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Extreme weather is opening the floodgates to nature invaders in the UK

  • Increased flooding in the UK is creating perfect conditions for the spread of damaging invasive non-native species (INNS) including well-known problem plants like Japanese Knotweed and Giant Hogweed
  • Flooding and increased temperatures risk the establishment of new invasive species in the UK, with animals including Red Imported Fire Ants known to spread via floodwater
  • As well as making invasive species spread more likely, excessive rainfall and flooding is hampering the work of local groups to control nature invaders

As Invasive Non-Native Species Week (20th – 26th May) gets underway nature groups including The Rivers Trust, Plantlife, and Buglife are calling for Government action to stop a flood of new nature invaders arriving and spreading in the UK due to more extreme weather. [1]

With the last 18 months being the wettest period in England since records began in 1836, experts are warning of a potential surge in the impacts of invasive species which are known to thrive in wet conditions. These include known problem species that are already established in the UK but which grow and/or spread quickly in wet weather, such as: Japanese Knotweed (which can cause structural damage), Giant Hogweed (with sap that can cause burns to skin) and Himalayan Balsam (which outcompetes native species and can cause riverbank damage and river-clogging causing higher flood-risk).

More extreme and frequent flooding from climate change also puts the UK at higher risk of some future nature invaders causing economic damage, harm to people, and declines in already struggling native species. The Red Imported Fire Ant (first discovered in Europe last year) has been spotted spreading in floodwater by joining together to create a raft. These ants carry a venomous sting and can cause structural damage from their nests. A warming climate is making European urban centres more susceptible to this species. The Golden Apple Snail is another species where flooding is a key path for their spread.

Richard Benwell, CEO of Wildlife and Countryside Link, said:
“As climate change brings increasingly volatile weather, the risk of invasive species spreading will grow. Invasive species are already one of the biggest threats to the UK environment, from smothering waterways to outcompeting native species. They also cause billions of pounds in damage a year to homes and businesses, and even pose risks to human health. Investment in a fully-funded inspectorate and a strong invasive species strategy could make a contribution to halting nature’s decline and creating a more resilient economy.”

David Smith, Social Change and Advocacy Officer at Buglife, said:
“We’re already seeing big impacts on native insects and invertebrates due to invasive species, including earthworms being predated by non-native flatworms to American Signal Crayfish damaging riverbanks and undermining flood defences. With highly invasive species known to spread via floodwaters, such as Red Imported Fire Ants, Chinese Mitten Crab & Chinese Mystery Snail, making their way across Europe, we must act now to stop further damage to our native wildlife.”

Dr Rob Collins, Director of Policy and Science at The Rivers Trust, said:
“This winter has seen some of the wettest conditions on record, and this has left our local Trusts across the country struggling to keep a wave of invasive species at bay. Invasive plants are growing faster than ever, and high floodwaters present a real risk of new species establishing and spreading to new areas. The Government must properly support local conservation groups nationwide who are working tirelessly to stop our waterways being smothered by nature invaders.”

Extreme wet weather in the UK is also scuppering ongoing local efforts to control invasive species. Chemical and biological controls (such as the use of mites) can be damaged by such extreme weather and heavy rain can also prevent the use of smart trapping devices around rivers due to safety issues. High rainfall has also led to a second wave of growth in some invasive plants which can mean chemical and other controls need to be used for an increased period, leading to higher resourcing and cost implications. Low budgets and uncertain funding are also leaving local groups with reduced capacity to control the spread of invasive species, as previously successful programmes are being forced to make difficult decisions including staff redundancies.

Erin Shott, Communications and Policy Officer at Plantlife, said:
“Many invasive plants, such as rhododendron ponticum and himalayan balsam were introduced deliberately for aesthetic value but are now having a dire impact on native British wildlife, including reducing populations of internationally rare species, particularly in vulnerable temperate rainforests. These species can dominate, suppressing regeneration and shading out characteristic flora, like Tree lungwort and Hutchens’ Hollywort. Rising flood risks and falling local resources to control invasive species could lead to a perfect storm for the spread of damaging species. It’s key that the UK gets ahead of the rising tide of invasive species as part of the climate crisis by providing the national leadership and local resources needed to tackle this issue.”

Invasive species not only harm our native wildlife, they also damage the economy, with a big price tag of around £4 billion per year for the damage they cause to infrastructure, industries such as fishing and forestry, our rivers and other water bodies, and homes. [2] Environmentalists are calling for the annual invasive species biosecurity budget to triple to £3 million with a further £3 million to fund a permanent dedicated invasive species Inspectorate. This could save the UK economy a total of £2.5 billion over 20 years, providing a return on investment of £21 for every £1 spent.

