Harvest mouse
Micromys minutus
The harvest mouse is Britain’s smallest rodent. It inhabits a variety of habitats typified by tall grasses, rushes and reeds. In certain areas it is thought to be predominantly a species of wetlands. It nests, during summer and autumn, off the ground in the stalk zone of the vegetation, returning to ground level during the winter. Its population can increase rapidly in suitable conditions, but it has a high over-winter mortality rate. This combined with its relatively limited dispersal ability may result in local extinctions and failure to re-colonise.
There is a concern that the harvest mouse might be undergoing a rapid population decline. A Mammal Society survey in 1997 resurveyed 250 sites nationally where the harvest mouse had been recorded in the 1970s. It was found to have disappeared from 72% of those sites.
The species is on the edge of its range in north-east England and locally there have been very few records, even historically. As recently as eight years ago there were no known current records. It had apparently disappeared from its last known sites since the 1980s. More recent research has unearthed about ten sites where harvest mice are still found in the wider Tees Valley area but some of these are likely to be small, isolated populations.
A captive breeding and release scheme has been run by Stockton Borough Council since 2001 and has resulted in introductions at four sites, all within the borough of Stockton. There is some evidence of limited success of this programme. The reasons behind its decline are not fully known but habitat loss through agricultural intensification and land drainage is thought to be a major factor. Habitat fragmentation, in particular, is a threat to those species with poor dispersal ability.
