On a global scale, the spread of non-native species 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. With less than 250 chalk streams globally, 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 and American mink. 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.

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. 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.

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. Over the last two years there has been a co-ordinated group of partners applying a top to bottom approach to ‘balsam bashing’ to prevent the pest from reseeding between Bishops Stortford and Harlow Mill Lock. This is a major task to stop Himalayan balsam out-competing native bankside flora, and the group have been eradicating 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. Together, The Trust, Whoosh Explore Canoe Club and Canal & River Trust and have taken the lead to address the issue with support from the Countryside Management Service, Friends of Castle Park, Harlow Conservation Volunteers and the Environment Agency. Despite the amount of help being provided, more hands-on deck are required and enquiries from groups such as scouting or youth groups, walking groups, green gyms etc. interested in adopting a patch would be very welcome. The aim is to get rid of the entire seedbank in two to four years – an ambitious target, but one that is achievable with the community behind it!

A physical approach to tackling Himalayan balsam is not the only tool in our armoury, a biological one is available too. Pathological rust fungus is a native parasite to the plant and early field trials are taking place to control growth of balsam. Currently, the Trust is trialling its inoculation at Tewinbury Nature Reserve and we hope to see its impact in this year’s growing season, which runs from May to October. This approach is being considered for the Stort Valley, alongside other rivers too.

Friends of the Rib & Quin have also been working hard to tackle balsam on these two rare and precious chalk streams, as well as bashing taking place on the River Beane coordinated by the River Beane Restoration Association. These efforts all help to make up a landscape-scale approach to eradicating Himalayan balsam from the River Lea Catchment.

Image: Our Living Rivers Officer, Sarah Perry next to a 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 second 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.

Image cc: Tom Hibbert

As well as threats from non-native plants 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 whilst activists may have welcomed their liberation, conservationists did not and nor do our water voles, who have fallen prey to the 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. We are part of an initiative called Water Life Recovery East which involves partners across the whole of East Anglia (and into Herts and Essex) 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 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.

Signal Crayfish by Linda Pitkin.

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. 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 these non-natives from our freshwater systems.  

More work is definitely needed to address the harmful effects of these non-native invasive species and much of that starts on the ground with individuals and groups volunteering. Can you help make a difference?