Pink salmon, Purple Asian clams, marine invertebrates that form “spaghetti-like colonies”, and a nematode worm that brings extensive mortality events to trees are among the new entries in experts’ watchlist of invasive species that pose a threat to Great Britain over the next ten years.

The latest version of the watchlist – which once again includes known problem species such as the yellow-legged (Asian) hornet, raccoon, and twoleaf watermilfoil, has been produced by a team of experts led by those at the UK Centre for Ecology and Hydrology (UKCEH) and commissioned by the British Government’s Department for Environment, Farming, and Rural Affairs (DEFRA).

These experts have identified some 145 non-native animals and plants that have the potential to become invasive in the near future, impacting biodiversity and ecosystems as well as – in some cases – human health and 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, meanwhile, are 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, however, is enabling many species from warmer climates to survive and spread in Europe.

From the 145 species identified as those of concern, the experts have compiled a ‘top 20’ that pose the greatest risk, in order to inform monitoring efforts, policies, and communications to the public, businesses and local authorities. Those posing a direct threat to British coastlines include the Pink salmon (Oncorhynchus gorbuscha), the Spaghetti bryozoan, Purple Asian clam, Veined rapa whelk, and Asian fan weed.

Pink salmon 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.

Spaghetti bryozoan (Amathia verticillata) is a colonial filter-feeding invertebrate that forms large, bushy colonies and outcompetes native species and disrupts 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.

The Purple Asian clam (the Corbicula largillierti) is a species that can reproduce rapidly, outcompete native molluscs and clog pipes, obstructing infrastructures such as drinking water facilities. 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.

The veined rapa whelk (rapana veriosa) is a voracious predator that feeds on native oysters, scallops, and mussels, outcompeting other predators and potentially resulting in economic impacts for fishing communities, while the 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 that were compiled 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.

Professor Helen Roy, an ecologist at UKCEH, lead author of 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.”

As well as marine life, the list includes the likes of beetles, hornets, even raccoons. They all join a growing list of over 2,000 non-native species already established in Britain with new species being introduced each year. While only 10 to 15% become invasive non-native species, those that do have serious impacts on the environment, cost the economy nearly £2bn 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,” said Olaf Booy, deputy chief non-native species officer at the GB Non-native Species Secretariat, part of the Animal and Plant Health Agency.

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

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