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Introduction This study was commissioned by the Caithness Biodiversity Group to determine the distribution of water vole, Arvicola terrestris, in Caithness. The European water vole is a UKBAP Priority Species. Although it is found throughout Britain it is one of our most threatened native mammals. The serious effects of predation by mink in the south, often accompanied by habitat loss or fragmentation, makes the future of water voles there uncertain. However, work in the north of Scotland (by the University of Aberdeen and others) indicates that this area might represent a vital stronghold for the species. Though found mainly in lowland areas in the south it has been found to be widespread in some upland areas in the north either where mink are not yet established or where they have difficulty in searching complex river catchments. Ecological background The water vole can generally be found along narrow waterways where there are soft beds and banks suitable for burrowing, as well as a high level of vegetation cover for feeding and protection. In many areas, where such suitable habitat exists, habitat fragmentation, whether it is naturally or anthropogenically influenced, has through time broken up their populations and resulted in severely reduced population sizes and ultimately a change to their distribution across the country. Such fragmentation leaves small populations of water voles extremely vulnerable to extinction. Water voles are known to disperse, about 2km on average, often moving through areas of unsuitable habitat in order to find suitable habitat and a mate (1). Although individually these populations may suffer high extinction risks, it has been suggested that the connecting regional processes of dispersal and recolonisation contribute to the long-term persistence of these patchy populations (2). Small populations are therefore considered to be equally as important as part of a larger collective population (3). Such a theory is commonly referred to as the metapopulation paradigm (Figure 1). Project background Caithness has been proposed for inclusion in the first tranche of Priority Areas where resources aimed at conserving water voles should be focussed (4). Water voles have been recorded in Caithness at a number of locations (5,6). However no systematic survey information on their distribution in the county existed prior to this study. Current evidence suggests that mink are not yet established in Caithness (although isolated sightings have been reported). This means that this area could represent a potential stronghold for water voles that might be defended against incursion by mink. For these reasons the Caithness Biodiversity Action Plan (7) identified an action to conduct a countywide survey of water voles and to develop a strategy for controlling mink. This project implements the first of these actions and forms part of the Highland BAP Implementation Programme. The overall aims of the project were:
Method Selection of survey locations The objective was to utilise the survey resource available (approximately 40 man days) to obtain the widest possible coverage of the county. Locations were selected according to the following criteria:
Landowner permissions were obtained for all locations surveyed. Deer cull restrictions meant that some 10 km squares in the south of the county could not be surveyed. Survey Methodology Suitability was determined by assessing the speed, depth, width and topography of the waterway as well as surrounding vegetation. Water voles inhabit burrows near still or slow flowing, reasonably shallow waterways that have soft banks with few rocks and stones. The vegetation usually consists of long grasses, sedges and rushes. GPS points were taken to mark the start and finish of suitability and to record the total area surveyed for future reference. It is however a fundamental feature of metapopulations that not all suitable habitats will be occupied. Occupation is indicated by the presence of recently made signs. Burrows are obvious but since they can persist for years in upland areas, more recent signs were required. Fresh latrines (droppings are usually about 8-12mm long and 4-5mm wide), footprints, vegetation clippings and runways determined whether an area was occupied since it is rare to actually see a water vole when surveying (Figure 2). It was not uncommon to find that field voles were sharing or were solely occupying suitable habitat hence it was important to distinguish between such signs. In comparison to the water vole signs, field voles have smaller and narrower faeces and leave much shorter vegetation clippings. Any signs of predators such as otter or stoat were also noted. (a) (c)
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(b)
(d)