Our use of Cookies

This site uses only cookies strictly necessary to ensure the site works correctly.

Please read about how we use cookies.

Hide this message

Strictly necessary and non-essential cookies

By clicking accept all cookies, you agree to our use of cookies and to our cookie policy.

We use third-party cookies on this site.

You have accepted necessary cookies only

You can change your cookie settings at any time
Hide this message


What are non-native species?

A non-native species (NNS) is a species that has been introduced into the country by human intervention (either deliberately or accidentally) since the end of the last ice age (approximately ten thousand years ago). The term 'non-native species' is synonymous with alien, non-indigenous, foreign and exotic.

How many non-native species are there in Britain?

A recent study has shown that there are nearly 2000 established non-native species in Britain.

Are all non-native species a problem?

Most non-native species do not cause problems, indeed the majority of our agricultural species (wheat, barley, sheep etc.) are not native to Britain. Only a minority (10-15%) become invasive and have negative impacts, some of which are seriously negative impacts on agriculture, forestry or biodiversity interests.

What are invasive non-native species?

Invasive non-native species are species that have been introduced (deliberately or accidentally) by people, which are having a detrimental impact on the economy, wildlife or habitats of Britain. There are far more non-native species present that are not invasive.

Where do most of our invasive non-native species come from?

Invasive non-native species come from all over the world, some from continental Europe (e.g. zebra mussels) some from Asia (e.g. Chinese mitten crab), some from America (grey squirrel, mink, ruddy duck, Ludwigia, signal crayfish), some from Africa (hottentot fig) and some from Australia (Australian flatworm, Australian swamp-stonecrop) and New Zealand (New Zealand flatworm).        

How do non-native species get to Britain?

Non-native species spread through a variety of pathways some of which are intentional (e.g. deliberate release of non-native fish species for angling) and some unintentional (e.g. by ‘hitch-hiking’ on food or other goods imported into Britain or by escaping from captivity (e.g. American mink)). Some marine species are transported large distances attached to ships or in ships ballast water (e.g. Chinese mitten crabs).  Many plant species are brought in intentionally as part of the horticulture trade but some then become established in the wild after they are disposed of irresponsibly or ‘escape’. 

Are they increasing and spreading?

There is already a large number of non-native species in Britain and the number becoming established is likely to increase due to the growth in world trade and global tourism. Climate change may also allow species that are currently benign in Britain to become invasive. The risks associated with invasive non-native species are therefore likely to remain a feature of our lives.

What are the main impacts on native species?

Invasive non-native species can have a huge range of negative impacts.  These include:

  • Transmission of disease to native species (e.g. grey squirrels with squirrel pox, signal crayfish with crayfish plague).
  • Competition with native species (harlequin ladybird).
  • Predation on native species (mink on water voles).
  • Increased flooding risk (invasive water weeds).
  • Infrastructure damage (e.g. to bridges, water treatment facilities).
  • Human health risks (oak processionary Moth (which have hairs that are irritants to humans), giant hogweed (that has sap that causes blistering) and terrapins (which can carry salmonella)).

Find out more about the impacts of invasive non-native species.

How much do they cost the British economy?

The cost to Britain is at least 1.7 billion per annum (Cabi, 2010). Read more in The Economic Cost of Invasive Non-native Species to the British Economy (PDF). This does not include less-quantifiable costs to biodiversity and ecosystem functions.

It is estimated that the Environment Agency spends approximately £1.5 million a year on invasive aquatic plants due to the flood risk they pose. British Waterways spend about £1 million a year on invasive non-native species (including riparian – riverbank – species). Floating pennywort is currently thought to be the most expensive aquatic weed, it is estimated that it costs over £2.5 million each year in control, and its impacts on leisure, recreation and flooding. Water primrose could be just as bad but we have acted quickly to try to stop it from establishing in Britain.

What is the worst invasive non-native species in Britain?

There are a number of species which have a significant detrimental impact.  Some of these are economic impacts while others have negative impacts on our wildlife.  Examples of some of the worst species are given below:

  • Japanese knotweed, which can grow through concrete, costs developers millions of pounds per annum, mainly on brownfield sites.
  • Rats have wiped some seabird species off a large number of British islands.
  • American mink are the main cause of the catastrophic loss of water voles in Britain.
  • A number of non-native aquatic weed plants clog watercourses, kill off fish stocks, prevent angling and can increase flood risk and damage structures such as bridges.

Are all non-native species going to be eradicated?

No. The first priority of the non-native species framework is prevention as this is by far the most effective means of tackling the problem of invasive non-native species. The GB Non-Native Species Strategy is concerned with addressing only those non-native species that are known to be, or are thought to present a significant risk of being invasive. It is internationally recognised that eradication is usually only practically feasible and financially affordable in the early stages of an invasion. Once invasive species are well established, it is usually a case of mitigation measures and control e.g. localised or possibly regular removal.

