Grey Squirrel. John Harding / BTO
Grey Squirrel, by John Harding / BTO

Tackling invasives: unpicking the definitions

BTO’s Head and Principal Ecologist David Noble breaks down what we really mean by ‘invasive species’.

David Noble

Head & Principal Ecologist

David is responsible for the strategic development of BTO’s monitoring programme and associated research, is the main contact for biodiversity indicators, surveys of taxa other than birds, and for conservation initiatives such as BAP and Birds of Conservation Concern.


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Today is the start of Invasive Species Week – seven days to raise awareness of the impacts of invasive species like Ring-necked Parakeet, Japanese Knotweed, Reeves’ Muntjac and Asian Hornet. This week is important: there is a huge body of evidence that non-native species can have strong adverse effects on the environment and other species in their new range. This makes them one of the most significant threats to biodiversity on a global scale, and one that is increasing despite stricter legislation and better practice to prevent their spread. 

As well as their effect on biodiversity, non-native species can also have significant impacts on human health and economies, with severe consequences for crops, timber resources or livestock, as well as transport infrastructure such as canals. Control and eradication programmes designed to deal with these pressures are a huge global drain on resources, estimated to cost billions – close to £2 billion annually in the UK alone.

On a global scale, the problem is particularly dire for island nations; high in endemic species, and low in natural predators and competitors, these isolated landmasses are hugely vulnerable to invasives. A well-known example is the local extinction of nearly all vertebrate species on the island of Guam after it was colonised by the Brown Tree Snake.

However, once you start talking about invasives, definitions can become surprisingly tricky.

Non-native species have been introduced to areas outside their natural range through human intervention.  
The Common Pheasant was introduced as a game bird in the UK as early as the 10th Century. Now, close to 40 million are released every year for the hunting season. 

Hitching a ride with humans

Non-native species are species that have been introduced to areas outside their natural range through human intervention. Synonyms used elsewhere include ‘non-indigenous’, ‘exotics’, ‘introduced species’ and ‘aliens’, many terms coming with negative or uncomfortable social connotations.

Introduction through ‘human intervention’ comes in many forms. Many species of plants and animals have been deliberately introduced to Great Britain and Ireland for food (for example, Rabbit), for hunting (Pheasants, Fallow Deer) and for aesthetics (Peafowl). Countless others arrive through escapes from wildlife parks and zoos, as pets or from aquariums, or as stowaways in cargo, food supplies, ship ballast or timber.

The definition also includes species that arrive through their own devices from introduced populations which are established elsewhere, as in the case of the Asian Hornet, brought to Europe on wood pallets from China but subsequently flying across to Great Britain. Sacred Ibis occasionally seen but not established in Great Britain may have flown over from the naturalised populations in France, although to date, most have turned out to be recent escapes from parks. 

The Asian Hornet was first recorded in France in 2005 after arriving in wood pallets containing pottery from China. It has since flown across the Channel, with the first British record in 2016.  
Established or naturalised populations of non-native species are self-sustaining and self-perpetuating, surviving and breeding without any deliberate human intervention.  
Populations of ornamental waterfowl, such as Black Swan, are not considered officially ‘established’, although they may become so; some pairs are forming free-living populations, albeit with little expansion. 

Putting down roots

The second issue is whether the species is established or synonymously naturalised, by which we mean whether the non-native population is self-sustaining and self-perpetuating. In other words, the population is surviving without any deliberate human intervention (e.g. food or shelter) and is breeding.

This is particularly relevant for plants, where there are hundreds of ornamental or crop species in gardens, parks and agricultural land, but the species is not considered naturalised unless it has established ‘beyond the garden wall’, meaning there are generations of self-sustaining populations in the wild.

This also excludes many waterfowl in city water bodies where they are being supplied regularly with food and shelter, like Black Swan in London’s Regents Park. The definition of establishment is important, despite its murkiness, because hundreds of species – usually single individuals – occasionally escape from homes, zoos or parks despite legislation meant to prevent this, and neither establish nor have much impact. Rogue pythons, Red Panda and even a Hippo which briefly occupied a canal before being recaptured are good examples.

Some escapes, however, are more worrying, because we know that the species has caused significant problems elsewhere, such as Raccoons which are well-established in Germany. The escape pathway is of special concern when the species is commonly kept in groups and escapes may include already breeding individuals.  

The global Convention on Biological Diversity defines invasive species as having a negative impact on other biodiversity, the economy or human health.  
Little Owl, by Liz Cutting / BTO
Little Owl was first reported breeding in the UK in 1879. It is now widely established but has little impact on native biodiversity, the economy or human health, so is not considered invasive. 

When does a non-native species become invasive?

The other slightly non-intuitive definition is for invasive non-native species.

