The declining water vole

The water vole has been declining in Britain generally since about 1900, although in some areas, notably Wales, the decline had set in even before then. The early decline can be explained by the overgrazing resulting from an increasing sheep population. In recent decades this trend has accelerated and the water vole is now our most rapidly declining mammal. This has resulted in its inclusion as a priority species under the UK Biodiversity Action Plan and partial protection under the Wildlife and Countryside Act 1981 (as amended).

In most areas where water voles survive, numbers are low and many local extinctions are predicted to occur in the near future. The decline is correlated over much of the species’ British range with the spread of introduced American mink (Mustela vison) and there is abundant evidence that mink predation is a major cause of water vole mortality in many areas. However, the decline started well before mink became widely established, leading to the conclusion that habitat loss, fragmentation and degradation is also a problem. The mink may simply have been the last straw.

Principal threats

In many areas, particularly in the lowlands, habitat loss, degradation and fragmentation have, over many years, contributed to the decline. These effects have resulted from various forms of bad riparian (riverbank) management, such as over-zealous or ill-considered vegetation cutting and unsympathetic ‘hard’ river engineering techniques, including canalisation and culverting long stretches of watercourse. Flooding can also be detrimental to water voles, especially in areas where there are no safe refuges to where the animals can retreat when the river is in spate.

Photo: Cleared ditch

Unsympathetic ditch management that has destroyed all the available water vole habitat

Sarah Norman

In many lowland pastoral areas water vole habitat can be prone to damage by over-grazing, resulting in close-cropping of the bankside vegetation and poaching of the riverbank. Conversely, an absence of grazing or manual control of the bankside vegetation can also be equally detrimental over time. The growth of dense thickets and uncontrolled scrub leads to a decline in the bankside grasses, reeds, sedges and rushes so favoured by the voles. Similarly, the heavy shade cast by closely-growing bankside trees also inhibits the growth of these important food plants.

Across Britain there is a strong negative association across Britain between the occurrence of water voles and the intensity of sheep grazing. This is because sheep graze the vegetation much more closely than any other species of domestic livestock. However, in much of upland Scotland, while over-grazing by sheep, deer and feral goats continues to prevent tree regeneration in many areas, the current level of grazing pressure does not appear to be damaging to water voles.

Photo: American Mink

American mink

Terry Whittaker

Water voles are preyed upon by a range of native predators including stoats, weasels, otters, foxes, rats, various birds of prey and herons. However, unlike the American mink, none of these seem to influence water voles at the population level. Female mink are smaller than males and are easily capable of following the rodents into their burrows. As the two species are both usually found close to water, they are much more likely to encounter one another than are water voles or stoats and weasels. A female mink with kits to feed is potentially, therefore a major threat to any nearby water vole colonies.

The threat posed by mink applies over most of the water vole’s range, although perhaps fortunately for upland-dwelling voles, mink tend to be scarce in open moorland and mountainous terrain, seeming to prefer the greater cover and abundance of prey that is to be found in the lower-lying parts of many rivers.

Although perhaps difficult to comprehend, there is some evidence of direct persecution of water voles. Some water gardens and nurseries, fish farms and game fisheries have shot or poisoned water voles to prevent or reduce the damage that the voles can do to the banks of their watercourses and holding ponds. There have also been reports of children shooting water voles with air rifles ‘for fun’.

Other potential threats include localised poor water quality and the ill-considered use of rodenticides for rat control in riparian areas.

The result of all these factors is the fragmentation of water vole populations. Consequently, for many river systems, the pattern of vole distribution shows the species as absent from most of the main stem and the larger tributaries. It is mainly restricted to the headwaters and smaller tributaries in the upper or peripheral parts of the catchment.