Introduced species of the British Isles


Islands, such as the British Isles, can be adversely affected by the introduction of non-native species. Often an island will have several distinct species not present on the nearest mainland, and vice versa. The native flora and fauna of islands which have been isolated for a longer period of time such as New Zealand or Hawaii are more vulnerable than islands such as Great Britain and Ireland, which became isolated more recently.
Many species have been introduced to Britain during historical times. Some species such as the midwife toad, rainbow trout, sweet chestnut and horseradish have been introduced with few adverse consequences. However, others such as the grey squirrel, signal crayfish, and Japanese knotweed have had a severe impact both economically and ecologically.

Problems caused

Economics

In 2010 CABI estimated that introduced species in United Kingdom cost £2 billion annually. The most costly species were listed as being the European rabbit and Japanese knotweed. The European rabbit, introduced to Britain by the Romans in 1AD, eats and therefore damages a wide variety of crops and cost the UK £263 million. Japanese knotweed, introduced as an ornamental garden plant in the late 19th century, the roots of which spread by underground rhizomes, can undermine and damage buildings, pavements and roads, cost £179 million. In fact, most mortgage lenders in the UK will demand proof of the plant's eradication from a home owner's property, for it can cause potential physical damage to one's estate. In 2023, it was confirmed that the number of non-native plant species introduced to the UK outnumbered the number of native UK plant species

Ecology

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, which either escaped or were released from fur farms, prey on native European water voles and are drastically reducing their numbers. 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.
Resource competition: The introduced grey squirrel is larger and more aggressive than the native red squirrel and displaces the native squirrel by competing for food and habitat. Rose-ringed parakeet 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. These large parakeets displace native birds species by competing for roosts and nest sites.
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. The European crayfish is susceptible to crayfish plague which is spread by the introduced signal crayfish.
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, 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 but in 2012 a "giant rat" was killed in County Durham, and authorities suspected that the animal was, in fact, a coypu.

Management of introduced species

Some species have adapted harmoniously into the ecology of the British Isles. For example, the little owl is not native to the British Isles but was first introduced in 1842, by Thomas Powys and is now naturalised there. However, the presence of some introduced species has proved disastrous for native flora and fauna. There is often a link between how well a species can integrate with an existing ecosystem, and the distance from their local range; i.e. species sourced closer to the sink site tend to cause less damage.
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Information on for aquatic species can be found at the website.

Example: grey squirrel

One notable example of a species introduced to the British Isles is the grey squirrel from North America, which out-competes the smaller native red squirrel, as well as carrying a virus that is fatal to the reds. The cost of attempting eradication was reported in 2010 to be £14 million. These attempts have been deemed unsuccessful and priority is now being given to preserving the remaining red squirrel habitats.
The following is a partial list of introduced species. Species marked with a dagger are controlled by The Invasive Alien Species Order 2019, which is the latest legislation.

Vertebrates

Mammals

Birds

Reptiles

Amphibians

Fish

Invertebrates

Molluscs

Crustaceans

Insects

Ants

Beetles

Butterflies and moths

Adoxophyes orana summer fruit tortrix mothArgyresthia cupressella moth from United StatesArgyresthia cupressella moth from United StatesBlastobasis adustella mothBlastobasis lacticolella moth

Planarians

Two species that prey on earthworms:Arthurdendyus triangulatus - from New ZealandAustraloplana sanguinea - from Australia

Stick insects

Termites

Reticulitermes grassei - identified in 1994 in Saunton, Devon. Declared eradicated in 2021

Arachnids

Euscorpius flavicaudis invasive; probably originating from Italy; there is a thriving colony in Kent

Plants

Resources

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