The slow invasion of England by the non-native Edible Dormouse (Glis glis): where and how many after 120 years?

Abstract

Since the Edible Dormouse (Glis glis) was introduced to England in 1902, it very slowly invaded a wider area west of London. A recent and more rapid increase in both numbers and distribution is reported. Regular population monitoring for 23 years by volunteers at a woodland study site using nestboxes, and microchipping individuals, confirmed consistently increasing numbers. Independent reports of culling in buildings (under Licence) also indicates an increasing level of conflict. As part of a formal assessment of all British mammals published in 2018, the Glis population size was unfortunately based on estimated density data from two decades ago, indicating <1/ha. This was in a non-breeding year, when evidence indicates many marked adult Glis are not available to be counted, but re-appear the next year. We compare more recent information using a variety of different methods of density estimation which each indicate the total population to be at least an order of magnitude greater at 10-20 adults/ha. Our interpretation of these analyses discounts the suggestion that nestbox presence alone has directly caused the population increase, as happens with some bird species. Additionally, evidence from culls in woodland match our live capture population estimates. The principal aim of the national Invasive Species Strategy is to minimise the risk and negative impacts of invasive non-native species in Great Britain. Edible Dormice damage trees, predate nests of woodland birds and cause problems in the built environment. Our updated estimates affect the seriousness with which this slowly invading species should be regarded.