News Release

Meet the next alien invaders to threaten nature and economies in Scotland

Reports and Proceedings

UK Centre for Ecology & Hydrology



Muntjac deer have a big impact damaging trees and shrubs, with knock-on effects on flora and fauna that rely on these habitats. Muntjac breed rapidly and the experts warn that should the species become established in Scotland, it would pose a significant threat to existing vegetation and plans for woodland expansion. Photo: GBNNSS

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Credit: GBNNSS

Raccoons, muntjac deer, plants that cause flooding, and mussels that clog up water pipes are among the new problem species that are likely to arrive in Scotland over the next decade and pose a serious threat to nature or people.

More than 1,000 invasive non-native plants, animals and other organisms are already established in Scotland including the grey squirrel, rhododendron, Japanese knotweed, mink and New Zealand flatworm.

Now, in an independent study for the Scottish Government, a team of experts, led by the UK Centre for Ecology & Hydrology (UKCEH) and NatureScot, has predicted other non-native species that could arrive, establish and have negative impacts on biodiversity and ecosystems, economies or human health over the next 10 years. They have also highlighted how these species are likely to arrive.

Out of the 171 possible new arrivals, the experts have drawn up a top 10 priority list (see notes and photos) including Asian clam, quagga mussel and zebra mussel, floating pennywort and parrot’s feather, which are freshwater species that outcompete native wildlife for food and space.

Freshwater biodiversity, which is culturally and economically important in Scotland, is considered to be particularly vulnerable because river and lake ecosystems already face other pressures such as climate change, overexploitation, sewage, nutrient pollution and recreation activities which can exacerbate the impacts of invasive non-native species.

For example, after the introduction of the non-native ruffe in Scottish waters as live bait by anglers in the early 1980s, its population expanded and, by feeding on the eggs of the powan, it caused declines in this rare native species at Loch Lomond.

The recent promotion of tourism in northern Scotland has led to exceptional increases in the number of visitors to this region, while national legislation has opened up access to watersports. The experts say there are therefore increased pathways for invasive non-native species to be transported into and throughout Scotland via vehicles, boats, equipment and clothing.

Other species on the priority list include muntjac deer, which could expand their range from other parts of Britain, as well as raccoons, which are kept within zoos and private collections and may escape or be released into the wild. Muntjac damage trees and shrubs and, the experts say, threaten Scotland’s plans for woodland expansion, while raccoons prey on native species, affect farm production and spread potentially fatal diseases including rabies.

The most common pathways for arrival of non-native species in a new region is through the import of produce and ornamental plants which are either invasive and then expand into the wild or which contain hidden species.

UKCEH ecologist Professor Helen Roy, an expert on invasive non-native species, led the ‘horizon scanning’ study, which is a systematic examination of information to identify potential threats and risks.

Professor Roy, who was also co-chair of a major global report on invasive non-native species for IPBES, explains: “Prevention is the best way to manage biological invasions , so we would urge people to follow simple biosecurity measures outlined in campaigns such as “Check, clean and dry”  and “Be Plant Wise”. Everyone can make a difference in preventing the introduction and spread of invasive non-native species.”

The report, Provision of horizon scanning and analysis of pathways of spread of invasive species into Scotland, will inform national and local strategies and action to reduce the threat of biological invasions, which are increasing year on year.

NatureScot’s invasive, non-native species specialist, Stan Whitaker, says:“Invasive non-native species are a serious threat to Scotland’s nature, damaging our environment, the economy and our health, and costing Scotland at least £499 million a year.

“The threat is increasing with the growth in international trade and travel. But we can all do something to help prevent these species from spreading by being plant wise and composting invasive pond plants, like floating pennywort and parrot's feather in our gardens, with care – or by reporting sightings of mammals like muntjac deer."

The report is available here.

Notes for Editors

In their study, the experts identified 171 new invasive non-native species that are considered likely to arrive in Scotland in the next 10 years, all of which negatively affect biodiversity. Some 27 of these species have impacts on human health and 47 on economies.

The experts highlighted 30 of these invasive non-native species that have a high risk of arriving, establishing and impacting biodiversity and ecosystems. From this, UKCEH and NatureScot then drew up a top 10 priority list (see photos in dropbox) which are:

  • Floating pennywort.
  • Parrot’s feather.
    These two invasive plants form dense strands that outcompete native flora for nutrients, sunlight and oxygen, and affect water quality and river flow, exacerbating flood risk. The species are widely distributed in England and Wales but have only been recorded on one site each in northern Scotland, and were removed.
  • The Reeve’s muntjac, which have already colonised parts of England and Wales. These small deer have a big impact damaging trees and shrubs, with knock-on effects on flora and fauna that rely on these habitats. Muntjac breed rapidly and the experts warn that should the species become established in Scotland, it would pose a significant threat to existing vegetation and plans for woodland expansion.


  • Raccoons. These highly adaptive omnivores were brought to Europe from America for fur farming and as prey for hunters, and are often kept as pets. They can spread diseases that are potentially fatal for humans and animals including rabies and a roundworm parasite, and have significant impacts on farm production by feeding on fruit crops and killing poultry.
  • Zebra mussel and quagga mussel. These two species outcompete native mussels and other organisms for space, nutrients and food, and also filter large amounts of water which has a negative impact on plankton, and therefore the aquatic food web.
    They feed on algae that compete with cyanobacteria, commonly known as blue-green algae, which allows these toxic algal blooms to flourish, This can have impacts on biodiversity, economic interests and health.

Zebra and quagga mussels can also clog up pipes of water treatment plants and ships.


  • Asian clam. These molluscs inhabit rivers and seas, and release phosphorus and nitrogen into the water through burrowing, feeding from the sediment and their excreta. Higher concentrations of these nutrients can increase growth of harmful algal blooms and reduce oxygen levels in water.
  • Flatworms. A group of invasive non-native species prey on earthworms, with knock-on effects on the birds and mammals that feed on them as well as on plant and crop growth. Once established there is currently no known method of removal or control. The experts say the Australian, Southampton and Brown Kontikia and Obama flatworms could follow the New Zealand flatworm in becoming established in Scotland.
  • Pheasant's-tail grass. This plant species grows quickly into dense clumps, so is a potential fire risk in vulnerable habitats. It is widespread and hardy, and can also prevent germination of seeds of other species.
  • Highbush blueberry. This tall and dense deciduous can have significant impacts on biodiversity and ecosystem impacts, through outcompeting lower-growing plants and possibly reducing water content of boggy soils.
  • Slipper limpet. This marine mollusc reduces populations of native shellfish, which can have significant impacts on the fishing industry. The limpets outcompete native species such as scallops for food, smother oyster beds and attach themselves to other molluscs, reducing their survival, growth and reproductivity. There are additional costs associated with cleaning shells and water.

The number of non-native species being introduced to new regions around the world is increasing year on year. Up to 14% of these species in the UK are considered invasive.

IPBES, the leading intergovernmental body that assesses the state of the planet's biodiversity and its benefits for people, has highlighted invasive non-native species as one of five major threats to biodiversity and ecosystem services, alongside climate change and land- or sea-use change direct exploitation of organisms and pollution. Combinations of these threats exacerbate changes in biodiversity.

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