Invasive salmon, clams and seaweed are next threats to biodiversity in Britain 

Pink salmon, Purple Asian clams, marine invertebrates that form spaghetti-like colonies and a nematode worm that causes extensive deaths of trees are among the new entries in experts’ watchlist of invasive non-native species that could threaten Great Britain in the next 10 years. 

The latest version of the watchlist, which again includes known problem species such as the yellow-legged (Asian) hornet, raccoon and twoleaf watermilfoil, has been produced by experts led by the UK Centre for Ecology & Hydrology (UKCEH) and commissioned by Defra. 

The experts identified 145 non-native animals and plants that have the potential to become invasive in the near future, impacting biodiversity and ecosystems and also, in some cases, human health and/or the economy. Some of these species, from a diverse range of taxonomic groups and habitats, have already been recorded in Britain but are not established, meaning they have not managed to sustain populations so far. Others have yet to arrive. 

Non-native species arrive in Britain through global transport and trade, either intentionally or are hidden in plants, other imported goods or ship ballast water. Climate change is enabling many species from warmer climates to survive and spread in Europe. 

Top 20 threats

From the 145 species of concern, the experts compiled a ‘top 20’ that pose the greatest risk (see table with impacts in Notes), in order to inform monitoring efforts, policies and communications to the public, businesses and local authorities. The new entries include: 

• Pink salmon (Oncorhynchus gorbuscha) is already recorded in some rivers in Scotland and northern England but it is not known if these populations are self-sustaining. It outcompetes already vulnerable native salmonoids, such as Atlantic salmon.  

• Pine wood nematode (Bursaphelenchus xylophilus), a threadworm that causes pine wilt disease,  which has resulted in extensive deaths of trees in all regions where it already occurs. Also Pine sawyer beetle (Monochamus galloprovincialis), a carrier of the nematode. 

• Spaghetti bryozoan (Amathia verticillata), a colonial filter-feeding invertebrate that forms large, bushy colonies, outcompeting native species and disrupting food chains by consuming large quantities of phytoplankton. It can grow on and smother seagrasses and foul boats and fishing equipment. When detached, it forms large drifting rafts that can clog intake pipes and aid dispersal of small motile non-natives that live on it.  

• Purple Asian clam (Corbicula largillierti), a species that can reproduce rapidly, outcompetes native molluscs and clogs pipes, obstructing infrastructures such as drinking water facilities, resulting in expensive repairs. It is closely related to the Asian clam (Corbicula fluminalis), which is also in the top 20, and the Asiatic clam (Corbicula fluminea), which is already invasive in Britain. 

• Veined rapa whelk (Rapana venosa) is a voracious predator that feeds on native oysters, scallops and mussels, outcompeting other predators and potentially resulting in economic impacts for fishing communities. 

• Asian fan weed (Rugulopteryx okamurae) is an invasive seaweed that displaces native species. When detached it can be deposited as decomposing masses on shores, smothering small invertebrates and affecting tourism and, potentially, human health. 

This is the third ‘horizon-scanning’ exercise to predict invasive non-native species that could be a threat to Britain, following lists in 2013 and 2019. It involves assessing many hundreds of species from around the world that have the potential to impact biodiversity and ecosystems in Britain. Compiling the best available evidence and combining this with scientists’ expertise to fill in gaps enabled the team to produce a list of priority invasive non-native species. 

Informing action

Professor Helen Roy, an ecologist at UKCEH, who led the report, said: “Prevention is the most effective approach to mitigating the threat of invasive non-native species. Eradicating them once they have arrived in a country and become established is very difficult and costly. 

“Horizon-scanning is therefore essential to identify species of particular concern, which can then inform monitoring and surveillance activities, and action plans, as well as raising awareness through communication. The commitment from the experts, who mostly contributed on a voluntary basis, was inspiring.  

“The public also play a vital role by recording and reporting sightings of invasive non-native species, helping to inform action.” 

