Shifting baselines for species in chronic decline and assessment of conservation status. Are hazel dormice Muscardinus avellanarius endangered?
Long-term data are beneficial for monitoring the conservation status of species. Assessments of population change over recent periods of fixed duration will, however, be subject to 'shifting baselines', where the accepted norm for the population at the start of the period already represents a reduction from historical levels. International Union for Conservation of Nature Red List criteria for categorizing conservation threat rely on assessing declines against quantitative thresholds, generally measured over 10 years, as indications of the likelihood of extinction in the near future. By contrast, legal frameworks such as the European Habitats Directive require states to achieve and sustain 'Favourable Conservation Status' for protected species, while domestic conservation legislation can have more diverse objectives and mechanisms, based on local contexts that extend beyond biological or quantitative criteria. We explore the challenges associated with assessing the risk of extinction and the conservation status that arise from the availability of long-term monitoring data for hazel dormice Muscardinus avellanarius in the United Kingdom. Numbers of adult dormice counted in the National Dormouse Monitoring Programme are in ongoing decline, amounting to an overall decline of 78% (95% confidence interval = 72%-84%) over 27 years, 1994-2020. If the observed annual rate of decline of 5.7% (95% CI = 4.7%-6.8%) were to continue unabated, dormouse counts would decline by >90% from 1994 to 2034. Despite this, the species would never be categorized as Endangered, under IUCN criteria, which specify a reduction of >50% within 10 years. While such chronic decline may not indicate imminent risk of extinction, justifying a higher Red List category, it is a demonstration of unfavourable conservation status at a national scale. Prioritization based on demonstration of such chronic declines might direct more effective action towards species conservation at a point when their recovery is more attainable, rather than attempting later to reverse a journey to the brink of extinction when the species is finally 'Endangered'.