Did you know that over 2,000 plants and animals have been introduced to Great Britain from all over the world? Many of these non-native species coexist with us without harm however, around 10-15% are problematic. On a global scale, the spread of species which have been introduced by humans rather than nature is one of the top five reasons for biodiversity loss. In Hertfordshire and Middlesex, they threaten our native wildlife, can be harmful to human health, destroy our landscapes and impact on us being able to enjoy the countryside in the way that we should be able to.

On a local level, The Trust’s Living Rivers project aims to protect and improve the precious chalk streams we have in Hertfordshire. There are only around 220 chalk streams globally, and around 10% of these are in our county. In prime condition, these should have gentle sloping banks and glass-clear water babbling over gravel. However, these natural gems and the wildlife that they support are threatened by non-native invasive species, with key culprits being Himalayan Balsam, Giant Hogweed, American Mink and Signal Crayfish. Fortunately, we are not alone in recognising how destructive these species can be and we are working with and alongside multiple organisations and volunteers on several active projects. It is a problem on a major scale and tackling it is labour intensive – there is plenty of room for more groups and individuals to get on board and help to protect our globally rare chalk river landscape.

Himalayan Balsam

Firstly, let’s take a look at Himalayan Balsam. Himalayan Balsam was first introduced to this country in 1839 as a garden plant. With its fast-growing nature and explosive seed pods, the plant quickly escaped and has gone on to invade wet habitats. It can now be found infesting every river in Hertfordshire. With its seeds travelling downstream, river banks have become infested and eroded by the plant, damaging the environment for the endangered Water Vole whose natural habitat is the river bank.

In the past, the River Stort, a tributary of the River Lea, in the Stort Valley is one that has seen plenty of action to tackle Himalayan Balsam. The Trust hosts the River Lea Catchment Partnership and since 2012, it has helped to encourage individuals, organisations and land owners to focus on invasive species amongst other restoration activities. Previously, a co-ordinated group of partners applied a top to bottom approach to ‘balsam bashing’ to prevent the pest from reseeding between Bishops Stortford and Harlow Mill Lock. This has been a major task to stop Himalayan Balsam out-competing native bankside flora, and the group worked to eradicate it along the Stort Navigation towpath, millstreams and brooks, and out across Thorley Wash Nature Reserve and other designated floodplain meadows which have been infested by the plant.

Now the project has evolved and thanks to funding from the Environment Agency and Affinity Water, we are looking to the future for a longer-term solution to the problem with the prospect of a biological approach to tackling Himalayan Balsam over a physical one.  Working with CABI, a nonprofit intergovernmental development and information organisation, focusing on agricultural and environmental issues, the Trust is trialling a novel biological treatment – pathological rust fungus, which is found in the native range of the plant in the foothills of the Himalayas. This is a naturally occurring fungus which grows on the leaves of the plant and stunts its growth, thereby reducing its height and dominance. The fungus has been tested to make sure it doesn’t impact on other species, is able to grow in our climate and is able to reproduce. Seeds taken from the area to be treated are grown on by CABI to see if they are susceptible to the controlling effects of the fungus.

It's early days and the Trust is learning from past trials but if CABI’s tests are deemed successful, the Trust will be trialling application of the pathological rust fungus in two areas on the River Stort, close to the charity’s Thorley Wash Nature Reserve.  Seeds will also be collected in the Ash valley with a view to a further trial there next year. Watch this space!

Giant Hogweed

Giant Hogweed originally comes from Southern Russia and Georgia and was introduced to this country by Victorian plant collectors. A close relative of Cow Parsley, this plant is invasive and its sap can be harmful to humans. The Environment Agency and Herts County Council have part-funded a 10-year programme to treat and eradicate an infestation on the Hunsdon Brook, close to Roydon in the Stort Valley, where the plant was likely first introduced to the area. This is the fourth year of treatment and a long-term approach is needed to the problem, as seeds of this plant can remain viable for up to 20 years.

The problem of Giant Hogweed infestation is also being addressed by the River Beane Restoration Association, who have had a professionally qualified volunteer treating the Beane Valley for the past five years, thanks to initial funding from the Environment Agency. Further funding from Affinity Water is enabling a second cohort of volunteers to train and qualify in Giant Hogweed eradication, an approach which is also being started on the River Rib too. And, in the meantime, our River Champion, Derrick Guy is working with farms at the top of the catchment to provide advice on managing and removing Giant Hogweed themselves.

American Mink

As well as threats from invasive plant species to our rivers, there is one animal in particular who has been responsible for endangering Water Voles who live on their banks and within wetlands in the floodplain. The American Mink escaped from fur farms in the 1950’s and 1960’s and subsequently Water Voles fell prey to this predator. Between the 1970’s and 1990’s, the species saw a 95% decline in their natural range across England.

Since 2005, saving the Water Vole from going extinct in Hertfordshire, hand-in-hand with mink control and habitat management, has been a core focus for the Trust, without mink control, Water Voles would likely now be extinct from Hertfordshire. We are part of an initiative called Water Life Recovery Trust which involves partners across the whole of Eastern and Central England committed to eradicating mink by bringing volunteers across the counties together to make it their mission. Locally, our Water Vole project is jointly funded by the Environment Agency and Lea Valley Regional Park which is allowing the Trust to provide, monitor and manage mink rafts, train and coordinate volunteer surveyors and invest in new technology with “smart rafts” now providing a time-efficient approach to mink control by sending a text message if triggered for volunteer surveyors to respond to. 

By recording presence and absence of Water Vole and American Mink along rivers and throughout wetland habitats, we are able to track species recovery and local population trends, to better target mink control and where Water Vole reintroductions might be possible. Hosting a mink raft is an extremely helpful part of this process and we welcome contact from landowners or volunteers who may be interested in supporting this project.

And then there is the Signal Crayfish…

Whilst we have concentrated on Himalayan Balsam, Giant Hogweed and American Mink, another very real threat to our rivers is the American Signal Crayfish. Originally imported for restaurant trade, these abandoned or escaped creatures are having a devastating impact on our chalk streams in the Lea and Colne catchments. These crayfish burrow deep into the river banks, releasing large amounts of sediment, undermining their stability and degrading water quality.

Information collated by a volunteer in a non-scientific survey estimates that one adult crayfish can be responsible for releasing a massive 12 tonnes of sediment over its lifetime! With chalk streams being low energy rivers, they simply can’t deal with these levels of extra erosion. The Chess Smarter Water Catchment has also been monitoring sediment ingress in to the River Chess and has found that bank erosion is the most significant contributor – there is speculation about whether this is the result of crayfish activity and the project hopes to research this further in due course. On a national level, there is currently little investment into research on the impacts that Signal Crayfish are having on rivers across the country or into methods of eradicating them from our freshwater systems.   

More work is definitely needed to address the harmful effects of these invasive species and much of that starts on the ground with individuals and groups volunteering. If you think you can help to make a difference, please visit our Living Rivers page to find out more, or contact your local river group. Thank you.