Trends in monthly abundance and species richness of carabids over 33 years at the Kaiserstuhl, southwest Germany

Abstract

Recent studies hint at consistent declines of insect abundance across taxa. However, detailed data from long-term surveys are rare in ecological studies, and yet are required in order to accurately infer trends and their causes. In the following, we analyse a dataset from pitfall traps sampled at a monthly resolution over a 33-year period (1979–2011) to investigate the activity density and species richness of ground beetles (Carabidae). The study site, an unmanaged xerophytic grassland amidst intensively managed vineyards in southwest Germany, underwent terracing for viticulture just before data collection began, which led the early community composition to be defined by succession. As this ecological situation initially conflates succession and other long-term trends, we define three successional phases (initial, early, and late), allowing us to separate trends over time. We use a generalised additive modelling approach to factor in the main drivers of the community trends in ground beetles, noticeably vegetation and climate. Both vegetation cover and climatic conditions had a small effect on carabid counts and species richness, and there was a notable decrease in both counts and species richness of 60% and 32%, respectively, during the late successional phase. An analysis of functional traits revealed an overall increase in community weighted mean biomass and a correlation of habitat preference with annual precipitation. Our results add to the growing volume of evidence pointing to a decline in insect populations both in central Europe and globally.