Quinag Wildlife Project – Water voles, and other mammals
Eilidh Summers and Ian M. Evans
Water voles, Arvicola amphibius, are the largest of the three voles found locally, weighing up to 300gms, but despite their size only live for up to a year and a half. They belong to a Scottish race, with glossy black fur, thought to be ‘descended from migrants from northern Iberia’ (Fig. 1). In contrast, those that colonised England and Wales following the last Ice Age were from South East Europe and have chestnut-brown fur.
Water voles suffered a catastrophic decline in numbers elsewhere in Britain during the 20th century. However they are thriving in Assynt, being widespread in the eastern half of the parish, with scattered outliers elsewhere. They are found on slow-running watercourses with beds and grassy banks that are suitable for burrowing into (Fig. 2); locally these conditions are found in small patches within a largely heather-dominated landscape which they avoid.
Assynt provides ideal conditions for studying water vole populations and research was initiated in 1997 by Xavier Lambin of the University of Aberdeen. IME had the pleasure of meeting him in Gleann Leireag on 25th April of that year. A continuing and very productive research programme has developed since, led by Xavier, who is now Professor of Ecology at Aberdeen. This research has, incidentally, provided a lot of data on other mammals that occur on Quinag.
An excellent general account of work up to 2018 is contained in a report in a Research Bulletin by Emma Bryce (view here), which contains distribution maps for water vole sites across most of Assynt (Fig. 3).
One of four main sites for this research has been the catchment of the Allt Sgiathaig on Quinag. Following a conversation in October 2020, Xavier and his staff at Aberdeen kindly made available copies of some of the raw data from this research, which ES has processed.
The live trap data, for 2015-2020 (view here), amount to 1352 lines (including repeats for the two commonest), with unique location records for eight species: common shrew (4), pygmy shrew (1), water shrew (3); field vole (168),water vole (715); wood mouse (1); stoat (1) and weasel (5).
The camera trap data for 2017-2019 (view here), amount to some 6402 lines (including, predictably, many repeats) with unique location records for 11 species: badger (6), fox (8), otter (16), stoat (1) and weasel (16); field vole (8) andwater vole (23); wood mouse (1); mole (1) and water shrew (8); mountain hare (1) and rabbit (1); red deer (23); there are also images of frogs (2) and ‘slugs’ (2), presumably the large black slug Arion ater.
Xavier’s research team also provided us with four key papers, which are summarised below.
1. Bryce, R., van der Wal, R., Mitchell, R. and Lambin, X., 2013. Metapopulation Dynamics of a Burrowing Herbivore Drive Spatio-temporal Dynamics of Riparian Plant Communities. Ecosystems 16, 1165–1177. https://doi.org/10.1007/s10021-013-9677-9 [please note this links takes you to an external site]
Water voles, Arvicola amphibius, are rodents that visibly impact riparian plant communities by grazing on surface and root vegetation and excavating long-lasting burrow systems. This species has a metapopulation structure and occurs across patches which are subject to frequent extinction and colonization events, causing spatially heterogeneous disturbances across the landscape.
Using a chronosequence of water vole occupancy in the Highlands of Scotland, we show that heterogeneity in plant community composition and structure, both within and between colony patches, was related to cumulative measures of past physical impact: burrow density and time since a patch was last occupied by voles, rather than to current indices of vole occupancy.
In our sample of 107 patches [in Assynt] monitored over 5 years, no fewer than 31 unique patch occupancy histories were found, each with potentially subtle differences in the accumulated influence of water vole herbivory and engineering. As a result, a patchwork of different plant successional stages occurs across the riparian landscape, which is both created and maintained by water vole extinction-colonization dynamics. We propose that the water vole vegetation system can be described as a metacommunity where dispersal by a higher tropic agent at the landscape scale influences the spatial dynamics of plants at the patch level.
2. Roos, D., 2016. Environmental legacy of water vole (Arvicola amphibius): An ecosystem engineering species affecting spatio-temporal patterns of nitrogen in a nitrogen poor environment. [Unpublished Honours Research Project Report, School of Biological Sciences, University of Aberdeen; view here]
This study is the only one known that explores the impact of burrowing ecosystem engineers within a water-saturated system. Traditionally, work that has been carried out on burrowing ecosystem engineers has focused on species in arid conditions whose engineering effects amount to the movement of soil from below ground to the surface. The study shows that ecosystem engineering effects can be much more subtle, involve influencing the nitrogen cycle around the burrow networks and are unconnected to the movement of soil to the surface.
The most likely mechanism by which water vole engineering influences the nitrogen cycle is through the aeration of soil, increasing soil microbe activity, and as a result, increasing the available nitrates for plants. Further potential to affect leaching, resulting in highly concentrated, localised areas of nitrates, or alternatively expulsion from the system, is an exciting possibility but requires further research to answer. Certainly the influence on the nitrogen cycle is responsible for plant heterogeneity found in the nitrogen-limited environment identified by Bryce et al. (2013).
