Alien Species are species which have been introduced to an area where they do not naturally occur. So many species have been introduced to the UK with around 1,400 alien species currently recognised.
A few examples:
American Mink: Escaped from fur farms where they were bred for fur. Now they destroy river ecosystems and threaten our native species.
Grey Squirrel: Brought over from America, now threatens the native Red Squirrel due to it carrying a virus (which our natives aren't immune to) and the greys outcompeting the native reds.
We want to outline the alien species so that you are aware of what they are and what they are doing to our native ecosystems.