Invasive Mammals
Credit: Padraig Kavanagh
Introduction
The Mammal Society is a membership-based charity which aims to bring people together who are interested in mammals; raise awareness of mammals and promote recording of them; and to encourage good conservation or management of mammals based on sound scientific evidence.
Invasive Non-Native Species (INNS) are one of the major threats to biodiversity globally. As well as affecting the environment, INNS can have a negative impact on the economy and human health. It is estimated that INNS, plants and animals, cost the British economy about £1.7 billion per year, but this could be significantly higher. Often, people are unsure which mammals are invasive and why. It is essential to have a clear understanding of the status and impacts of invasive mammals to determine the appropriate management. The Mammal Society is not actively involved in INNS management, but it is an important co-ordinator of monitoring and science that underpins INNS management. This is done by encouraging accurate recording and reporting of INNS, initiating mammal surveys and producing reports, for example the Review of the Population and Conservation Status of British Mammals (2018) and the Atlas of the Mammals of Great Britain and Northern Ireland (2020), to inform best practice.
Summary
The Mammal Society supports efforts to stop the introduction, release or spread of invasive mammals which involves collaborative work with a range of stakeholders through the following actions:
Encouraging the recording and reporting of all mammals, using apps such as Mammal Mapper, to identify changes in distribution and populations, help in the detection of potential or new invasive mammals, reporting of established invasive mammals and management options.
Initiating and supporting survey and monitoring programmes to find out more about the status of mammals in Britain.
Promoting legislation with restrictions on transport, breeding, selling and cultivation/breeding of number of species or release of invasive species captured.
Promoting biosecurity measures to reduce the introduction or spread of invasive mammals, as proposed by the GB NNSS.
Supporting programmes to reduce or mitigate the effects of established invasive mammals in Britain where this is carried with suitable concerted planning, such as catchment area removal of mink for a clear purpose such as water vole conservation, and where control techniques are considered humane.
Collaborative working with a range of individuals and organisations, including members of the public, game keepers and land managers, charities and statutory organisations.
Increasing awareness of mammals in general, by our members, the public and in the media, as well as encouraging clear, accurate and up-to-date sharing of information.
Definitions
There is often much confusion of what INNS are. The following definitions aim to provide clarity:
Native species – a species that originates from Britain since 6,100BCE when Britain became separated from mainland Europe.
Non-Native Species – are plant or animal (terrestrial, freshwater or marine) species that have moved from their place of origin by the assistance of humans, whether intentional or accidental. Non-native species in Britain have been brought here from overseas and it is estimated there are around 2000 species, but the number is increasing every year.
Naturalised species – are those that were introduced by people since 6,100BCE, but have been in Britain since before the 12th Century — so long that they are considered part of our fauna and ecosystems. In practice, most of these species were introduced in Norman times or earlier. Naturalised species can also be INNS.
Invasive Non-Native Species – INNS have also been moved from their place of origin and brought into Britain by humans but also have a negative impact on the environment, the economy and health. It is estimated that around 10-15% of non-native species are INNS, and as the number of non-native species is increasing every year, so is the number of INNS.
Invasive Alien Species – is the term used by the European Union (EU) for INNS.
Vagrant species – individual species that move outside their normal range are vagrants. This includes mammals that make their own way into Britain, such as bats or sea mammals, like the walrus (Odobenus rosmarus), as recorded in Wales and Cornwall in 2021. They are not classified as INNS because they have made their way here without the assistance of humans.
Why invasive mammals are important
Invasive mammals can have negative impacts on the environment, the economy and human health:
Environmental impacts – predation on native species e.g. the American mink (Neovison vison) on water voles (Arvicola amphibius) or the spread of disease e.g. American grey squirrel (Sciurus carolinensis) spreading the squirrel pox virus to native red squirrels (Sciurus vulgaris).
Economic impacts – damage to forestry e.g. by deer grazing, or squirrel bark damage.
Human health – causing illness or injury to humans e.g. the spread of disease by rats or road traffic accidents caused by collision with deer.
Invasive mammals may have such a negative impact because they often have no natural predators or competitors, are highly mobile, have a high reproductive rate and are generalists with a range of food sources.
Introduction pathways
Mammals have been introduced to Britain in different ways. This may include: deliberate introductions, e.g. European rabbits for food; American mink for fur; deer for hunting or accidental introductions, e.g. house mice (Mus musculus) as stowaways on boats. Pet and zoo escapes or the deliberate release of captive mammals are key risks of introduction into the wild e.g. edible dormouse (Glis glis) or American mink.
Legislation
The key pieces of legislation relevant to invasive mammals are:
The Wildlife and Countryside Act 1981 (section 14(1)) makes it an offence to release or allow to escape into the wild any animal which is not ordinarily resident in Britain and is not a regular visitor to Britain in a wild state or is listed in Schedule 9 to the Act.
