Independent: Lord Howe, an idyllic island off the Australian mainland, carefully conserves its natural treasures. The World Heritage-listed chunk of rock has strict quarantine laws, and limits the number of tourists who may visit. But its unique birds, insects and plants are under threat from an implacable foe: the black rat. Accidentally introduced in 1918 when a ship ran aground, rats are blamed for the extinction of five endemic bird species. Wildlife experts warn that 13 other native birds, …