Voluntary recording scheme reveals ongoing decline in the United Kingdom hazel dormouse Muscardinus avellanarius population

1. In order to conserve threatened species, knowledge of the status, trends and trajectories of populations is required. Co-ordinating collection of these data is challenging, especially for inconspicuous species such as the hazel dormouse Muscardinus avellanarius. 2. The UK National Dormouse Monitoring Programme (NDMP) is comprised of nest box recording schemes organised by volunteers. The number, size, and coverage of these schemes has varied over time. Such changes risk conflation of genuine population trends with covarying artefacts, including survey effort and expansion into sites of variable quality. 3. We provide a robust analysis of count data from 400 NDMP sites from 1993 to 2014 and demonstrate that changes in counts are not an artefact of survey characteristics. In relation to the International Union for Conservation of Nature (IUCN) Red List criteria, we conclude that dormouse counts in nest boxes are an index of abundance appropriate to the taxon and allow the inference of population reduction of 72% (95% confidence intervals 62-79%) over the 22 years from 1993 to 2014, equivalent to a mean annual rate of decline of 5.8% (4.5-7.1%). This decline is ongoing. 4. We highlight difficulties in assigning an IUCN Red List conservation category to a population, given variation in apparent trends over consecutive time-periods. In eight out of 13 sliding window intervals of 10 years from 1993 to 2014, the 95% confidence intervals overlap a decline of 50%. While average population decline over 10-year periods suggests that the hazel dormouse should be classified as Vulnerable, a precautionary approach would not rule out the category of Endangered in the United Kingdom, given the lower bounds of population change estimates, the mean annual rate of decline and ongoing decline. 5. Ongoing decline in the hazel dormouse population is despite a high level of species protection and widespread conservation measures. The hazel dormouse is a UK Biodiversity Action Plan Priority Species and a European Protected Species, and the causes of population reduction are not well understood and may not have ceased. An urgent appraisal of dormouse conservation is required to ensure the species' favourable conservation status.