Carabus granulatus (Coleoptera, Carabidae) body size becomes less dependent on habitat latitude as soil organic carbon increases across mid-latitude zone

Abstract

The study examined the relationship between soil quality and the size characteristics of ground beetle Carabus granulatus as bioindicators of ecosystem health. Soil quality is defined by its ability to sustain biological productivity, the surrounding environment, and the health of living organisms, with biological (especially soil invertebrates), chemical, and physical soil properties playing a key role. The aim of the research was to identify the influence of soil factors on the size variability of ground beetles. Using regression analysis, we studied the dependence of beetle size on organic carbon content in the soil (0–5 cm layer) and the geographical latitude of the habitat, taking into account the sex of the insects. Carbon content data were extracted from an existing global distribution map generated using machine learning. A total of 8107 specimens from 13 regions (42,9–56,8° N) were examined. The findings revealed a statistically significant influence of both latitude and organic carbon content on beetle size, with an interaction effect identified. It was established that males are 1 mm smaller than females. Additionally, for each degree increase in latitude, the length of the elytra decreases by 0.13 mm, and for each tonne of organic carbon in the 5 cm soil layer, it decreases by 0.14 mm. The results demonstrate a compensatory effect: as organic carbon content increases, the influence of habitat latitude on insect size weakens. These findings expand our understanding of the mechanisms shaping insect size variability in relation to soil conditions and geographical location, which is important for ecological monitoring and bioindication.