Population Trends of the Adder (Vipera berus) in Sweden:A Rapid Review and Meta-Analysis

BackgroundThe conservation status of the common European adder (Vipera berus) in Sweden is ambiguous. While the species is officially classified as "Least Concern" (LC) on the national Red List, localised reports and long-term studies suggest significant population declines, creating a need for a consolidated evidence summary. ObjectiveTo systematically review and synthesise available scientific and official evidence to determine the overall population trend for the adder in Sweden and to identify the principal drivers behind this trend, with a specific focus on habitat, urbanisation, and procreation. MethodA rapid review of the literature was conducted using a defined Boolean search strategy across academic databases (Web of Science, Scopus, PubMed), preprint servers (bioRxiv), government archives, and citizen science portals. The research question was structured using the PECO framework. The search yielded 255 initial hits, which, after screening, resulted in 12 core sources for inclusion. Evidence was weighted using a four-level hierarchy to assess its robustness. The rapid nature of this review carries an inherent risk of time bias, as studies published after the search date are not included. ResultThe analysis reveals a significant discrepancy between the species official national status and the findings from high-quality longitudinal studies. While the national Red List assesses the species as "Least Concern," the most robust evidence indicates severe and rapid local population declines. The primary identified threats are habitat fragmentation from infrastructure and urban development, and acute effects of climate change, particularly extreme drought. A critical gap in national environmental monitoring was also identified, with no dedicated, systematic programme for reptiles. ConclusionThe probable national trend for the adder in Sweden is a slow, cryptic decline driven by local extirpations. The official "Least Concern" status risks being a lagging indicator that masks underlying threats. DiscussionThe findings highlight a paradox where the most reliable, high-resolution scientific data conflicts with broad-scale national assessments. This suggests that the current conservation status may engender a false sense of security, potentially delaying necessary conservation interventions. The lack of a national monitoring programme is a major limitation to evidence-based management. Recommendations include establishing such a programme and integrating habitat connectivity into landscape planning to mitigate the impacts of fragmentation.