Plot: Adam Henson and Ellie Harrison head for Lough Erne in Northern Ireland where they find out about the fight against the zebra mussel, a foreign invader threatening the future of the marine environment, and help a farmer move his rare-breed pigs to their summer grazing on an island in the middle of the Lough. John Craven investigates why the number of pollinating insects is declining at such an alarming rate and what is being done to reverse the situation