Long-term drought triggers severe declines in carabid beetles in a temperate forest

Evidence for widespread declines in arthropods is growing and climate change is one of the suspected drivers. Recent droughts in Europe were unprecedented in the previous centuries and we are only beginning to understand the impacts on ecosystems. We analysed a 24-year dataset of carabid beetles from a temperate forest area in northeast Germany and investigated linear and non-linear trends in carabid abundance, biomass, diversity and species traits. We were especially interested if and how these were linked to droughts at different temporal scales using the standardized precipitation evapotranspiration index (SPEI). We found significant linear declines in abundance and biomass with annual rates of -3.1% (0.95 CI [-5.3, -1]) and -4.9% (0.95 CI [-9.4, -1.6]), respectively. Non-linear trends were closely related to the SPEI when considering the climatic water balance of the previous six years and showed severe declines between 2015 and 2022 (-71% abundance, 0.95 CI [-84, -61] / -89% biomass, 0.95 CI [-97, -59]). However, there remained a significant annual background-decline of -2.1% (0.95 CI [-5.7, -0.2]) and -3.1% (0.95 CI [-6.5, -0.1]), respectively, which occurred independently of drought. We observed negative trends in standardized carabid diversity metrics and a shift in species assemblage that were less directly linked to droughts. Declining and drought-sensitive species tended to be larger predators with low dispersal abilities. This study is among the very first to investigate the impacts of the current unprecedented drought on forest insects in central Europe. Our findings add to the concerning amount of evidence for widespread declines in arthropods while pointing towards weather anomalies and climate change as one important driver.