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NATURE AND ENVIRONMENT
The Distribution of the Water
Vole in Caithness
One of the major contributors to
the demise of water voles across the country is the spread of American
mink. There have been sightings of mink as far north as Cape Wrath, but
only as far as Dornoch on the east coast. (8). There has been no evidence
of mink in Caithness and so the water vole populations that were surveyed
can be assumed to be unaffected by mink predation. In addition, habitat
loss, fragmentation and degradation have been recognised as important
contributors to the decline (4). It is worth noting that, particularly in
the peatlands, the habitat remains relatively undisturbed and spared from
modifications such as watercourse realignment, drainage and pollution.
This survey, therefore, represented an opportunity to obtain distribution
data on a water vole population that has not been affected by mink
predation or significant habitat alteration or loss. These characteristics
are shared with very few places in the UK, one of which is Assynt in
Sutherland. The water vole population in Assynt has been studied by
researchers at Aberdeen University since 1998 and provides vital evidence
on population dynamics in the absence of mink predation and habitat
It was noted that in farm and croftland sites with poor habitat, grazing by livestock was absent at the water’s edge. A strong negative correlation has been found across Britain between the presence of water voles and the density of sheep grazing (4). However, the growth of uncontrolled scrub and thick undergrowth can also lead to a reduction in favoured bank side vegetation (4). ‘Poor’ habitat in Caithness was characterised by the presence of thicket vegetation. This habitat could be improved by allowing a low level of grazing to control dominant vegetation and encourage growth of grasses and sedges. An example of the effects of grazing on vegetation was demonstrated at North Calder. In part of the surveyed area, the watercourse was fenced off from access by livestock. Further downstream livestock had unlimited access to the water’s edge. Although the habitat was potentially suitable on the whole, the ground was poached where livestock had been grazing. Alternatively, where grazing was prohibited, thistles and other scrub vegetation dominated. It is therefore recommended that in occupied areas of farm and croftland, grazing could be controlled but not eliminated, where necessary.
In relation to the metapopulation theory, it should be remembered that water vole populations are very dynamic and occupancy patterns are subject to change. In areas such as Torran Water, where the habitat was considered suitable, dispersal into this patch can be as likely as a populated patch becoming empty. Extinction–recolonisation events are common between years, but can happen at any time due to a large number of animals dispersing between patches (2). Recolonisation such as this was found to occur within the timescale of this project. Although a first survey of Loch Hollistan found no fresh signs, a resurvey three weeks later found a low level of occupancy. This nature of occupancy has also been recorded before. Ecologists at the University of Aberdeen examined 50 known water vole habitat patches in the Grampian Mountains and recorded 7 occupied and 43 empty patches on a first survey in 2003. A subsequent survey three weeks later, revealed that only five of the occupied patches still contained water voles and that two of the 43 empty patches had been recolonised (10). This indicated that the extinction-recolonisation dynamics of water voles can occur over very short timescales indeed. Loch Hollistan is very close to the border with Sutherland which, although not surveyed for this project, is known to have a thriving population, from which dispersal could have occurred. Considering the above, it is therefore crucial that future surveys monitor these patches of unoccupied habitat considered suitable in the original survey, especially Loch Garbh, Torran Water, Allt Chaiteag and Allt nam Sealbag.