Does beetle size depend on carbon in soils? (The case study in Carabus granulatus L.)

The research explored how soil quality influences the size characteristics of the ground beetle Carabus granulatus L. (Coleoptera: Carabidae), which serves as a bioindicator of ecosystem health. Soil quality, understood as its capacity to support biological productivity, maintain environmental stability, and ensure the well‑being of living organisms, depends crucially on biological (particularly soil invertebrates), chemical, and physical properties.

The primary goal was to determine how soil factors shape the size variation observed in ground beetles. To achieve this, regression analysis was applied to examine the relationship between beetle size and two key variables: the organic carbon content in the top 0–5 cm soil layer and the geographical latitude of the habitat. The analysis also accounted for the sex of the insects. The organic carbon data were sourced from a pre‑existing global distribution map created with machine learning techniques.

In total, 8107 beetle specimens from 13 regions spanning 42.9 – 56.8° N were studied. The results confirmed that both latitude and organic carbon content exerted a statistically significant effect on beetle size, and an interaction between these factors was also detected. The study found that male beetles are, on average, 1 mm smaller than females. Furthermore, it was quantified that for every one‑degree increase in latitude, the length of the elytra decreased by 0.13 mm, while each additional tonne of organic carbon in the 5 cm soil layer corresponded to a 0.14 mm reduction in elytra length.

An important compensatory pattern emerged: as the organic carbon content rose, the impact of latitude on insect size became less pronounced. This research represents the first step to the large scale interspecific comparative study. It may provide insights into how the current biogeografic profile of Carabidae communities across the Palearctic zone has been formed.