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What are Invasive Non-Native Species (INNS) and their impacts?

As part of Invasive Species Week, we have produced a guide to the plants and animals that impact the environment and enjoyment of our waterways.

Deer In Pennywort

Floating pennywort can grow very quickly.

About 2,000 plants and animals have been introduced, often accidentally, to the UK from all over the world.

Many are harmless, but some species have had a detrimental impact on our native species as well as our own health and the activities we enjoy.


The wrong plant in the wrong place can be damaging to the environment, impacting on the wildlife that depend on our native species.

The non-native species secretariat (NNSS) has a very useful Be Plant Wise guide on its website.

Below are some of the species that affect paddlers, specifically.

Floating pennywort, introduced as an ornamental pond plant, can grow exponentially and clog up waterways, making it hard to paddle.

It can deplete oxygen levels, killing fish and invertebrates as well as outcompeting native plants. 

Other damaging species include Himalayan balsam, which can out compete native species, particularly on riverbanks. 

Giant hogweed, which can grow up to 5m, has a toxic sap which can cause a blistering of the skin upon exposure to sunlight. 

Australian swamp-stonecrop can form dense impenetrable mats and grow 200 times faster than native pond plants.

Water primrose is an alert species, which means if found you must report it to iRecord, here. They are at just a few sites, but can produce hundreds of seeds per year.

Water fern, a small free floating plant, can cause the water’s surface to appear solid. 

Parrots feather, which has blue-green feather-like leaves, can block ditches and dominate ponds. 


Signal crayfish's lobster-like appearance makes them easy to recognise (c)GBNNSS

What about animals?

Paddlers need to be aware of one particular pair…

When killer shrimps enter a new waterway they quickly kill native invertebrates and small fish. They breed fast and are difficult to control.

Killer shrimp can survive for up to 16 days in damp paddling equipment and clothing.

Signal crayfish, which pose a huge threat to our native white-clawed crayfish, eat aquatic plants, invertebrates and fish.

They also burrow into river banks causing erosion to those areas.

Paddlers should report sightings of signal crayfish. Also, please check your craft and equipment during a clean.