For example, the scientists predicted in their first watchlist in 2013 that the yellow-legged hornet (Vespa velutina), also known as the Asian hornet, would arrive in Britain and pose a threat to pollinating insects. It was added to the list of alert species for Britain, monitoring and surveillance systems were implemented and communication campaigns raised awareness and encouraged people to submit potential sightings.  

The yellow-legged hornet was first recorded in Britain in 2016 and there have been subsequent sightings every year but sustained early detection and then rapid response from the Animal and Plant Health Agency’s National Bee Unit in eradicating insects and nests found has so far prevented it establishing here. It is critical that everyone remains alert to the threat of yellow-legged hornets and continues to report sightings of concern because the threat of this species to remains high.  

Economic costs

The list again includes beetle species that pose a significant risk to trees through their feeding and tunnelling habitats – Emerald ash borer, Asian longhorn and Citrus longhorn. It also again includes raccoons which may threaten bird species and fruit crops, displace native carnivores and carry rabies and a roundworm parasite that is potentially fatal to humans. 

Olaf Booy, Deputy Chief Non-Native Species Officer at the GB Non-native Species Secretariat, part of the Animal and Plant Health Agency, said: “There are over 2,000 non-native species already established in Britain and new species are introduced each year. While only 10-15% become invasive non-native species, those that do have serious impacts on the environment, cost the economy nearly £2 billion a year, and even harm our health.

"Horizon scanning is a crucial part of preventing new arrivals by helping us to predict in advance which species are likely to be introduced, establish, spread and have a harmful impact in future.” 

The report, Horizon-scanning for invasive alien species with the potential to threaten biodiversity and ecosystems, human health and economies in Britain, and is available on the horizon-scanning section of the Non-native species Secretariat webisite.

Invasive Species Week runs from 12-18 May. Find out more at nonnativespecies.org/invasivespeciesweek  

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Media enquiries  

For interviews and further information, please contact Simon Williams, Media Relations Officer at UKCEH, via simwil@ceh.ac.uk or +44 (0)7920 295384.  

Notes to Editors 

The new report was led by UKCEH and involved consultation with around 40 experts including workshops and written responses. The experts divided invasive non-native species into five groups: plants, vertebrates, terrestrial invertebrates, freshwater animals and marine species. They then assessed the potential risk posed by each species, based on the likelihood that they will become established without action and the threat they would pose to biodiversity, human health and/or the economy.  

The table below shows the top 20 species of concern – there is no individual ranking. 

