, it dies back over winter leaving banks vulnerable to erosion. Whilst bees love its nectar this means they aren’t pollinating as many of our native plants.


Native species support a greater number of other local organisms than Invasive Non-Natives. Plants that have evolved here often have specific invertebrates or fungi for example that rely on them. When they are replaced with INNS these species will not have the food and other resources they require. Also, INNS often dominate large areas, reducing the number and variety of species that would usually be present.
Example: Rhododendron bushes change woodland soils, increasing acidity and reducing the number of native woodland plants and trees.


The introduction of new predators can be a serious issue, native species are not able to adapt and are susceptible to the new predators.
Habitat loss and the predation by American mink (INNS) are believed to be the main cause of Water Vole decline.

Introduction of new disease

Native species may have limited immunity to new diseases introduced by non-native species. These diseases can easily be passed between rivers (see how to stop this below!).
Example: The Signal Crayfish (INNS) is a carrier and host of the crayfish plague which can kill our native white clawed crayfish.


Some species can breed with other related but distant species creating hybrids and one species can become lost.
Example: Our Native Black poplar trees hybridised rapidly with introduced American poplar, reducing their numbers dramatically.
The impact of INNS on the water environment is hugely challenging. Rivers are at significant risk from INNS with the natural flow of water helping disperse seeds and plants to spread downstream. INNS species can displace the native species, can interfere with navigation, flow, impact on water quality, effect stability leading to erosion, contribute to flooding, and threaten biodiversity.

Which INNS are found in your local rivers and on land next to the river?

Invasive non-native species are widespread across a range of habitats including your waterways/rivers here in the Arun, Rother & Western Streams. Learning how to spot them will help stop the spread.

Common invasive plants on your rivers and banks include:


Common invasive animals include:

Signal Crayfish

American Mink

What is ARRT doing to help?

ARRT carries out river walkover surveys mapping features including INNS. Identifying the spread of INNS across our catchment helps us direct our efforts and track our impact.
We work in Partnership with stakeholders, landowners and our Catchment Partnership to tackle INNS and a range of other issues affecting our water environment.
INNS can cause widespread damage to our water landscape, there is much to do and we cannot do this vital work without your help and support.

What can you do to help?

Don’t pick or transport INNS – in some cases this is an offence.
Report any sightings of INNS to your local Biodiversity Record Centre, or record on the National Biodiversity Network Gateway or on other species apps such as iRecord.
If you are entering or boating on different rivers you need to be sure you don’t transfer them between rivers. See guidance below on how to help.


We can all help to prevent the spread of INNS by following a simple 3 step process……..

Check Clean & Dry!

Be Plant Wise

We can help stop the spread of non-native plants by understanding and knowing what plants are in our gardens and ponds, by keeping our plants in our gardens & ponds, and disposing of unwanted plants, roots and seeds responsibly…..Be Plant Wise!

Volunteer with ARRT

We run events to tackle INNs and plan to do more, volunteers can help us survey invasive plants, record how far they have spread. If you would like to help out, please sign up as a volunteer to be added to our volunteer mailing list.

Further INNS information:

Download ARRT’s INNS Plant Species Leaflets

Download all 10 Leaflets

Further information:

GB Non-native species secretariat NNSS  (