The EU Regulation (1141/2014) on invasive alien (non-native) species, which has been retained in UK law through The Invasive Non-native Species (Amendment etc.) (EU Exit) Regulations 2019, lists the following mammals that cannot be imported, bred, bought or sold, or transported: small Asian mongoose (Herpestes javanicus), muntjac deer (Muntiacus reevesii), coypu (Myocastor coypus), coati (Nasua nasua), raccoon dog (Nyctereutes procyonoides), muskrat (Ondatra zibethicus), raccoon (Procyon lotor), grey squirrel, fox squirrel (Sciurus niger), Siberian chipmunk (Tamias sibiricus).
Biosecurity
Biosecurity involves a range of measures that can be taken to prevent the introduction of new or spread of existing INNS. Legislation provides restrictions on transport, breeding, selling and cultivation/breeding of a number of species. The GB Non-Native Species Secretariat (GB NNSS) is producing a series of Pathway Action Plans including the UK Non-Native Species Pathway Action Plan: Zoos. The GB NNSS Great Britain Non-Native Species Strategy (2015) advocates a three-stage process:
Prevention – preventing the introduction and establishment of new INNS is the most effective approach and a high priority in Britain.
Early detection and rapid response – requiring accurate and up-to-date information, including surveys and monitoring, to determine management options and if eradication is possible.
Long term management and control – determining if management is possible or feasible including large scale eradication, containment, control or mitigation.
Key Invasive Mammals in Britain
There is often confusion over what is a native or invasive non-native mammal, for example many people consider grey squirrels, rats and rabbits as native. Although many of these species are considered naturalised and have been present in Britain a long time, they were introduced by humans and have one the most damaging economic impacts.
Key invasive mammals and their impacts are outlined below:
Rodents (Rodentia)
Rodents are among the most abundant British mammals and four of them are considered invasive:
Grey squirrels have rapidly expanded their range following their introduction in the 1870s. Although a popular mammal to many, as easily seen, they are considered a significant pest, causing damage to forestry and predating on eggs and chicks. They outcompete native red squirrels for food resources and rest spaces and carry squirrel pox virus which is fatal to our native squirrel. Grey squirrel control is carried out for forestry protection, by game keepers and for the conservation of red squirrels.
Edible dormice were deliberately introduced into the wild in 1902 in Tring and have expanded into almost all of the Chilterns woodland area and beyond. Various monitoring and control programmes have been carried out to review impacts on native wildlife and to protect damage to trees and as it is considered by many as a pest in homes.
House mice are widespread in Britain. They have been present since Neolithic times and are closely associated with humans, arriving as stowaways with goods on boats. They are common in urban and rural areas, are pests to crops and in dwellings or storage areas and may be controlled on a small scale.
Brown rats (Rattus norvegicus) are highly adaptable and widespread in Britain and arrived as stowaways in boats in the 1720s. Rats are generally not a popular mammal and have a wide range of impacts including spreading disease, preying on native species, contaminating stored food or damaging crops. Rats have long been subject to control programmes on small and larger scales.
Black rats (Rattus rattus) were widespread in Britain after being introduced probably by the Romans, but are now thought likely to be extinct. There are no known populations of black rats as they were out competed by the larger brown rats and due to control. Former island populations including Lundy Island (2006) and the Shiant Isles (2018), were eradicated for seabird conservation. There are occasional reports near ports and dockyards, but this could be misidentification. In the absence of a systematic survey, they can not yet be classified as extinct.
Deer (Artiodactyla)
Four of the six deer found in Britain are invasive and all can impact on forestry, be involved with road traffic accidents and be subject to some form of control. The Mammal Society’s Review of the Population and Conservation Status of British Mammals (2018) indicates that all deer are increasing in population size and range.
Fallow deer (Dama dama) were deliberately released into the wild for hunting by the Normans and are fairly widespread across Britain. Woodland habitat can be significantly impacted on and they are frequently involved in road traffic accidents.
Sika deer (Cervus nippon) have been present in Britain since 1860 and have a patchy but expanding distribution across Britain. Sika deer can cause damage to forestry and crops but also have a negative impact through hybridisation with the native red deer.
Muntjac (Muntiacus reevesi), also known as Reeves’ muntjac, were deliberate or accidental releases after being brought here for private collections in the 1830s. It has a patchy presence in Britain, being more common in England, sparse in Wales and with only a few records in Scotland. It has impacts on biodiversity, forestry and can be involved in road traffic accidents.
Chinese water deer (Hydropotes inermis) became established in the wild in the 1940s from private collections. The population is predominantly limited to the east of England but has shown some expansion and, as their name suggests, show a preference for wetland habitats, e.g. along rivers or reedbeds. Britain forms a high proportion of the global total as they are declining in their native countries. Chinese water deer can cause damage to agricultural crops.
