South East Rivers Trust (& the Wandle Trust)

It’s Invasive Species Week!

What are they and how can you help ‘Stop the Spread’?

Invasive species are non-native animals and plants (terrestrial, freshwater and marine) that have established themselves in the UK and are causing problems for our native wildlife and ecosystems. Invasive non-native species (INNS) are globally recognised as a major threat to biodiversity, second only to habitat loss. Economically, their impact is also highly significant. A recent study in the UK concluded that the direct cost of controlling these species is at least £1.7 billion every year, with researchers admitting that this is likely to be a gross underestimate. This week we are trying to raise awareness of the problematic invasive species found in our rivers here in the South East and what you can do to help.

Exotic plants have been introduced to the UK for centuries! Plants that now seem commonplace to us were once brought here from far and wide for their attractive flowers or leaves, medicinal qualities or for food. Not all of these are a problem, and the benefits they have brought with them far outweigh any negatives. However, some of them have aggressive seed dispersal, are fast growing and aren’t eaten by any animals in the UK, meaning that they can out compete our native flora and take over.

Rivers can act like corridors for dispersal of seeds, which means that dense populations of these troublesome plants can often be found clogging rivers and dominating plant life along the banks. You can find out about the BIG FOUR invasive plant species we have here in the South East on our website:

The GB Non-Native Species Secretariat is a national body with a responsibility for helping to coordinate the approach to INNS in Great Britain. We encourage you to follow their advice and ‘be plant wise’ when choosing plants to go in your garden to help stop the spread of invasive species:

Our amazing team of River Rangers have just completed their first INNS survey on the River Wandle for 2018. These surveys help identify and locate INNS on our waterways which allows us manage their removal in the long term. If you’re interested in becoming a River Ranger please get in touch on

But it’s not just plants that are having a big impact on our rivers: animals introduced in the past are also now playing havoc with the natural balance of life. With no predators these species can take over, spreading disease and competing with native wildlife for food and resources.

A classic tale is that of American signal crayfish which was introduced the 1970s for export to Scandinavia where it’s a sought-after food. Unfortunately, they are carriers of crayfish plague and they can pass this deadly disease to native white-clawed crayfish whilst remaining healthy themselves. The signal crayfish is much larger than its native relatives and is a voracious predator feeding on a variety of fish, frogs, invertebrates, plants, and even eating other signal crayfish. It didn’t take long for the signal crayfish to escape from commercial fisheries and begin to outcompete their UK cousins for habitat and food.  Since their introduction they have decimated the native crayfish populations wherever they are present, and cause further problems by burrowing into river and canal banks causing erosion, bank collapse and sediment pollution. There have been anecdotal reports of signal crayfish spotted on the Hogsmill River recently.

You can help stop the spread of INNS like the signal crayfish by following the ‘check, clean, dry’ process on all your clothing and kit after visiting a river:

Hitchhiking eggs, seeds and disease can be spread between rivers if these rules aren’t followed, having a devastating effect on our wonderful native wildlife. But everyone can play their part in stopping the spread! Please be careful to check, clean and dry after your next visit to the river.

Have any Question or Comment?

4 comments on “It’s Invasive Species Week!

Thomas Hatton

With the correct equipment and license would I be able to catch signal crayfish on the river Medway ?

Jess Mead

Dear Thomas, As well as having a license and correct equipment you will need the permission of the landowner and any fishing clubs that own the rights to fish along the stretch you’d like to trap along. You can find more information here:


I’m surprised that Signal crayfish have only anecdotally been seen in the Hogsmill, given the insanely huge numbers of them in other local waterways such as the Wey in Guildford.

Jess Mead

Hi Chris, this blog was written a couple of years back and since then we’ve unfortunately seen more concrete proof of the presence of signal crayfish on the Hogsmill.

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