Alien species are of considerable conservation concern because of the impact some of them have on other fauna and flora. Among mammals in Britain, Grey Squirrels Sciurus carolinensis, American Mink Neovison vison and Reeve's Muntjac Muntiacus reevesi are major problem species. Some aliens, though, seem relatively benign, and they do not always become pests. The Red-necked Wallaby Macropus ruforiseus seems, in Europe at least, to be one of these.