When Weasels Eat Water Voles

June 2020 Student of the Month - Victoria Curley

Undergraduate Research at University of Aberdeen

To get in touch: victoria.curley.15@abdn.ac.uk   or  @VictoriaCurley2

Background

Water voles (Arvicola amphibius) are the largest species of vole in the UK and are our fastest declining mammal. They occupy a variety of habitats from upland moorland to low-lying farmland, though within these areas they are primarily restricted to narrow slow-moving watercourses. They prefer streams with heavily vegetated banks that provide food and shelter, and that have soft substrate suitable for making their burrows. Research suggests that water vole numbers in the UK decreased by 78% between 1989 and 1998 and current population estimates show a further 50% decline (Mathews et al., 2018). The primary threats to water voles are habitat destruction through agricultural intensification, pollution and watercourse developments, and predation by the invasive American mink (Neovison vison). Mink are highly adaptable semi-aquatic predators that are capable of entering the bank-side burrows of water voles (unlike some of their native predators). Having not evolved alongside the American mink, water voles have no natural defences against them, and this has resulted in catastrophic water vole declines. There has been much positive conservation work across the UK for water voles in terms of river restoration and mink control; however, this charismatic rodent is still very much under threat.

The Assynt water voles and my undergraduate research project

I am a recent Conservation Biology graduate from the University of Aberdeen and for my undergraduate thesis, I was lucky enough to assist in studying the delightful water vole.  Since 1998, with support from local landowners, the university have been monitoring water vole populations in the Assynt region of North-west Scotland; studying many aspects of their ecology, population dynamics and disease transmission. These upland water voles are extremely important for the species conservation as populations here are thriving existing beyond the current range of the American mink. Due to the pristine nature of these populations, the Assynt water voles can be used as conservation reference points. Findings relating to their ecology and population dynamics can be used to inform conservation management decisions for water voles elsewhere in the UK.

The focus of my research project was looking at the impact of one of our native predators, the common weasel (Mustela nivalis), on water vole population dynamics. Similar to other water vole populations in the UK, the Assynt water voles exist as lots of small sub-populations. They inhabit suitable grassy patches separated by heather moorland and are connected by the migration of voles between these patches. Such movements are important as each year many sub-populations will naturally go extinct, but dispersing water voles allow the re-colonisation of these patches (called a “rescue effect”). From previous research, it has been observed that extinctions of sub-populations are spatially correlated (Sutherland et al., 2012). If extinction occurs at a patch, then neighbouring patches are also more likely to go extinct due to fewer incoming voles. However, it has also been suggested that the scale of these extinctions could be related to predation by native predators such as weasels. Spatially correlated extinctions have been observed in lowland water vole populations exposed to mink (Telfer et al., 2001); however, the effect of native predators had not yet been studied. Using data from water vole live trapping and from camera traps, I wanted to investigate if the occurrence of weasels was related to the observed patterns of extinction in water vole populations.

Summer fieldwork

Last summer, I joined the university’s Assynt field team for six weeks of fantastic fieldwork. For the first couple of weeks we carried out water vole surveys, looking for signs such as latrines (piles of droppings) and fresh vegetation clippings along the banks of lochs and streams. It was during these surveys I saw my first water vole (something I will never forget), as well as a whole host of other amazing wildlife (including red deer, ospreys, white-tailed sea eagles and black-throated divers). I loved being out in the hills searching for signs of water voles, it felt like I was a wildlife detective! Once we had identified sites where water voles were present, we then left out traps to catch and count the voles. We used Elliot traps baited with carrot and potato and, once caught, tagged and released the voles. We also weighed and sexed them and removed any ticks and fleas for disease studies. This was all done as quickly as possible, to prevent stressing the water voles. I felt really lucky to get some hands on experience with one of our most elusive mammals and was pleasantly surprised by their very different personalities!

Camera trap images

Camera traps were also set at several of our survey sites to catch images of both weasels and water voles. These were extremely useful as they captured images all year round, even when we were not in the field. I spent much time during the winter months analysing these camera trap images and, amongst the thousands of pictures (mostly grass blowing in the wind!) there was a huge variety of species recorded. As well as weasels and water voles, there were images of otters, foxes, badgers, stoats, water shrew and many species of bird. It was quite exciting each day not knowing what I was going to find, and this kept me going through the thousands of images.

Findings and future work

By analysing the camera trap images, I found some evidence to suggest that weasel occurrence is related to the observed patterns of water vole population extinction. If a weasel was detected at a patch, then there was an increased likelihood that water vole colonies at nearby patches would go extinct. This occurred within the scale of spatially correlated extinctions and could mean that weasel predation is driving these patterns. However, due to the small number of camera traps, this finding was only an indication and could not conclusively prove that weasel predation is contributing to the spatially correlated extinctions. Future work would benefit from the deployment of more cameras across the study area as, although camera traps proved effective in detecting water voles, weasels (with their low population density and large home range sizes) were much more challenging to detect. This was also found in a recent pilot study by the Vincent Wildlife Trust (Croose & Carter, 2019). Greater spatial coverage of camera traps could provide more detections of weasels and therefore more conclusive evidence of their potential role in influencing water vole population dynamics.  

Overall, I thoroughly enjoyed working on all aspects of my honours project thesis and later this summer, I hope to return to Assynt and join this year’s field team (if COVID restrictions allow). In the meantime, during lockdown, I am enjoying the wildlife in my own garden and I am now hooked on camera traps! I am especially loving watching videos of our resident hedgehog. Thank you all for reading my blog, I hope you enjoyed it, and that you are staying safe and making the most of the local wildlife in your area.

References

Croose, E. & Carter, S.P., 2019. A pilot study of a novel method to monitor weasels (Mustela nivalis) and stoats (M. erminea) in Britain. Mammal Communications 5: 6-12, London.

Mathews, F., Kubasiewicz, L.M., Gurnell, J., Harrower, C.A., McDonald, R.A., Shore, R.F., 2018. A Review of the Population and Conservation Status of British Mammals. A report by the Mammal Society under contract to Natural England, Natural Resources Wales and Scottish Natural Heritage. Natural England, Peterborough. ISBN 978-1-78354-494-3.

Sutherland, C., Elston, D.A. and Lambin, X., 2012. Multi‐scale processes in metapopulations: contributions of stage structure, rescue effect, and correlated extinctions. Ecology, 93(11), pp.2465-2473.

Telfer, S., Holt, A., Donaldson, R. and Lambin, X., 2001. Metapopulation processes and persistence in remnant water vole populations. Oikos, 95(1), pp.31-42.