Species	Group	Impact	Invasion history
Raccoon (Procyon lotor)	Vertebrate	Raccoons may threaten vulnerable bird species and displace native carnivores, as well as raiding fruit crops. They carry rabies and a roundworm parasite potentially fatal to humans.	Native to the Americas, raccoons are commonly kept in captivity in Britain. Some have escaped or been released, and they have been recorded in the wild here since the 1970s.
Yellow-legged hornet   (Vespa velutina)	Terrestrial invertebrate	Aggressive predator of honeybees and wild pollinating insects.	First recorded in Britain in 2016. There have been  subsequent sightings every year.
Asian longhorn beetle (Anoplophora glabripennis)	Terrestrial invertebrates	Feeds on a wide range of trees reducing diversity and altering the composition of woodlands. Tunnelling by larva reduces the structural integrity of trees and leads to death within 3-5 years.	Found in imported wood but not recorded yet in the wild in Britain.
Emerald ash borer (Agrilus plannipennis)	Terrestrial invertebrate	Larvae burrow into bark and feed on tissues that transport water and nutrients through the tree, causing extensive – and sometimes fatal - damage.	Native in parts of Asia, the species has killed millions of trees in the USA but has not yet been recorded in Britain.
Citrus longhorn beetle   (Anoplophora chinensis)	Terrestrial invertebrate	The larvae tunnel through trees, leaving them susceptible to disease and wind damage.	There have been interceptions of individual beetles in Britain but no breeding populations.
Pine wood nematode (Bursaphelenchus xylophilus)	Terrestrial invertebrate	Causes pine wilt disease which can result in extensive tree deaths.	Originating in North America, this nematode is absent from Europe except for a population in a small area of Portugal.
Pine sawyer beetle (Monochamus galloprovincialis)	Terrestrial invertebrate	Carrier of the Pine wood nematode.	It is widely distributed in Europe and has been intercepted several times in Britain.
Pink salmon  (Oncorhynchus gorbuscha)	Freshwater	Outcompetes native salmonoids for food and space, reducing populations which could impact rural areas with high dependence on fishing., Their carcasses can result in excessive nutrients, affecting water quality.	Native to Pacific Ocean and coastal rivers of North America and Asia, this species is present in at least 20 rivers in Scotland and Northern England but is not yet considered to have established in Britain.
Asian clam  (Corbicula fluminalis)	Freshwater	Reproduce and grow rapidly, outcompeting native molluscs for resources, while their high filtration rates remove nutrients and oxygen from the water, reducing plankton and other food sources for organisms. Accumulations of shells can block pipes at power plants and drinking water facilities, resulting in expensive repairs.	Native to eastern Asia. this species is now widespread in several European countries and has a high likelihood of arrival in Britain.
Purple Asian clam (Corbicula largillierti)	Freshwater	Similar impacts as Corbicula fluminalis.	Native to eastern Asia and established in Europe but not yet as widespread as Corbicula fluminalis.
Chinese pond mussel (Sinanodonta woodiana)	Freshwater	Outcompetes native mussels and other invertebrates as for food, space and hosts, are a source of parasites, as well as ecosystem engineers, altering habitats.	Not yet recorded in Britain but widespread in continental Europe.
Salmon fluke (Gyrodactylus salaris)	Freshwater	This parasite has decimated wild salmon populations in many Norwegian rivers.	Not yet recorded in Britain.
Marbled crayfish (Procambarus fallax. f virginalis)	Freshwater	A generalist diet and ability to reproduce asexually means this species could potentially establish and spread rapidly, outcompeting native crayfish.	Intercepted in Britain but not yet recorded in the wild, which it has been in some European countries.
Kagoshima blood cockle (Anadara kagoshimensis)	Marine	A significant pest in the Mediterranean, which can displace native speciesIt could be a carrier of a herpes virus that can spread to other species and kill oysters.	Native to Asia but now widespread in the Mediterranean. Not yet recorded in Britain.
Asian shore crab (Hemigrapsus sanguineus)	Marine	Aggressive and opportunistic omnivore that may significantly affect native crabs, fish and commercially important shellfish populations by disrupting the food web.	Small number of reports in Britain since 2014.
Veined rapa whelk    (Rapana venosa)	Marine	A voracious predator that feeds on native oysters, scallops and mussels. Reduces food for native whelks, crabs, birds, fish and starfish. A significant decline in shellfish affects local habitats because they provide ecosystem benefits such as water filtration and biogeochemical cycling.	Not yet recorded in Britain.
Spaghetti bryozoan (Amathia verticillata)	Marine	A colonial, filter-feeding invertebrate that forms large, bushy colonies, outcompeting native species and disrupting food chains by consuming large quantities of phytoplankton. It can grow on and smother seagrasses and foul boats and fishing equipment. When detached, it forms large drifting rafts that can clog intake pipes and aid dispersal of small mobile non-native species that live on it.	Not yet recorded in Britain.
Asian fan weed  (Rugulopteryx okamurae)	Marine	An invasive seaweed that displaces native species. When detached it can be deposited as decomposing masses on shores, smothering small invertebrates and affecting tourism and, potentially, human health.	Native to the northwestern Pacific Ocean, it has become invasive along southern European coastlines but has not yet been recorded around the British Isles.
Wireplant (Muehlenbeckia complexa)	Plants	Its ability to spread quickly on the ground and as a vine means it can smother vegetation, and it has an increasing ability to thrive in our warming climate.	Introduced as an ornamental plant, it has been reported in several locations in the wild, near the coast in Britain.
Twoleaf Watermilfoil (Myriophyllum heterophyllum)	Plants	Dense stands reduce light to other aquatic plants, and its decomposition reduces oxygen levels thereby impacting fish and aquatic invertebrates	Originally from North America, this species has been classified as invasive in Germany, France and the Netherlands.