Should we celebrate the arrival of new species to the UK or drive them out? Tom Heap and Helen Czerski investigate the role of alien invasive species in the British countryside and beyond. Tom and Helen hear about non native earthworms invading North America, posing a threat to forests by changing the soil. They discuss whether it's now time to be less judgmental about alien invasive species with Professor Chris Thomas, an expert in Anthropocene Biodiversity at the University of York. Rare Earth meets conservationists 'holding the line' against Grey Squirrels on Tayside, to give Red Squirrels a chance to re-establish. Ecologist Conor McKinney shares his experiences of trying to keep the remote St Kilda archepelago, off Scotland, free of invasive predators which could decimate the islands' globally important seabird breeding colonies. Produced by Sarah Swadling for BBC Audio Bristol in conjunction with the Open University