Caithness CWS - Nature and environement - highland Biodiersity - Watervole Survey

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 modification.

In general, a wide distribution of water voles was found across the county of Caithness, showing presence in 65% of the 10km squares surveyed. The highest number of occupied sites was found in peatland habitat. Although this was only slightly higher than the number of occupied sites in farm and croftland, it is important to note that the proportion of suitable, typical habitat in peatlands far exceeds that in farm and croftland. In addition, a minority (1 out of 9) of occupied peatland sites had a low level of occupancy compared with the majority (5 out of 7) of farm and croftland sites. This preference for peatland habitat is not uncommon in the Scottish uplands, for example in Assynt.

As described in the results section, each positively occupied survey site was assigned an occupancy level depending on the quantity and frequency of signs. The distribution of sites that have high, medium and low occupancy is very interesting, particularly in reference to the way water vole populations are thought to function (i.e. the metapopulation theory). Generally, in metapopulations, there are one or more core populations that are either stable or expanding in numbers, surrounded by smaller populations which are maintained by the larger ones through dispersal. Small and isolated populations can, however, become extinct if they are not ‘replenished’ by dispersers from nearby populations. It can be seen from Figure 10 that sites with a high level of occupancy are fairly centralised with several occupied sites in the surrounding area. A possible exception to this is Allt na Craoibhe where evidence of occupancy is lacking to the west of the site. However, it can also been seen from Figure 10 that there were no survey sites in this area, and so the presence of further occupied sites cannot be ruled out. For example, there are previous records of water vole occupancy at Raffin Burn (near Dunbeath Water) (9) which was not re-surveyed in this project. Surveyed sites with low occupancy were generally found in outlying areas and/or where habitat suitability occurred only in a small patch. The 3 water vole populations along the north coast of the county (found at Loch Hollistan, River Forss and Burn of Rattar) are the most vulnerable to extinction because of their isolation. Of the sites surveyed nearby, few were occupied meaning that dispersal in to these populations could be limited. Nevertheless, it must be remembered that water voles could disperse from Sutherland, as may have already occurred during the project (at Loch Hollistan). The remaining 3 low occupancy sites (Little River, Achsinegar and Burn of Latheronwheel) are not as isolated but their small populations are most probably explained by the limited expanse of suitable habitat available at these sites. Water voles have been found to inhabit sites with relatively poor habitat, while areas of good habitat remain unoccupied. It has been suggested that where water voles are living at high population densities, some individuals may disperse to smaller, poorer areas of habitat nearby (2). This could explain the small population sizes found at Little River and Burn of Latheronwheel which were adjacent to the highest occupied areas. The results suggest that the area of Caithness is well populated; however estimates of abundance would have to be verified using techniques such as capture-mark-recapture.

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.