Nature & Environment Index Page

CAITHNESS BIODIVERSITY GROUP
November 2005

The Distribution of the Water Vole in Caithness
E Fraser, D Glass and S Hogg

Results

A total of 34 sites were surveyed, 19 (56%) of which were found to be occupied with water voles and the remaining 15 (44%) unoccupied. Occupied sites were found in 11 (64.7%) of the 17 10 km squares chosen for surveying. Table 1 shows the location of all survey sites along with the 10 km square that each survey site was located in and the extent of area surveyed at each site. Figure 3 shows the locations of all surveyed sites, as numbered in Table 1.

The survey sites were classified, in terms of habitat type, using the Caithness Habitat Map from the Caithness Biodiversity Action Plan (February 2003). The surveyed sites covered 4 main habitat types, bog and moorland (from now on referred to as peatland), farm and croftland, forest and woodland and urban and unclassified. In total, 44.1% (15/34) of the surveyed sites were peatland, 41.2% (14/34) were farm and croftland, 11.8% (4/34) were forest and woodland and 2.9% (1/34) were urban and unclassified (Figures 4 and 5). Of the occupied sites, 47.4% (9/19) were peatland, 36.8% (7/19) were farm and croftland and 15.8% (3/19) were forest and woodland. Forty percent (6/15) of unoccupied sites were in peatland, 46.7% (7/15) were farm and croftland, 6.7% (1/15) were forest and woodland and 6.7% (1/15) were urban and unclassified. In one case, a site was initially determined as unoccupied but was re-surveyed 3 weeks later and was found then to be occupied. This occurred at Loch Hollistan which has been classified as occupied in the analysis.

All occupied sites had a defined flow of water, such as a burn or ditch, except Newtonhill Community Woodland. Here, signs of occupancy were found in a marshy area with standing water. Bank vegetation was varied. In general, peatland areas had a high level of vegetation cover which was commonly a mix of tall grasses, sedges and rushes, for example Allt na Craoibhe (Figure 6). In areas of farm and croftland, where grazing had been allowed up to the water’s edge, vegetation was short with discrete tussocks of rushes or sedges (as found at the Little River, Tacher). In other areas of farm and croftland, grazing was not permitted near the water because of fencing. This had led to the area becoming densely overgrown, with plants other than grasses and rushes tending to dominate. An example of this is the area surveyed on Thrumster Estate. In forest and woodland areas, suitable habitat and positive occupancy were recorded in open areas where growth of ground vegetation was not restricted by tree cover, for example in Achairn forest. There was no positive occupancy in areas categorised as urban and unclassified. For a list of example plant species found in suitable and unsuitable habitat see Table 2.

This variety of vegetation found at occupied sites led to a further classification which divided suitable habitat. ‘Typical’ habitat had good vegetation cover of grasses, sedges and rushes, with few shrubby plants e.g. Allt na Craoibhe or Achairn Forest. ‘Poor’ habitat had adequate vegetation cover which was generally tall, but overgrown and dominated by plants such as meadow sweet, thistles and nettles. Grasses and rushes were present but commonly only near to the water’s edges. A higher proportion of stones/rocks in the water or intermittently on the banks was also characteristic of ‘poor’ habitat e.g. Thrumster estate or Achsinegar (Table 3). It should, however, also be noted that in positively occupied sites, the habitat was generally not continuously suitable across the complete length of the survey.

Unoccupied areas were also further classified in to 2 groups: sites that had suitable habitat but were unoccupied and sites that were unsuitable. Suitable habitat, such as Loch Garbh and Loch Saorach outlet burns, followed the same description as above. Habitat was classed as unsuitable if the river banks were continuously rocky, solid or high e.g. Thurso river and Dunbeath water, water was fast flowing, there was a permanently high water table e.g. Broubster Leans, and vegetation cover was minimal e.g. Rumster forest, and/or dominated by woody or shrubby plants and bushes such as heather or gorse (Figure 7).

Occupied sites were 73.7% (14/19) typical and 26.3% (5/19) poor habitat. Suitable habitat was also found at 53.3% (8/15) of unoccupied sites. At the remaining 46.8% (7/15) of unoccupied sites, habitat was unsuitable (Figures 8 and 9).

Estimates of abundance have, in the past, been predicted using a correlation between number of latrines and total population of adults and juveniles (3); however, this correlation may not be valid in all habitats, particularly if water vole populations are small and fragmented (3). Estimates of abundance have therefore not been attempted but instead each occupied site was assigned a crude level of occupancy according to the quantity and distribution of signs that were found. There were 3 categories: low, medium and high. Sites classified as having low occupancy were those where only a few, isolated signs were found, for example a couple of droppings up to 3 latrines. Medium occupancy was used to describe sites where signs were numerous but had a patchy distribution. High occupancy was determined by a continuous stretch of signs over a distance of at least 100m. The majority (57.9%) of occupied sites had a medium level of occupancy. 31.6% of occupied sites had a low level of occupancy and 10.5% were highly occupied (Figure 10).

Loch Burn

Habitat/

Suitable

plant group

‘Typical’

‘Poor’

Unsuitable

Plant group

peatland