Long‐term population changes for the <scp>UK</scp> stag beetle <i>Lucanus cervus</i> —Evidence from citizen science surveys and museum collections

Abstract The stag beetle Lucanus cervus is a European Protected Species reported to be declining in parts of its range. A better understanding of its population change is important for directing conservation efforts and, more widely, as a flagship species for other saproxylic beetle species. Partly because it remains relatively widespread, long‐term stag beetle population trends are difficult to obtain using surveys at a national scale or over sufficient timescales. Data from citizen science schemes are increasingly used to fill these types of data gaps. Here, we use 82,883 distribution records of the stag beetle Lucanus cervus in the United Kingdom, collected as part of People's Trust for Endangered Species' citizen science project, to compare the area of occupancy in two periods: 1998 to 2002 and 2018 to 2022. We also obtained historic data from museum records of this species and compare their geographic distribution with that of the citizen science data. Results describe a broadly stable distribution of stag beetles in the United Kingdom over the last 25 years and similar to that of historic records going back over a century, although with a moderate, recent occupancy decrease in the western part of the species' range. In addition, we use mean counts per record of adult beetles and, separately, larvae from survey data, plus the temporal distribution of observation date each year, as indications of changes in abundance and phenology respectively. Annual counts per record showed a significant decrease for adult beetles and an increase for larvae but should be interpreted with caution because of the lack of standardised, abundance‐focused recording. Median observation date showed no overall change between 1998 and 2022, but the slope of the cumulative frequency curve for observation date increased significantly over this period, which might suggest a shorter activity period of adult beetles in recent years. These citizen science data provide important long‐term information on the distribution of stag beetles in the United Kingdom and could be developed further to improve the monitoring of this species and the targeting of conservation management actions.