Evidence for regional-scale declines in carabid beetles in old lowland beech forests following a period of severe drought

Context

Evidence for declines in insect populations is growing with climate change being one suspected driver. Forests, however, are still underrepresented in the relevant research. Recent droughts (2018–2020) have severely affected forests in Central Europe and have been linked to declines in carabid abundance, biomass as well as changes in species traits at the local scale.

Objective

We tested drought effects on forest carabids at regional scale. We additionally investigated whether variability in drought effects could be explained with the initial community composition and the local environmental context.

Methods

We used generalized linear mixed models to compare data from 1999 to 2001 and 2020 to 2022 across eleven old beech forest sites of high conservation interest in North-East Germany and investigated changes in carabid abundance, biomass, Hill numbers and selected species traits. We then tested additional community-related and environmental predictors to explain spatial variability in changes in biomass.

Results

We found significant declines in biomass of 65% and in abundance of 51%. There were no significant changes in Hill numbers. We found consistent evidence that declines affected especially larger and less mobile species. Declines and changes in species traits also occurred in strictly protected old-growth beech forests. Among environmental predictors, landscape composition explained local variability in biomass declines best with stronger decline at forest sites with less forest area in their vicinity.

Conclusions

Our findings reveal large-scale declines in forest carabids in the context of recent droughts and highlight the exceptional role of landscape composition in this regard. Future insect conservation strategies need to incorporate the landscape context and potential exposure to extreme weather.