Some of Ireland\'s oldest mammalian inhabitants are facing serious threats and possible extinction because of foreign species, researchers say. Indigenous species including red squirrel, Irish hare and red deer are among the species under threat from introduced foreign species, researchers at Queen\'s University Belfast said. They spent two years looking at the impact of two introduced species -- the bank vole and greater white toothed shrew -- on two native small animals, the wood mouse and the pygmy shrew, a university release said. The found the pygmy shrew has completely vanished in parts of Ireland where both invasive small mammals are found, while wood mouse numbers have decreased by more than 50 percent in areas where the bank vole is established. \"The introduction of alien mammals to Ireland over the last 100 years has had major detrimental effects, threatening our indigenous habitats and species,\" researcher Ian Montgomery said. \"The American grey squirrel, for example, passes a deadly virus to native red squirrels whilst European hares threaten the ecological and genetic integrity of the native Irish hare through competition and interbreeding.\" The cumulative effects of invasive mammal species on indigenous species has been called \"invasional meltdown.\" \"It is no longer tenable to treat each invasive species as an isolated case,\" Montgomery said. \"We should establish a realistic plan identifying the mammal species that are key to maintaining our unique biodiversity and ecology and those that we should eliminate or control.\"