Can we really do anything about non-native species?

Yes, there is a lot that can be done both by government and the public to help reduce the risks of introducing or spreading invasive non-native species. The strategy sets out the measures the GB Administrations will take to raise awareness and understanding of the risks, to gather information and to minimise the risks of further introductions or establishment of invasive species. Key organisations covering a wide range of interests from business to hobbies can help inform their members about these issues and everyone can generally adopt more responsible attitudes and behaviours when it comes to using non-native species in gardens or keeping exotic pets. Government can ensure that there are adequate inspections on imports to detect invasive species and can support research and management of invasive species where appropriate.

Find out more about how you can help

What should I do if I see a non-native species?

Any non-native species records can be submitted to iRecord. It’s particularly important to record alert species. Find out more about other ways to record non-native species.

What can I do to help?

There are simple ways that everyone can help prevent the spread of non-native species.  

What are the penalties if people introduce these into the wild? 

Find information on legislation.

What research has government carried out on invasive non-native species?

GB administrations and agencies have been carrying out research on a range of non-native species issues for many years.  This has ranged from strategic research (e.g. assessing the use of fertility control techniques) to work on individual species (e.g. Rabbits, Parakeets, Ruddy Duck, Mink) including looking at impacts on native species and methods of control. 

Why is the last ice age used as a baseline?

When the Ice Age ended over 10,000 years ago the ice that covered most of Britain retreated northwards. Following behind this retreating ice were waves of plants and animals that slowly colonised Britain as conditions warmed up. These plants and animals got to Britain under their own steam as there was still a connection (the land bridge) attaching us to the European mainland.

However, as the ice melted sea levels rose and the connection was flooded. This effectively stopped any more colonisation by species that couldn’t cross the water. All these plants and animals – the ones that established themselves in Britain naturally - are called native species.

Some species (like elk and lemmings) died out naturally as the climate continued to warm up, while other species such as wolves and beavers were eradicated by man only relatively recently – in the last few hundred years.

People first arrived in Britain about 8,000 years ago and virtually all new land animals and plants that have become established since this date have been brought here by people.  These are all non-native species.

Which deer species are native and which are non-native to the UK?

There are six species of deer in the UK. Red and roe deer are the only native species. fallow deer (Dama dama) are a long-standing naturalised species and sika deer (Cervus nippon), muntjac deer (Muntiacus reevesi) and Chinese water deer (Hydropotes inermis) were introduced in the last 150 years.

Why are deer numbers increasing?

It is thought that deer are more abundant and widespread now than at any time in the past 1000 years. There is also evidence of increasing deer numbers in the expansion of their geographic range. Rapid increase in deer numbers in recent decades is a result of:

  • Increased woodland cover
  • Milder winters leading to increased fecundity
  • Changes to agriculture such as increased area of winter crops
  • Escape from parks and farms
  • Greater connectivity between green space in urban areas

Why are deer culled?

Deer are culled (approximately 350,000 per year) to manage the populations and to try to prevent the problems they can cause.

What problems are associated with deer?

In the countryside, excessive deer densities cause over-grazing and excessive browsing and trampling. In ancient woodland this can lead to:

  • Loss of characteristic woodland plant species such as the oxlip and bluebell
  • Declines in characteristic woodland bird species such as the nightingale due to loss of plant structural diversity and food supply
  • Declines in invertebrate abundance and diversity
  • Prevention of adequate  tree regeneration  and traditional coppicing management
  • Other serious problems include disease transmission to humans and livestock, animal welfare in dense populations, poaching and human safety.

Increasing numbers of deer in urban areas has lead to several emergent problems including:

  • Road traffic accidents,
  • Damage to gardens, allotments and parks,
  • Attacks on pets by muntjac deer and vice versa
  • Deer trapped in railings, canals and waterways requiring emergency service assistance
  • Violent attacks on deer by humans
  • Illegal deer coursing and poaching

Are there many road traffic accidents involving deer?

A nationwide survey from 2000-2005 collected reports of over 30,500 deer-vehicle collisions (DVC), of which 1,150 resulted in human injury and 20 in human fatality. The survey emphasises that DVC are significantly under-reported. In 2006 over 3,500 live deer casualties were reported.

What are the estimated benefits and costs to the economy?

Wild deer are a significant economic and social resource in the Scottish Highlands, contributing an estimated £170 million and the equivalent of 2,500 full-time jobs in Scotland. Actual costs of damage caused by deer are very difficult to quantify, however, the cost of managing deer in Scotland alone has been estimated at £4.5 million per year by the Forestry Commission Scotland. Defra estimated the cost to agriculture in England at £4.3 million.