Invasive species in 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 (which have been isolated for millions of years) are more vulnerable than islands such as Britain and Ireland, which became isolated more recently (8,000 years ago at the end of the last ice age).

Many species have been introduced to Britain during historical times. Some species such as the midwife toad (Alytes), rainbow trout (Oncorhynchus mykiss), sweet chestnut (Castanea sativa) and horseradish (Armoracia rusticana) have been introduced with no adverse consequences. However, others such as the eastern grey squirrel (Sciurus carolinensis), signal crayfish (Pacifastacus leniusculus), and Japanese knotweed (Fallopia japonica) have had a severe impact both economically and ecologically.

In 2010 CABI (Centre for Agricultural Bioscience International) 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 in the 12th century, 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(if signs of it being present are noticed), for it can cause potential physical damage to one's estate.

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 (Neovison vison), which either escaped or were released from fur farms, prey on native European water voles (Arvicola amphibius) 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.

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