Displacement of native species

In addition to the economic costs incurred by management, some introduced fauna displace native species. This can occur by predation, competition for resources, or the spread of disease.

Predation: American mink (Neogale vison), which either escaped or were released from fur farms, prey on native European water voles (Arvicola amphibius) and are drastically reducing their numbers.^[5] It was reported that since the late 1980s, 90% of the UK population of the European water vole has been lost, primarily due to displacement and predation by the American mink.^[6]

Resource competition: The introduced grey squirrel is larger and more aggressive than the native red squirrel (Sciurus vulgaris) and displaces the native squirrel by competing for food and habitat. Rose-ringed parakeet (Psittacula krameri) populations, originally an Afro-Asian parakeet, have become established in Britain from introduced and escaped birds. There are two main populations: the largest is based around south London, where they can be regularly seen in places such as Battersea Park, Richmond Park, and Greenwich Park; the smaller population can be seen in Surrey and Berkshire, and by 2005 consisted of many thousands of birds, known as the Kingston parakeets.^[7] These large parakeets displace native birds species by competing for roosts and nest sites.^[8]

Disease: Some introduced species carry diseases to which native species are susceptible. The grey squirrel is a carrier of the squirrel pox virus which kills red squirrels but not grey squirrels.^[9] The European crayfish is susceptible to crayfish plague which is spread by the introduced signal crayfish.^[10]

Unlike some other environmental problems such as pollution, the effect of an introduced species is not a single event. Once a species has been introduced to an island, the problems may persist and escalate as the species spreads further.

Environmental damage

Coypus (Myocastor coypus), large semi-aquatic rodents native to South America, were introduced to the British Isles in 1929 when fur farms were set up in Sussex, Hampshire, Devon and Norfolk. The farms were sited mainly in lowland areas rich in rivers and streams. During the 1930s coypus escaped from captivity and despite repeated attempts to control them, they adapted well to the British habitat, breeding successfully in the countryside of East Anglia. Their habit of building large burrows in river banks caused great erosion damage and threatened the tourism industry where boating is a popular recreation, and caused great damage to drainage works. Coypu were declared to have been successfully eradicated in December 1989,.^[11] but in 2012 a "giant rat" was killed in County Durham, and authorities suspected that the animal was, in fact, a coypu.^[12]

Butterflies and moths

• Large chequered skipper butterfly from continental Europe to Channel Islands (subsequently lost)
• Large copper butterfly (Lycaena dispar rutilus) from Continental Europe (subsequently lost)
• Large copper butterfly (Lycaena dispar batavus) from The Netherlands (subsequently lost)
• Geranium bronze butterfly from South Africa via Southern Europe on geranium (not established)
• Map butterfly (subsequently eradicated)
• Psychoides filicivora moth from the Far East
• Azalea leaf miner moth from east Asia
• Argyresthia cupressella moth from United States
• Brown house moth from Asia
• Tachystola acroxantha moth from Australia
• Coleotechnites piceaella moth from United States
• Cotoneaster webworm moth from United States
• Blastobasis adustella moth
• Blastobasis lacticolella moth
• Adoxophyes orana summer fruit tortrix moth
• Carnation tortrix
• Light brown apple moth (Epiphyas postvittana) from Australia
• Codling moth
• Horse-chestnut leaf miner
• Box tree moth (Cydalima perspectalis) from east Asia^[29]
• Common forest looper (Pseudocoremia suavis) a New Zealand endemic found west Cornwall in 2007, possibly not established.^[30]
• Oak processionary moth (Thaumetopoea processionea)^[31]
• Musotima nitidalis, from Australia and New Zealand and first found in Dorset in 2009. It has since spread across southern England to Essex and is thought to have been originally imported with tree ferns.
• Simacauda dicommatias, a South American moth found in Cornwall in 2020 but probably here, since the introduction of the larval food plant Chilean myrtle (Luma apiculata) over a hundred years ago).^[32]

Planarians

Two species that prey on earthworms:

• Arthurdendyus triangulatus - from New Zealand
• Australoplana sanguinea - from Australia

Stick insects

• Unarmed stick insect (Acanthoxyla inermis) from New Zealand
• Prickly stick insect (Acanthoxyla prasina) from New Zealand

Termites

• Reticulitermes grassei - identified in 1994 in Saunton, Devon. Declared eradicated in 2021^[33][34]