The water vole is Britain's fastest declining mammal, disappearing from 70% of known sites in only seven years between national surveys in the late 1980s and early 1990s. In 1998 there were estimated to be only 875,000 individuals.
In pre-Roman times, the water vole was actually Britain's commonest small mammal. It has been estimated that in the late Iron Age there were 6.7 billion water voles in Britain, but that the development of agriculture in Britain around that time began a long history of decline. Nevertheless, in the 1900s, water voles were a common animal, familiar to everyone. Their decline in the 20th Century is attributable to two causes: degradation and loss of habitat, and predation by the American mink, which became naturalised in Britain from 1930 onwards. Where these declines have resulted in the local extinction of the vole population it is possible to reintroduce water voles as demonstrated by the Trust on the River Dore.
Genetic evidence suggests that water voles in Scotland are a distinct race of the species, probably resulting from a separate colonisation route, but they are threatened by the same two factors. Paradoxically, Arvicola terrestris is still a common species in much of continental Europe, where a field-living (fossorial) form exists alongside the riverbank (riparian) form that we know as the 'water vole'. It is unclear whether the riverbank form is on the decline in mainland Europe, but the field-living form reaches nuisance levels on occasion.
In Britain, habitat degradation and loss can be addressed through appropriate management which can be supported under current agri-environment schemes. There was no demonstrably effective solution to mink predation until 2002, when the GWCT introduced its GWCT Mink Raft. At a national level, strategy for the conservation of water voles in Britain is now developing continuously. Work by the GWCT and others has demonstrated the feasibility of mink control on progressively larger geographical scales.