Introduced species of the British Isles

Some species such as the midwife toad (Alytes), rainbow trout (Oncorhynchus mykiss), sweet chestnut (Castanea sativa) and horseradish (Armoracia rusticana) have been introduced with few adverse consequences.

However, others such as the 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.

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,[3] 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.

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.

[6] 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.

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.