 Introduced predators are a threat to vulnerable island populations . In addition to the extirpation of the Icelandic race , mink have been responsible for marked declines in the populations of water rails and other ground @-@ nesting birds in the Hebrides , where the mainly fish @-@ eating otter was the only native carnivore . The mink derived from fur farms on Lewis , from whence they spread through Harris , North Uist and South Uist . Mink and ferret eradication programmes have enabled the rail to return to islands including Lewis and Harris , and further projects are ongoing or planned on the Scottish mainland . Locally , habitat may be affected by the drainage of marshes , canalisation of water courses and urban encroachment , or by pollution . 
