Relationships between the regional distribution of carabid beetles (Coleoptera, Carabidae) and the abundance of their potential prey

We studied regional variation in abundance of carabid beetles in relation to both abiotic factors and prey availability in 19 forest sires, with a focus on the relationship between the distributions of the carabid beetle Abux ater Villers and its prey, earthworms. No significant correlation was found between the abundance of Abax ater and any of the abiotic factors considered. On the other hand, litter thickness was strongly associated with the composition and structure of the whole carabid community: an increase in litter thickness was accompanied by a decrease in both total carabid abundance and carabid species richness, and by an increase in the proportion of forest species. The relationship between the regional distributions of abundance of carabid beetles and their potential prey was very different for soil-dwelling and litter-dwelling prey. There was a significant pattern of association between low soil prey abundance and low carabid abundance, which suggests that carabid abundance is partly controlled by prey numbers in the soil. On the other hand, there was a significant pattern of association between high prey density per volume of litter and low Abax ater abundance, thus showing a negative relationship between the abundance of this species and prey density in the litter. We propose two complementary hypotheses to explain this pattern: prey depletion by predation in the litter. and biased sampling of carabids due to differential mobility. The prey depletion hypothesis considers that soil acts as a refuge from predation, so that prey numbers in the soil control carabid abundance, while dominant carabids in turn deplete prey and control their abundance in the litter. This hypothesis suggests a complex relationship between the abundances of carabids and their prey.