Britain's post-glacial mammal fauna is relatively improverished because of our island status. The number of species present has waxed and waned at the hands of man for over a thousand years. Extinctions such as the Aurochs, Bos primigenius, Wolf, Canis lupus, Brown Bear, Ursus arctos, and European Beaver, Castor fiber, have been replaced by the introduction into the wild (accidental or otherwise) of many non-native mammals.