It seems that the marsh frog (Pelophylax ridibundus) was introduced to Britain in 1933 when twelve were intentionally released at Romney Marsh in Kent. I cannot find out why they were released but the fact does seem to have been documented at the time. Since then this colony has grown and spread and there have been further introductions and there is now a colony at at least one site in Dorset. Whether this was a result of natural colonisation or introduction I have no idea. It occupies a breeding sites not used by other native amphibians and can be found in dykes, ditches and fen so the reed beds of Dorset suit it well. The marsh frog, however, is a voracious predator and further introductions are forbidden under the Wildlife and Countryside Act 1981.
Marsh frogs are most noticeable by the "very loud, raucous sequence of duck-like quacks which build to a loud laugh-like cackle (ridibundus means laughing) heard day and night." [Britain's Reptiles and Amphibians - Howard Inns]. They like to bask on the edge of their chosen water supply and jump spectacularly into the water when alarmed.
My thanks to Ian Andrews for the 'tip-off' about marsh frogs at West Bexington and for his photograph.
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