In its prime, the Thames seems an open book. But off the main river it still has plenty of hidden places colonised by Canada geese and another bird introduced for its looks. Secluded backwaters are ideal duck domain, and one tree-dwelling foreigner is multiplying fast here - the exotic mandarin duck. It's probably doing better here than in its native China. But not all foreign imports are so harmless. The American mink was brought here to be farmed for its fur, but it soon absconded and has brought a reign of terror to the riverbanks. It's just a fifth the size of an otter but a much less fussy eater. If eel or fish are scarce, mink simply go further afield in search of rabbits and other small mammals. Mink now outnumber otters on the Thames by more than 100 to 1. Cute as they look, mink are born to kill, and they have decimated Wind in the Willows country. What would Kenneth Grahame have written about today? 90% of Britain's water voles have disappeared in the last few decades, leaving their empty riverbank tunnels as hollow reminders of what used to be.