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

Abstract

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.

Functional Niche Partitioning Occurs over Body Size but Not Nutrient Reserves nor Melanism in a Polar Carabid Beetle along an Altitudinal Gradient

“…Smaller individuals of A. pacificum occurred at lower altitudes, larger ones occurred at 600 m asl, and individuals with intermediate body sizes were found at 900 m asl. Since body size and body mass are often correlated in carabids [ 61 ], we were expecting a quadratic relationship between altitude and body mass. However, the observed trend was not significant.…”

Long‐term drought triggers severe declines in carabid beetles in a temperate forest

“…1). An additional crossed random intercept for year (factor) was used to account for temporal pseudoreplication and year effects (Chaves 2010, Knape 2016, Daskalova et al 2021, Weiss et al 2023a). We also fitted a negative‐binomial generalized additive mixed model (GAMM) to investigate non‐linear trends, which followed the same structure as the GLMM with the only difference that the main predictor (year) was fitted with a smoothing term.…”

“…In the AIC‐based model selection we fitted all candidate models with maximum likelihood instead of restricted maximum likelihood (Fox et al 2015). We ran a sensitivity analysis for abundance and biomass models to test the robustness of estimates by iteratively excluding data of single plots and years and refitting the models (Weiss et al 2023a). Predictions for all GLMMs and GAMMs (incl.…”

“…The model with the lowest AIC was subsequently used to account for drought effects (if ∆AIC > 2 compared to the initial GLMM, Burnham and Anderson 2004). We calculated biomass with size-weight equations of Szyszko (1983) and Booij et al (1994) following the approach proposed by Weiss and Linde (2022). Average species sizes needed for the calculation were based on minimum and maximum sizes stated in Müller-Motzfeld (2004).…”

“…Data are available from PubData: https://doi.org/10.48548/pubdata-46 (Weiss et al 2023b). The R code is available at https://github.com/fabioweiss/forest_carabid_declines.…”