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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 an experts' watchlist of invasive non-native species that could threaten Great Britain in the next 10 years.

The , 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).

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 list of the top 20 that pose the greatest risk, 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 whether 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, the 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 the dispersal of small motile non-natives that live on it.
• Purple Asian clam (Corbicula largillierti), a species that can reproduce rapidly, outcompetes native mollusks 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 plays a vital role by recording and reporting sightings of invasive non-native species, helping to inform action."

For example, 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 remains high.

Additional threats

The list again includes beetle species that pose a significant risk to trees through their feeding and tunneling habitats—the 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 the future."