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About water voles

Introduction

The water vole, or 'water rat' as it is often mistakenly called, was formerly one our most familiar and abundant riverside mammals. Sadly, it is now one of our most threatened native mammals. The species has undergone a dramatic decline, particularly during the latter part of the twentieth century and voles have been lost from many areas where they were formerly common. The decline is correlated with the spread of introduced American mink (Neovison vison) and there is abundant evidence that mink predation is a major cause of water vole mortality in many areas. However, as the decline started well before mink became widely established, it is clear that habitat loss, fragmentation and degradation is also a problem. The mink may simply have been the last straw.

The water vole is the largest of the British voles with many Scottish animals having black fur rather than brown, which is more typical elsewhere.

Where do they live?

In lowland areas, water voles are now most likely to be found in small slow-flowing or static burns, backwaters, canals, ditches and overgrown field drains, sometimes in intensively-farmed and urban areas. These watercourses are usually less than 3 m wide and 1 m deep and do not show extreme fluctuations in water level. Water voles prefer sites with steep (>35º) or stepped bank profiles into which the vole can burrow and create nest chambers above the water table. Soft, easily excavated soils are preferred. The amount of bankside and emergent vegetation cover is very important, with the best sites offering a continuous swathe of tall and luxuriant riparian plants. Sites excessively shaded by shrubs or trees (>20% bankside tree cover) are less favoured.

In the uplands, voles are able to survive in narrow moorland burns on flat or gently sloping ground (average gradient no more than 3%)with extensive thick deposits of peat. Such conditions may be encountered near the headwaters of a river system or in the upper reaches of a glaciated valley where a small burn meanders across a high altitude marshy floodplain. Here, conditions are often suitable for stands of the water vole's preferred food, i.e rushes and sedges.

Principal threats

In many areas, particularly in the lowlands, habitat loss and fragmentation have contributed to the decline. These effects have resulted from damaging riparian management, such as over-zealous or ill-considered vegetation cutting and 'hard' river engineering techniques such as canalisation and culverting long stretches of watercourse.  In many lowland pastoral areas over-grazing can be problem, 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 be equally detrimental over time. The growth of dense thickets, uncontrolled scrub and trees leads to a decline in the bankside grasses, reeds, sedges and rushes so favoured by the voles.

Water voles are preyed upon by a range of native predators, but unlike the introduced 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 and, say, stoats or 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 in upland areas mink tend to be scarce or absent allowing the water voles to persist.

As a result of these pressures, in many river systems, water voles are now as absent from most of the main stem and the larger tributaries, being restricted to the headwaters and smaller tributaries in the upper or peripheral parts of the catchment.



Last updated on Monday 1st August 2016 at 14:59 PM. Click here to comment on this page