How to estimate carabid biomass?—an evaluation of size-weight models for ground beetles (Coleoptera: Carabidae) and perspectives for further improvement

Biomass is an important metric for monitoring carabid populations and serves as an ecological indicator. Models that predict carabid weight based on body size represent a simple and straightforward method to estimate biomass and are therefore commonly used. However, such models are rarely evaluated against independent validation data. In this study, we evaluated the two widely used size-weight models by Szyszko (1983) and Booij et al. (1994) drawing on previously published independent data. Additionally, we developed and tested four new models to also evaluate the potential effect of taxonomic parameters; and compared model predictions with actual measurements of biomass using relative deviation graphs and observed versus predicted from regression. We show that the two models by Szyszko (1983) and Booij et al. (1994) contain systematic bias towards larger and smaller carabids, respectively, suggesting restricted applicability of such models. Additional taxonomic parameters improved weight predictions, indicating one possible solution to the issue of restricted applicability. We discuss further relevant limitations of size-weight models and their application and recommend a combined use of the models of Szyszko (1983) and Booij et al. (1994) for carabids ≥ 11.8 mm and < 11.8 mm, respectively. Implications for insect conservation: Size-weight models are a suitable and simple method to estimate the biomass of carabids and have great potential to be used in monitoring schemes, the investigation of long-term trends and ecological studies. It is, however, essential that researchers pay special attention to potential restrictions in their applicability and methodological limitations.