Not only has been it been demonstrated that water voles act as ecosystem engineers, but it has also been shown that the engineering effect has a temporal component. Since water voles in the Highlands of Scotland show classic metapopulation dynamics, this means that water vole patches of occupation do not have fixed, binary effects of ecosystem engineering, but rather dynamic effects dependent on length of occupancy.
The interaction between soil microbes, neighbouring burrow proximity and time lags in nitrogen components suggests that the time required for the extinction debt to be reached may be substantial. Above all, this highlights the need to protect the species, but also that small mammals may play an increasingly important role in nutrient cycling within environments with limited nutrients.
3. Curley, V., 2020. When Weasels Eat Water Voles: Investigating The Impact of Bite-Sized Predators on an Upland Scottish Arvicola amphibius Metapopulation
[Unpublished Honours Research Project Report, School of Biological Sciences, University of Aberdeen; view here].
1.The role of specialist small mustelid predators in driving the population dynamics of small mammal species has been extensively studied, with research predominantly focusing on small Microtus voles as prey species. Comparatively less work has been conducted on how these predators impact the larger microtine rodent the water vole (Arvicola amphibius) which, in upland Scotland, exists as a metapopulation, undergoing frequent extinction and re-colonization events. Small-scale spatially correlated extinctions have been observed, and at a scale that is consistent with patterns of small mustelid predation. Despite this, investigations into these potentially related processes have not been carried out.
2.The purpose of this study was to investigate the role of the native small mustelid the common weasel (Mustela nivalis) in causing the spatially correlated extinctions in an upland Scottish A. amphibius metapopulation [in Assynt]. It was explored whether patterns of local extinction in A. amphibius could be predicted by evidence of occurrence of M. nivalis. Additionally, the temporal scale over which M. nivalis was perceived to cause extinctions of A. amphibius populations was investigated.
3.Detection histories of A. amphibius and M. nivalis from camera-trap images (Fig. 4) and parameter estimates from multi-season occupancy models (which account for imperfect detection) were used to explore the effect of weasel detections on water vole extinction probability. This was investigated at the local patch level and the neighbourhood level (circa 0.89km) and effects of other important covariates such as patch size and connectivity were considered.
4.The study provided only minimal support for the hypothesis that M. nivalis predation is causing the small-scale spatially correlated extinctions in Scottish upland water vole populations. More extensive research is required as, given the current status of A. amphibius in the UK, it is important to fully understand these patterns of population extinction.
[The survey area in Assynt comprised four blocks, two of which were on or immediately adjacent to Quinag. Of 130,423 images captured on camera-trap during 2017-2019, 4288 included water voles and 490 weasels. The paper contains images of weasels which had captured water voles (Fig. 4).]
4. Sales, NG., McKenzie, MB., Drake, J., et al., 2020. Fishing for mammals: Landscape-level monitoring of terrestrial and semi-aquatic communities using eDNA from riverine systems. J Appl Ecol. 2020; 00:1–10. https://doi.org/10.1111/1365-2664.13592 [please note this links takes you to an external site]
This research assessed the efficiency of eDNA metabarcoding to detect the semi-aquatic and terrestrial mammals of river systems in two areas, Assynt and the Peak District. It evaluated eDNA results from both water and sediment samples against historical data for the areas, and synchronic survey methods (latrine surveys and camera trapping). The site surveyed in Assynt included the catchment of the Allt an Sgiathaig on Quinag, and also areas to its east and south-east (Fig. 5).
In Assynt, the wild species identified were: red deer (18/18 sites); water vole (15/18); field vole (13/18); wood mouse Apodemus sylvaticus (9/18); pygmy shrew Sorex minutus (4/18); wild/domestic cat Felis spp. (4/18); mountain hareLepus timidus (4/18); rabbit
Oryctolagus cuniculus (3/18); water shrew Neomys fodiens (3/18); common shrew Sorex araneus (2/18); edible dormouse Glis glis (2/18) [sic]; grey squirrel Sciurus carolinensis (1/18) [sic]; pine marten Martes martes (1/18); brown rat Rattus norvegicus (1/18); red fox Vulpes vulpes (1/18) and badger Meles meles (1/18). Otter and weasel were not identified, although they were both captured on camera traps.
All of these species are distributed around/within Assynt, with the exception of the edible dormouse and the grey squirrel, which are unequivocally absent from the region. The false positives for these ‘potentially arose due to sample carryover from a previous sequencing run on the same instrument’ and can be discounted.
Particularly interesting in a conservation context are the records for wild/domestic cat Felis spp, which occurred ‘in sediment samples in multiple sites in Assynt’. It is suggested that these ‘may be historical rather than contemporary’. Field Club records include sightings of possible wild cats in the Inchnadamph/Skiag Bridge area as recently as 2003.