Invasive Non-native Species

Invasive Non-native Species

The Severn Estuary Partnership and Association of Severn Estuary Relevant Authorities is currently working with stakeholders across the estuary to produce a Marine Biosecurity Plan for the area. This work is due to be completed in early 2025. For more information, or to be added to our stakeholder list please contact us at

The Severn Estuary and surrounding areas, like many different regions worldwide, has experienced the arrival of a number of ‘non-native species’. These are animals or plants which have been carried into the UK, either deliberately or inadvertently, and integrate into the natural environment with native species. In the UK there is currently a recorded 3,224 non-native species, 2,010 of which are established and breeding in the wild1. Species are classed as non-native if they arrived in the UK after 1500. While some non-native species introduced hundreds of years ago have contributed positively to the heritage of Britain, others can have devastating effects on the environment, often outcompeting or preying upon native species. These are called ‘invasive species’, which the United Nations Convention on Biological Diversity (CBD) identifies as a major threat to biodiversity.

The routes by which non-native species are introduced are varied. In the marine and estuarine environment, species can latch onto ships, become transported in ship ballasts, escape from aquaculture and many more. As we have seen more and more movement of people and goods across the planet, we have seen increasing numbers of non-native species in the UK.

Invasive non-native species

Before the formation of the modern British Isles, the UK was connected to Europe via a land bridge called ‘Doggerland’. This meant that a variety of species, including mammoths, could simply walk to the UK and colonise the land. After the Ice Age, glaciers began to retreat from Europe as the earth warmed up. This caused enough sea level rise to submerge Doggerland and transform the UK into an island, meaning the population of flora and fauna in the UK became isolated from the rest of the world and became a unique ecosystem.

As a result, species within the UK have adapted to survive within its ecosystems but are poorly adapted for places elsewhere. When foreign species are introduced to the UK, they have different adaptations  and can have an advantage over native species, which leads to the phenomenon of invasive species.

An example of a zebra mussel encrusting a surface (Photo credit Gene Wilburn, 2007)

Invasive species don’t usually have predators, meaning their numbers can grown unchecked. They can also introduce new diseases, prey upon native species, and take food sources away from native species. There are usually health and economic consequences for people as well. Zebra mussels and killer shrimps within Cardiff Bay have weakened marine architecture, causing sinking of buoys, weakening of structures and build-up within pipes.

Non-Native Species detected within the Severn Estuary area

There are many examples of both land-dwelling and freshwater non-native plants and animals immediately inland of the Severn Estuary. These range from the widespread species such as mink and Japanese knotweed, to those with currently less extensive distribution, such as the zebra mussel.

The zebra mussel is endemic within bodies of water across the UK. In 2004, zebra mussels were first discovered in Cardiff Bay and are thought to have lived in the Bay since 2003. Approximately 4 tonnes of Zebra mussels are removed from Cardiff Bay each year and this species must be carefully managed within the Bay. Zebra mussels were carried to the UK on the hulls of ships from the Black and Caspian seas in south-eastern Europe. The mussels grow up to 5cm long and create large colonies, attaching to structures and impeding sluices and water gates. By feeding on plankton, zebra mussels clear the water and further stimulate the growth of invasive weeds.

Bugulina stolonifera is a species of bryozoan (a living ‘moss animal’ which filter-feeds in the marine environment) which has been found in the ports of Barry and Swansea. This small animal grows 30-40 mm long and originates from the east coast of America. These bryozoans can collect in large colonies, causing surface fouling (accumulation of unwanted material on solid surfaces) and decreasing the efficient by which boats and marine structures operate.

The killer shrimp is an invasive species found in Cardiff Bay which has earned its name for good reason. The shrimp has a voracious appetite for native shrimp, fish, and fish eggs, though it sometimes leaves its prey uneaten! Originally from the region around the Black and Caspian Seas, the killer shrimp has spread across Western Europe since the late 1990s. Their fast spread is due to the killer shrimp’s rapid spawning rate, with an average of 150 eggs per brood! As such, the killer shrimp can have a devastating effect on the ecosystem by wiping-out native species.

Procedures such as the ‘Check, Clean, Dry’ help prevent the spread of non-native species. For more guidance on preventing the spread of non-native and invasive species, visit the NNSS (Non-native Species Secretariat) biosecurity & prevention webpage.

Climate change and non-native species

The changing climate and conditions of the Severn Estuary will affect the flora and fauna which live within. it Warmer ocean temperatures may encourage species from further afield to migrate north into the UK’s waters, such as the pacific oyster Crassostrea gigas. Most non-native species that have managed to enter the UK are opportunistic generalists and have a wide range of environmental tolerances to survive extreme climatic events (ECEs). These characteristics means that non-native and invasive species can take advantage of climate change events, such as increased rainfall. As these changes accelerate, non-native species that currently do no harm to the environment may rapidly increase in number, becoming invasive.

Litter Picking Groups

Find out how you can help keep your local beaches, estuaries and bathing waters clean for everyone to enjoy.

An example of a bryozoan (Photo credit Karen Montgomery, 2008)

Get involved and report non-native and invasive species

For more information on the wide variety of non-native species in the UK and how to identify and prevent their spread, Living Levels has a pocket ID guide which can be found below.

There is also a free online training course available at the Non-native species secretariat website, designed for anyone ranging from government field staff to recreational water users and anyone who is curious! The course can be found below.

If you spot any invasive species, you can submit images to the Local Record Centre at

Alternatively, you can submit via the LERC Wales biological recording app available for Apple and Android

For more information, visit:

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