Our local environment is undeniably beautiful. Near the rolling hills of the South Downs, we're surrounded by some of the country's most stunning views. This unique landscape is also home to rare wildlife found only in these areas, as well as several chalk streams—an exceptionally rare and biodiverse habitat, with 85% of the world’s chalk streams located in southern England.

Yet this wonderful landscape that we share is undergoing subtle changes driven by unwelcome visitors — Invasive Non-Native Species (INNS). These are plants and animals introduced from other areas that outcompete native wildlife. You might be familiar with the grey squirrel? This non-native species eradicated the red squirrel, that is now only usually seen on the Isle of Wight, safely away from their more dominant grey non-native species.

INNS cost the British economy nearly £2 billion annually through damage to infrastructure, agriculture and natural resources, while silently weakening the biodiversity of our local and unique ecosystems.

As we mark Invasive Non-Native Species week from 12th-18th May 2025, it’s a reminder to address this challenge, as the good news is getting clued up on INNS is an important first step to managing the impacts.

What are Invasive Non-Native Species

Non-native species are plants and animals that have been introduced to the UK from all over the world. Most are harmless, but around 10-15% are invasive which spread and have a harmful impact.

These species could be introduced to an area outside their native range deliberately, for example as ornamental plants or exotic animals; or accidently as ‘hitchhikers’ – for example with goods transported from overseas or when returning from a holiday abroad.
Once these invasive species have been introduced, they are extremely difficult to control as they spread rapidly through eggs, young, seeds, spores or fragments.

Here in the UK, they threaten the survival of native wildlife and damage our natural ecosystems by preying on or out-competing other plants and animals, disrupting habitats and ecosystems, or even spreading harmful diseases.

The impact of INNS on water

Here at Portsmouth Water, we’re not exempt from the threat that INNS pose. Around 40% of Britain’s 200+ registered INNS are known to be aquatic, threatening the health of the local water bodies. We take from groundwater, so while this has no risk to drinking water, it’s important we understand what invasive species are present and the impacts so we can:

• Manage the spread of INNS
• Enhance biodiversity in local habitats such as chalk streams
• Build resilience within our water supply as a healthy environment benefits the quality of the water we abstract.
• Comply with the law as it’s illegal to release, plant or allow to spread any non-native species into the wild under the Wildlife and Countryside Act 1981.

Taking Action tackling the spread of Invasive Non-Native Species

The main species we tackle is Himalayan Balsam, introduced to the UK in the 1800s as an ornamental plant due to its beauty, it has since become one of the most invasive species.

Himalayan Balsam spreads along riverbanks, forming dense bushes that release floating seeds and infect entire riverine systems. Its presence causes erosion, increasing sedimentation and affecting the water quality.

Our abstraction along the River Itchen is infected annually by seeds floating down from upstream sources of the plant. Total eradication is extremely difficult, so we manage balsam on our section of the river, annually removing it in the summer months, which encourages plant growth of native species. We do this at our annual conservation working parties by pulling the plant, before it can spread its seeds – many hands make light work!
On the hermitage stream, which runs on our land at our head office in Havant, we employ contractors and support local conservation groups to remove the plant. As a result. much less Himalayan Balsam is growing, and with a few more years we hope to see the plants presence much more removed along the stream.

We have also identified Japanese Knotweed and Crassula on our land. These species are much harder to eradicate as tiny fragments of these plants can start whole new invasions. Therefore, we work to manage their spread by careful application of chemicals and controlling the area of infection to reduce its spread.

What can you do

We work to manage and eradicate INNS where we can, protecting the rich and diverse natural flora and fauna we’re surrounded by. However, we need your support to achieve lasting success to ensure habitats are not continually re-invaded.

You can help prevent the spread of invasive non-native species by following some simple steps.

Remember: Check, clean, dry

Invasive species are often spread accidently, particularly via recreational water activities. These can be spread on equipment, vehicles and clothing or in the movement of soil, water or garden waste.

You could accidentally be spreading them without realising through your work or hobbies - particularly water-based activities such as fishing, boating and paddle boarding.

You can help protect the environment and activities you enjoy by keeping your kit free of invasive plants and animals, whenever you leave the water remember to: