Edible dormouse (Glis glis) by Roger Trout - Invasive Species Week
Roger competed a PhD on harvest mice whilst at King’s College, London before joining the Pest laboratory of the Ministry of Agriculture in 1973 and Forest Research in 2000. Currently running research projects on both species of dormouse, each exceeding 20 years. His experience on small mammal management of both pest and rare species includes work for national companies, charitable trusts, individual landowners and Government bodies in Britain, Europe and Australasia.
You might have never heard of an Edible dormouse unless you live in the Chilterns. This species was introduced to Britain in 1902 in Tring, west of London. It is a long lived, arboreal, nocturnal rodent that hibernates underground for c. 7 months and only breeds when there is an abundance of tree flowers. They look somewhat like a half sized grey squirrel and in early summer may call like one … but only at night. The literature and evidence indicates Glis to be a slow-spreading but invasive pest species causing damage to forestry interests, harmful impacts on protected native species but also invades gardens and buildings where it may cause significant infrastructure damage (cables) and nuisance e.g. noise or drowning in water tanks. Historical distribution data of Glis culling as a pest in properties from local authority reports and latterly Defra indicates both spread and consolidation: they now apparently cover almost the entire Chilterns woodland area bounded by the M25, M40 and Chilterns scarp with current verified populations/records beyond, including north of Reading, eastwards towards London, south towards Slough, in west Essex and Ascot in southeast Berkshire.
Regular – largely unfunded - population monitoring for 25 years (1996-2020) by volunteers at a woodland study site (using over 200 nestboxes and microchipping more than 25,000 individuals) showed consistently increasing numbers. During non-breeding years, many adults are not recorded in nestboxes but known-to-be-alive since they reappear the following year. The first 5 breeding years yielded 97-175 marked individuals, the last 5 breeding years 1373-2048 individuals (c.40-60/ha). Concerns have been raised that the introduction of ‘extra’ nest box refuges may actually cause artificially high population indices. However, four different assessments indicate this not to be the case, including the fact that lifetime recapture rate of adults (some survived over 10 winters) averaged once in ten inspections; and in the 2 August nestbox inspections in a breeding year the proportion of adults of those known-to-be-alive that August was only 20% - so 80% used other refuges.
The completely independent Defra reports from culling under Licence have also shown a similar increasing trend in numbers. Fewer than 100 Licences were issued in 2008 with 423 Glis killed but nine years later 453 licences resulted in 2,541 Glis. Both were non-breeding years. On an estate of 62 houses 41 were infested and recorded 452, 380 and 468 (30, 25 and 31/ha) culled in successive years. Additionally, recent woodland cull trials over only 10 days of trapping across c. 12 ha resulted in 14/ha. being removed in a non breeding year and 22 adults/ha in a breeding year.
Our nestbox studies also indicate up to half of hole nesting birds nests were predated in Glis breeding years and reports suggest Hazel dormice and bats are also affected.
It is difficult to estimate the population; the 2018 published estimate of the UK population (23,000 based on density estimates of under 1/ha from 1-2 decades ago) differs with other independent recent estimates of at least an order of magnitude larger; 10-20+adults/ha. Could the annual cull totals reported to Defra from buildings realistically represent over a tenth of the entire UK (woodland based) population? This invasive species (predating bird nests, impacting protected mammal species, causing tree damage and increasing problems in buildings) urgently needs further pragmatic research. Why not walk through a Chilterns wood at night in late June/early July to count them calling and report to the Mammal Mapper app?
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