Live trapping to monitor small mammals in Britain

ABSTRACT 1. The use of live trapping in Britain to monitor small mammal distribution and population fluctuations is reviewed. Four small mammal monitoring programmes from Britain, the results of which are either partially or wholly unpublished, were examined to determine the nature and extent of information that can be gathered from long‐term and/or large‐scale live‐trapping programmes, the degree to which such information can be extrapolated across habitats, and the likely precision with which long‐term changes in numbers can be quantified. The most extensive of the surveys examined were The Mammal Society National Woodland Small Rodent Survey and the ADAS Hedgerow Survey. These monitored wood mouse Apodemus sylvaticus , bank vole Clethrionomys glareolus and, in the ADAS survey only, common shrew Sorex araneus population fluctuations on a broad spatial scale. 2. The studies provided evidence of population synchrony and demonstrated quantitatively the effects of mast crops and local food supplies on rodent dynamics. Evidence of population synchrony between sites for wood mice and shrews suggests that it may be possible to extrapolate population changes from site‐specific to national scenarios. However, this may be problematic for some species and across habitats, although comparison of the data from the woodland and hedgerow surveys suggests that there may be a degree of long‐term synchrony in bank vole populations between the two habitats. 3. Analysis of the capture data from the two surveys indicated that annual changes of 3–11% from the population mean would be detectable with 85% power if monitoring is carried out for 10 years on 50 sites; increasing site replication further is unlikely to result in a significant increase in precision. 4. The nature of the information provided by live trapping with grids, as carried out in The Mammal Society and the ADAS surveys, is compared with that obtained from line trapping and from a range of indirect techniques. It is suggested that future monitoring will need to take an integrated approach in which information from direct and indirect monitoring of small mammal populations is linked with broad‐scale data on changes in habitat availability.