Anne McElvoy talks to the biologist Ken Thompson who dismisses attempts to control invasive species and questions the veracity of dividing plants and animals into 'native' and 'alien'. However the Director of the Kew Innovation Unit Monique Simmonds warns that alien pests and diseases can have a devastating effect on much-loved plants, and that it's vital to maintain and support diverse environments. The farmer John Lewis-Stempel records the passage of the seasons in his account of the life of an English meadow and he laments the decline of some of his favourite birds from his childhood. The woolly mammoth used to be native in Europe before it became extinct, and the palaeontologist Victoria Herridge confounds expectations by identifying the smallest mammoth ever known to have lived. Producer: Katy Hickman.