Harvest Mice - On the (Road) Verge?
October 2020 Student of the Month - Ellie Smith
Master's Research at University of Lincoln
Email: ellie_smith97@yahoo.com LinkedIn: https://www.linkedin.com/in/eleanor-rose-smith/
I have recently completed an MSc Evolution and Ecology by Research at the University of Lincoln, working alongside the Lincolnshire Wildlife Trust, and over the last two years I have had the pleasure of studying Britain’s smallest (and in my opinion, the cutest!) rodent – the harvest mouse Micromys minutus.
Background
Weighing up to only 6 g – even less than a 2 pence piece, the harvest mouse can be identified by its distinctive russet orange fur and contrasting white underside. It is the only mammal in Britain to have a prehensile tail, which it uses to climb between tall grasses and reeds (Trout and Harris, 2008). Due to its small size and elusive nature, the species has long been under-surveyed leading to discrepancies in accurate population estimates, but in recent decades it is believed that it has become progressively scarcer. The harvest mouse is incredibly susceptible to a range of threats, including habitat destruction and degradation – primarily as a result of agricultural intensification – and extreme weather events such as floods and prolonged frosts (Perrow and Jowitt, 1995), which are becoming increasingly frequent as climate change progresses.
Consequently, harvest mice have been identified as a Priority Species by the UK Biodiversity Action Plan (UK BAP, 2007) and a Species of Principle Importance in England and Wales (Natural Environment and Rural Communities (NERC) Act 2006, s.41-42). The current population estimate of harvest mice in Britain is 566,000 (95% CI = 288,000-934,000; Matthews et al., 2018), but data regarding population density and distribution are incredibly limited and so the reliability of this estimate is low. The need for more surveying has therefore been highlighted, to gain a better understanding of their current status and in turn a better idea of how best to protect them.
Research Project
My research aimed to improve current knowledge on the status of harvest mice, both locally in Lincolnshire and nationally, using a range of techniques.
First, I wanted to assess the current distribution of harvest mice around Lincolnshire through direct surveying. Despite being the second largest county in England and home to ideal harvest mouse habitats such as grasslands and fens, just 143 harvest mouse records were documented throughout the entire county in the last 40 years. Using a combination of owl pellet dissection and Longworth trapping, I established their current distribution across the county, which I then compared with previous record locations using QGIS. Although records were relatively infrequent (they certainly lived up to their elusive nature!), presence appeared to have largely persisted across the county, which was really promising.
I also wanted to see what influenced their distribution, so I analysed their presence in relation to both the presence of other small mammal species and various environmental landscape features. Only field vole Microtus agrestis abundance was found to effect harvest mouse presence, having a significant negative relationship, which may indicate competition for food sources or differences in habitat requirements. Regarding environmental factors, increased area of uncultivated land (which included grasslands, marshes and fens) significantly increased harvest mouse presence. Increased surrounding hedgerow length negatively influenced harvest mouse presence whereas increased surrounding road length had a surprisingly positive effect, perhaps indicating the importance of grassy road verges as a harvest mouse habitat.
The second part of my project involved using population viability analysis to assess the minimum population size and habitat area required for population viability, as well as overall population resilience, using a programme called VORTEX. The model outputs demonstrated that harvest mice were actually incredibly resilient to fluctuations in demographic rates and “catastrophic events” (e.g. floods), only going to extinction in extreme circumstances. Additionally, demographic viability was achieved in relatively small populations of 50-200 individuals, which corresponded to realistic requirements for habitat patches. However, when genetic viability was considered, population viability was not even reached in populations of 2000 individuals. This corresponded to requirements for vast areas of habitat, which were unfeasibly large given the fragmentation British landscapes have faced due to the expansion of agriculture. This therefore indicated to me that connecting remnant habitat patches may be critical in maintaining genetic diversity and enabling harvest mice to persist in the future.
Areas for Future Research
My research demonstrated that harvest mice are generally very resilient and able to persist in suitable habitats. However, particularly when in small or poor quality habitats, connectivity may be crucial in order to maintain the genetic diversity of harvest mouse populations, thus helping to secure their existence in the long term. Further surveying, particularly focused on habitat corridors such as wide grassy margins along roads and fields, would be beneficial both for establishing the distribution of harvest mice along verges like these, and for identifying the factors responsible for determining their habitability for harvest mice. Gaining a better understanding of their habitat requirements could help to develop “harvest mouse friendly” verge management schemes (e.g. phased mowing – check out Hata et al., 2010) which may be key to connecting otherwise isolated populations.
Thank you for taking the time to read my blog, I hope you found it interesting! Please feel free to get in touch if you have any questions (email: ellie_smith97@yahoo.com).
Acknowledgements
I’d like to say a massive thank you to everyone at the Lincolnshire Wildlife Trust who provided support and guidance throughout this project. Thank you also to everyone who collected barn owl pellets on my behalf and allowed me to carry out small mammal trapping on their land. Finally, thank you to my supervisors Dr Carl Soulsbury and Dr Graziella Iossa, and to all of the fantastic student volunteers who helped me to collect the data for this project.
References
Hata, S., Sawabe, K. and Natuhara, Y. (2010) A suitable embankment mowing strategy for habitat conservation of the harvest mouse. Landscape and Ecological Engineering, 6(1) 133-142.
Mathews, F., Kubasiewicz, L. M., Gurnell, J., Harrower, C. A., McDonald, R. A. and 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. Peterborough: Natural England. Available from http://publications.naturalengland.org.uk/publication/5636785878597632 [accessed 05 October 2020].
Natural Environment and Rural Communities (NERC) Act 2006, s.41-42 (c. 16) Available from: http://www.legislation.gov.uk/ukpga/2006/16/contents [accessed 05 October 2020].
Perrow, M. and Jowitt, A. (1995) What future for the harvest mouse? British Wildlife, 6(6) 356-365.
Trout, R. C. and Harris, S. (2008) Harvest Mouse. In: S. Harris and D. W. Yalden (eds.) Mammals of the British Isles: Handbook, 4th Edition. The Mammal Society, 117-125.
UK Biodiversity Action Plan (BAP) (2007) UK BAP List of Priority Terrestrial Mammals. JNCC. Available from: https://hub.jncc.gov.uk/assets/98fb6dab-13ae-470d-884b-7816afce42d4#UKBAP-priority-terrestrial-mammals.pdf [accessed 05 October 2020].