Long-term agricultural management reduces abundance and alters community structure of ground beetles (Coleoptera: Carabidae)

Carabid beetles (Coleoptera: Carabidae) are beneficial predators and bioindicators of ground-dwelling arthropod diversity in agricultural landscapes. We studied changes in community composition, activity density, and diversity of carabids in a long-term agroecosystem study spanning 30 years in the US Midwest. We contrasted carabid community metrics measured by pitfall trap sampling in two conservation-oriented treatments in a corn-soybean-wheat rotation. One treatment uses conventional practices but with reduced agrochemical inputs (Reduced Input), while the other has no synthetic inputs (Biologically Based). Since a 1994–95 study on the same site, in 2019 overall carabid activity density had declined a minimum of 58–76 % with the four previously dominant species (all predators) declining 94–98 % and becoming uncommon to rare. In addition, carabid species richness, activity density, and community structure have diverged significantly under the two management regimes. In 2019 sampling, activity density decline in the Biologically Based treatment was mitigated by large increases in the abundance of two granivorous species (Harpalus compar (LeConte) and Harpalus pensylvanicus (DeGeer)). In contrast, carabid activity density in the Reduced Input treatment remained low. After decades of management, the Biologically Based treatment supported greater diversity and activity density of carabids compared to Reduced Input, and community structure shifted from predatory toward granivorous species. This long-term study contributes to the growing literature on insect decline in agricultural landscapes and demonstrates that changes in abundance and species turnover of the carabid communities can occur even under conservation-oriented management regimes.