The impact of an extreme climatic event on adder (Vipera berus) demography in southern Sweden

Climate change has been shown to have significant negative effects on wildlife populations owing to both gradual shifts in mean climatic conditions and an increased frequency of extreme climatic events. Based on a long-term study spanning 40 years, we explore the demographic effect of the exceptionally hot and dry summer of 2018 on the demography of an isolated adder (Vipera berus) population in southern Sweden. In spring 2019, we observed a dramatic decline in adder relative body mass and a concomitant 50% reduction in population size. Our results strongly suggest that the negative impacts on adder demography were the result of a combination of low prey abundance and adder dehydration caused by the extreme climatic conditions during the previous summer. The study provides an example of the significant negative effects of ongoing climate change on vertebrate populations and emphasizes the importance of long-term studies to document the demographic effects of extreme climatic events.