In Great Britain, we follow the lead of the global Convention on Biological Diversity in defining invasive as having a negative impact, by harming other biodiversity, causing economic damage or by impacting human health. Hence it is not the ecological meaning of invasive (rapidly spreading and colonising new areas or habitats) that is used, but the extent of a species’ negative effect.

It is of course virtually impossible for any non-native species to have absolutely zero impact, but where impacts are considered minimal (as for Little Owl or Red-necked Wallaby in Britain – indeed, 85% of non-native species) they are not considered to be invasive. In some cases, the species may still be too rare to have much impact, but if it is known to be problematic elsewhere, it would be included.

Can a native species be invasive? Yes, but only in the ecological sense, in being able to rapidly colonise and dominate new habitats – Bracken could be described in this way. 

The impact of Ring-necked Parakeet on native cavity-nesting species in the UK is still uncertain although it is thought to out-compete Nuthatch and Noctule Bats elsewhere in Europe. The British population of this introduced species are the most northerly  breeding parrots in the world. 
The cultural value of species such as Reeves’ Muntjac and Canada Geese should be weighed against evidence for their negative impact elsewhere. BTO research has revealed Reeve's Muntjac have strong adverse effects on native woodland biodiversity,  and high concentrations of geese – with their droppings – can affect water quality. 

Can non-native species be beneficial in their new environment?

The damaging consequences of non-native species are clear, and the most cost-effective solutions include promoting awareness to prevent their introduction and spread through bio-security measures. However, the question of whether non-native species can be beneficial is often raised. Evidence of this is rare apart from instances of high cultural value, particularly in urban environments, where much of the public enjoys seeing and interacting with a suite of non-native species such as Ring-necked Parakeet, Canada Goose or Grey Squirrel.

For some of these culturally-valued species, these benefits must be weighed against strong evidence of damage to native diversity (as for Reeves’ Muntjac and other non-native – and native – deer on woodland plants, insects and birds) but in other cases, such as for Ring-necked Parakeet, the impact appears still to be a potential threat rather than a current pressure, at least regarding ecological effects.

In general, pragmatism rules: early eradication of new arrivals, such as American Bullfrog, is both more cost-effective and risk-averse than trying to control already widespread and problematic species such as Signal Crayfish or Grey Squirrels.

In Great Britain, there are more than 2,000 established non-native species of animals or plants; of these, 15% are classed as invasive.  
Initially present in the UK as an ornamental plant, Water Primrose started to cause severe damage to native biodiversity and waterway transport routes. Its sale is now banned in the UK. 

Monitoring and tracking invasive species 

In Great Britain, there are more than 2,000 established non-native species of animals or plants. The majority have had minimal impact, but around 15% – the invasive non-native species – have spread and are having a negative impact on biodiversity and/or human health or economy. These are largely the species in the GB Non-Native Species Indicator, which tracks the aggregated geographical spread of these species across the past six decades. Of these established non-native species, the majority (around 75%) are plants, followed by insects, and then vertebrates, with most found in the terrestrial environment.

We know this mainly because of the high effort in biodiversity recording that goes on in the UK, via a plethora of different monitoring schemes, most of them volunteer-based. In addition, a number of particularly problematic species – including Raccoon, Water Primrose and Asian Hornet – are on the ‘alert list’ for Great Britain because their arrival and establishment could cause huge problems for native wildlife, or to human health and our economy. 

The public can submit sightings and photos of any of these species through iRecord or the Non-Native Species Information Portal. Non-native species records submitted as part of other monitoring schemes (such as the BTO/JNCC/RSPB Breeding Bird Survey or BirdTrack) are also subsequently forwarded for use to the National Biodiversity Network Atlas and relevant organisations.

Invasive Species Week: get involved! 

Invasive Species Week is organised by the National Biodiversity Network (NBN). It is an annual national event to raise awareness of the impacts of invasive species, and how everyone can play a part in preventing their spread, from learning how to be Plant Wise when gardening to looking out for alert species like the Asian Hornet. To find out how you can get involved, follow the links above for recording non-native species, visit the GB Non-native Species Secretariat’s website, or explore the content below.

BTO is a National Biodiversity Network Trust Member. Read more about NBN, and their work on invasive species, on their website.

Read more

Nightingale, by Edmund Fellowes / BTO

BTO research suggests that Common Nightingale and Willow Tit are being affected by increasing populations of non-native deer.

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Collaborative ‘horizon-scanning’ research involving BTO has identified species with the potential to threaten Great Britain’s biodiversity.

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Featured Publication
Peer-reviewed papers
Ring-necked Parakeet, Sarah Kelman

The potential for analyses of monitoring scheme data to inform about the impacts of invasive on native species

2022 | Pringle, H. & Siriwardena, G.

Biological Invasions

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We’d love to hear from you! 

Leave your comments and questions below. 

David Noble, 15 May 2023

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