Reintroduction of hazel dormice Muscardinus avellanarius to Treswell Wood, Nottinghamshire, England


The hazel dormouse Muscardinus avellanarius is an uncommon native British species that has disappeared from half its UK historical range in the past 150 years. As part of English Nature's Species Recovery Programme, a series of reintroductions were carried out, focussing on the counties where dormice had become recently extinct, one of these being Nottinghamshire in central England. Rope (1885) listed dormice as present in the county and a dormouse was reported from near the town of Worksop in the Victoria County History of Nottinghamshire, but no recent records have been confirmed.

Study site: Treswell Wood (manged by the Nottinghamshire Wildlife Trust) was selected as a potentially suitable introduction site for hazel dormice and although it was not ideal, no better woodland could be found locally. This site is dominated by ash Fraxinus excelsior a tree which offers dormice little to eat (except the ash keys/seeds). This wood is situated about 100 km north of the main area of native dormouse distribution (although there are a few more northerly sites) in the UK.

Dormouse releases: Captive bred dormice became available at short notice, and 23 were individually marked with ear tattoos and installed in 15 release cages. These were opened on 12 July 1994 after a few days acclimatisation. Supplementary feeding was maintained until mid August, by which time at least 17 of the 200 or so available nest boxes installed in the wood had been located and used by dormice.

In 1995, an additional 29 marked captive bred animals were released using the same methods.

Dormouse survival: In September 1994, a nest box survey revealed that of the 23 dormice released at least eight were still alive, representing a minimum of 35% survival. Young dormice were present too, with more in October, confirming that food resources were adequate to allow breeding. At least five were still alive in October, representing a minimum 22% survival rate over the first three months, similar to that observed for captive bred animals in a previous release at Brampton Wood, Cambridgeshire (see Case 90).

At least four animals released in 1994 survived to 1995, when an additional 29 captive bred animals were released. In September 1995, five litters were present in nest boxes, including the first confirmed second generation site-native offspring. But despite an encouraging start, the Nottinghamshire population did not flourish. Only occasional animals were found in 1996 and 1997. By 1999 there was little evidence of any dormouse activity. A single animal was found alive in 2001, and a dead one in a tawny owl Strix aluco nest box in 2002 (see also Update, below).

This new dormouse population did not immediately die out, but its future appears uncertain. Small founder populations face a high probability of extinction but in this case there were additional confounding problems. Several of the release animals had become obese in captivity or were badly affected by mites. Their viability was open to question, as was the desirability of releasing them but inadequate facilities were available for retaining them in captivity. Some of these animals are thought to have died soon after release as they were not recorded during subsequent monitoring.

Coppicing: A further problem was the difficulty of reconciling the needs of dormice with the practical and commercial interests of coppice contractors. The total area of suitable habitat was reduced in the autumn and winter when coppicing was undertaken and as a consequence, dispersal of the dormice was also impeded.

Pig introduction: In 2004, the situation at Treswell Wood was exacerbated by the realease of domestic pigs Sus scrofa into the woodland. Unless removed during the winter months, their trampling and rooting activities may eliminate dormice (as they hibernate on the ground). Pigs were not part of the original woodland management plan, otherwise if this had been the case dormice would not have been released.

Ongoing monitoring Ongoing monitoring will establish whether any dormice still persist at this site (see Update, below).

Update November 2005: The discovery of three individuals in 2004 means that despite the unsuitable conditions of the habitat (and the site being on the edge of the natural range of the species), a relict population has survived. The rediscovery has lead to a revision of the management plan which is ongoing. One result in this is that large areas of the coppice will be managed on a longer rotation, increasing the likelihood of hazel Corylus avellana fruiting for the benefit of the dormice.

Another reason, infact, for considering the wood unsuitable for dormice introduction was that the plan for the wood was to restore the coppice rotation, and this work was already underway at the time of the releases. Commercial coppice workers are employed and the work is regulated to a conservation management plan, and overseen by a Wildlife Trust Reserve Officer. Howevr, it is believed that this will improve habitat quality for woodland flora and fauna in the longer term. Specifically for dormice, more hazel will become available, along with conditions providing more bramble Rubus fruticosus and honeysuckle Lonicera periclymenum. These are all important food plants for hazel dormice.

Charles Langtree
Head Of Estate Management and Development
Nottinghamshire Wildlife Trust

Note: If using or referring to this published study, please read and quote the original paper. Please do not quote as a case as this is for previously unpublished work only.

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