Carnivores (Carnivora)
American mink were brought into Britain for fur farms in the 1920s and escaped or have been deliberated released into the wild. Mink are widespread but appear to be abundant in some areas and scarce and declining in others. Mink are semi-aquatic with a wide range of prey including water fowl. They are best known for their link to the devasting decline of water voles. Large and smaller scale mink control programmes are carried out for water voles and bird conservation by various organisations.
Rabbits (Lagomorphs)
European rabbits were introduced by the Romans for food and fur and, although considered naturalised and are important in our ecosystem, they are an invasive species with significant impacts on agriculture, horticulture and forestry. Rabbits are widespread in Britain but have been impacted by myxomatosis and rabbit haemorrhagic disease on a large scale and are controlled by shooting and ferreting on a smaller scale. The population in Britain appears to be declining and rabbits in Wales are classed as near threatened and in Scotland as vulnerable by the Mammal Soceity review. Internationally rabbits are classed as ‘near threatened’ by the International Union for the Conservation of Nature.
Island invasives
INNS can have a greater impact on islands as native island species, especially endemic species, are more vulnerable. Native island species can be small, localised or highly specialised and easy food sources for adaptable invasive species.
Invasive mammals can include those that are native to mainland Britain but have been introduced (accidentally or intentionally) to an island such as hedgehogs (Erinaceus europaeus) stoats (Mustela erminea) and rats (Rattus norvegicus). Control programmes on islands or removal (especially of hedgehogs) is often done for bird conservation.
Domestic mammals
Domesticated or tamed animals that live with humans can also have an impact on native species:
Domestic cats (Felis catus) are now the single biggest threat to native wildcats (Felis silvestris). Wildcats can successfully hybridise with domestic cats, which threatens the genetic purity of the species. In Britain, wildcats are only found in Scotland, but they have suffered a dramatic decline and are extremely rare. Hybridisation was probably not the initial cause of this decline but hampers their recovery.
Ferrets (Mustela putorius fero), often called domesticated polecats, can breed with native polecats (Mustela putorius) which are currently recovering from a former population crash. Hybridized animals have poorer survival rates and can be difficult to distinguish between without the aid of genetic testing.
Eradicated invasive mammals
There are some unusual examples of other invasive mammals in Britain such as coypu (Myocastor coypus), muskrat (Ondatra zibethicus) that were present, but have been successfully eradicated.
Coypu were introduced for fur farming in the 1930s and there was a significant population in East Anglia. Coypu are semi-aquatic, impacting on waterways by causing erosion and damaging flood defence and impacting on crops. Copyu were successfully eradicated by 1989.
Muskrats were also introduced for fur farming and are semi aquatic causing similar problems as the coypu. They have also been successfully eradicated.
Positive aspects of invasive mammals
Some invasive mammals can easily be seen, are much loved by the public with positive social impacts, especially in urban areas e.g. the grey squirrel, and are a key species that people engage with. In a limited number of cases invasive mammals can have a positive impact on our environment, for example European rabbits; they are ecosystem engineers that have an important grazing role e.g. on heath lands and are an important prey species for several mammalian and avian predators. In some areas, especially in Scotland, deer stalking brings large economic gains. A limited number of people believe that new invasive species should be welcome as they increase biodiversity; but this is not the view of the Mammal Society.
Horizon scanning
Horizon scanning was commissioned by the GB NNSS to identify the most threatening potential INNS that could become a problem in Britain. The Mammal Society participated in these exercises and examined pathways of spread to the wild, adaptability and impact. Of 243 species reviewed, two mammals were included as high risk in the top 20 list. Both mammals can impact on biodiversity, the environment and act as reservoirs for disease, with potential impacts on human health. The two mammals identified as high risk were:
Racoon (Procyon lotor) – Urban and potential agricultural pest and competitor and predator of native species. Adaptable and most likely to escape from captive collections.
Racoon dog (Nyctereutes procyonoides) – Reports of these mammals as exotic pets in the news in 2019, but they are not suitable pets and, while existing owners can keep these animals, further breeding or sale is now banned. Predator of birds and amphibians and competes with other carnivores. Adaptable and most likely to escape from captive collections.
References
Atlas of the Mammals of Great Britain and Northern Ireland (2020), to inform good management. MATHEWS, F., KUBASIEWICZ, L. M., GURNELL, J., HARROWER, C. A., MCDONALD, R. A. & SHORE, R. F. 2018. A review of the population and conservation status of British mammals. A report by the Mammal Society under contract to Natural England, Natural Resources Wales and Scottish Natural Heritage. Natural England, Peterborough. ISBN 978-1-78354-494-
Horizon-scanning for invasive alien species with the potential to threaten biodiversity and ecosystems, human health and economies in Britain Helen E. Roy*, Jodey Peyton, Stephanie Rorke UK Centre for Ecology & Hydrology, Wallingford, OX10 8BB, UK
Great Britain Non-Native Species Strategy 2015, GB Non-Native Species Secretariat
Useful links
GB NNSS http://www.nonnativespecies.org
Mammal Society https://www.mammal.org.uk