Assessing The Current Status of Harvest Mice (Micromys Minutus) in Lincolnshire, UK.
Creators: name: Author, Unknown
Publisher: name: University of Lincoln
Year: 2023
Subjects: C180 - Ecology

description: The harvest mouse Micromys minutus is Britain’s smallest rodent, and is a species that is notably under-surveyed. As a result, little is known about its current status across Britain and population size and change are poorly understood. Harvest mice are thought to be susceptible to a range of threats including habitat destruction and degradation from agricultural intensification. This thesis assessed the current status of harvest mice, the factors that affect their survival, and the conditions required to maintain viable populations in Lincolnshire, a largely agricultural county where harvest mice are poorly recorded. Chapter 2 aimed to compare current harvest mouse distribution with previous records and investigate the effects of landscape-scale habitat factors and presence of other small mammal species on harvest mouse presence. Chapter 3 then aimed to assess the population resilience of the species in response to demographic and environmental fluctuations, and determine the minimum viable population size (MVP) and minimum area of suitable habitat (MASH) required to support demographically and genetically viable harvest mouse populations in various known habitats. Firstly, the presence of harvest mice throughout the county was established using owl pellet analysis and Longworth live trapping. A random sample of up to 20 pellets from each location were selected – in total, 944 barn owl Tyto alba pellets from 62 locations around Lincolnshire were dissected. An additional 25 sites across Lincolnshire were selected for live trapping using opportunistic sampling methods, and a total of 2,715 trap nights (trap count x trap sessions) were carried out. Surrounding landscape features were then categorised using aerial imagery and the presence and absence data, supported by additional data from recent records, were then evaluated in relation to these habitat factors. Using pellet analysis data only, the effect of other small mammal presence on harvest mouse presence was also assessed. Increased area of surrounding uncultivated land positively influenced harvest mouse presence, which was supported by other landscape scale studies, but surrounding cultivated land area had no significant effect. Increased hedgerow length negatively influenced harvest mouse presence whereas increased road length had a positive effect, perhaps indicating the importance of grassy road verges as a harvest mouse habitat. In addition, the abundance of field voles Microtus agrestis – but not wood mice Apodemus sylvaticus, bank voles Myodes glareolus and common shrews Sorex araneus – had a significant negative effect on harvest mouse presence. However, the use of owl pellets introduces a sampling bias, as barn owls exhibit prey and habitat selection, which likely influenced the results. Using demographic information from pre-existing literature, a population viability analysis (PVA) was then carried out to determine the MVPs required for harvest mouse populations to remain both demographically and genetically viable, in addition to the MASHs necessary in different habitats to support said populations. Fluctuations in mortality and reproductive rates and inbreeding depression effects were modelled to determine the resilience of harvest mouse populations. As harvest mice are susceptible to extreme weather events, the effects of catastrophe events were also modelled. Harvest mice were found to be mostly resilient to fluctuations in mortality and reproductive rates, as well as catastrophic events, only going to extinction in the most extreme circumstances. Demographic viability was achieved in relatively small populations of 50-200 individuals, which generally corresponded to realistic spatial requirements for habitat patches. However, genetic viability was not even achieved in populations of 2000 individuals, which corresponded to unfeasibly large spatial requirements for habitat patches. Harvest mouse habitats, such as reedbeds and fens, are increasingly small and fragmented due to agricultural expansion, and so conservation attention should consider the connectivity of isolated patches in order to facilitate gene flow between populations and maintain genetic diversity. This thesis demonstrated that harvest mouse populations are highly resilient, but that habitat connectivity may be critical in preserving the species in British landscapes. Continued surveying, repeated annually and using a range of survey methods, would help to build a better picture of the species’ current status in Lincolnshire. Further research into hedgerows and road verges as harvest mouse habitats and dispersal corridors would help to develop optimal methods of establishing connectivity between existing habitat patches. As harvest mice can be vulnerable to changing conditions, establishing a network of suitable and connected habitats across Lincolnshire’s fragmented arable landscapes may be key to ensuring stable populations in the years to come.