Nature experts have made the following key recommendations for the next Parliament to tackle invasive species. Read more on these here

  • The Government should secure long-term and ambitious funding for INNS management, including funding for Local Action Groups (LAGs) to create a biosecurity “citizens army”, as recommended by the Environmental Audit Committee.
  • Triple the invasive species biosecurity budget to £3 million and providing a further £3 million to fund a permanent dedicated Invasive Species Inspectorate. The Government should also increase the powers of the inspectorate so that it may carry out border inspections to prevent new invasive species entering the UK.
  • Reform the process of listing GB Invasive Species of Special Concern by speeding the process up to rapidly respond to new threats, as well as proactively managing the listed species that are already widespread and causing harm.
    Increase public awareness of INNS, supporting the GB NNSS to deliver a strong and robust public communications and engagement plan to target the general public as well as private companies and businesses.
  • Create a publicly available evidence base for all identified and potential INNS, kept up to date with climate change modelling and intel from our closest trading partners.
    Increase the powers of the invasives species inspectorate so that it may carry out border inspections to prevent new invasive species entering the UK, and access home dwellings.

Table 1 – invasive species already established in UK, which spread and thrive in excessively wet conditions


Effect by heavy rain and flooding

Japanese KnotweedRapid growth leaves native plants struggling for space and light. Can also cause damage to buildings (though not to the extremes sometimes reported) and make homes harder to sell.Weather related disasters such as flooding is a significant cause of Japanese Knotweed spread. Fragments of rhizome break away from the main plant and are swept away to take hold in new locations.
Himalayan Balsam

It is fast-growing and spreads quickly, invading wet habitat at the expense of other, native flowers. The plant also increases flood risk as when it dies in the winter it leaves large areas of eroded exposed riverbanks.High flood waters help carry the seeds into new areas, as they drift along waterways after dispersal.
Giant Hogweed
Spreads across river banks where its seeds are transported by the water. Can cause burns to people’s skin. Further impacts include reduced native plant populations and inhibited access to rivers.Floodwaters have been documented leading to a rise in Giant Hogweed including across Gloucestershire.
Rhododendron ponticum
Chokehold on temperate rainforest habitats (particularly in the north coast of Scotland), densely shading out native trees and key rainforest species, including internationally rare lichens and bryophytes.Thrives in wet weather conditions.
Zebra Mussels
Colonies impede operation of canal gates and outcompete native mussels. Filtration of water also stimulates growth of weeds. Damage to water pipes is thought to cost £8million a year.Spread primarily via clinging to watercraft, but experts have warned that floods can help the mussel spread to new waterways.
Chinese Mystery Snail
Two populations identified in Sussex and Hampshire. In the US they have been found to carry parasites and diseases and outcompete native species for food and habitat.Flooding of outdoor ponds thought to have facilitated spread of snails into rivers in the US.

Table 2 – invasive species on the horizon, which are known to spread through flooding

SpeciesCurrent statusImpactEffect by heavy rain and flooding
Golden Apple Snail
Listed in the top 100 invasive species worldwide mostly for its impacts on cropsRestrictions on importing the golden apple snail for UK aquariums were lifted in 2021. A warming climate could increase the risk of it establishing in the wild.Flooding facilitates its spread along waterways.
Red Imported Fire Ant
An established wild colony in Europe was seen for the first time in Italy in 2023. Risk of arriving in the UK via contaminated plants or cargo ships.One of the most damaging invasive species with a range of negative impacts on biodiversity, including ground-nesting birds, buildings and public health.Spotted travelling in floodwater by joining together to create a raft which drifts across the surface to new areas.


Notes to Editors:

  1. Read the full report Stemming the Flood of Invasive, Non-Native Species in the UK here
  2. Analysis by Link on the economic costs of invasive species was first undertaken in 2021, and has been updated for this year following on from other reputable analysis. The economic impact has been found to be approximately the same as it was in 2021, though the impending arrival of new species (exacerbated by warming temperatures and extreme weather) could see a sharp rise in coming years. 
    1. This figure includes impacts from invasive fungi such as Hymenoscyphus fraxineus which causes Ash Dieback (which has an annual cost alone of £883.5m. 
    2. When fungi are not included the figure is closer to £2 billion.

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