While some sites class any species which wasn't found in Britain when it was cut off from Europe, here we class any which were introduced by human help, and not those which made it here naturally. A category for all species that are not native to Britain, but are living in the countryside as a result of introduction. The Grey Squirrel is a famous example.

Some species have had a major impact on other wildlife (such as the American Mink and Himalayan Balsam), some have little apparent impact, but still do impact native wildlife (such as the Light Brown Apple Moth), and others have become so established in Britain, many don't realise they are aliens. (such as the Rabbit).

Species such as the Fallow Deer have been in Britain for hundreds of years, but others have only recently been introduced to Britain (such as the Western Green Lizard and Western Conifir Seed Bug).

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