Can carabid biomasses be estimated from pitfall traps?

Biomass estimations are currently needed to explain the insect decline of the past 30 years. As pitfall trapping is the most frequently performed method in carabid studies, it is necessary to question whether this method is appropriate for biomass studies. The results of pitfall trapping have often been accepted without considering the limits of the method. They were assumed to correspond to species density, although many studies in the past found that pitfall trapping could not be used to describe density or biomass. Ecosystem analysis is mainly based on area-related data, while pitfall trap data reflect a combination of density and activity that, moreover, varies depending on species composition and ecosystem structure. The present study is based on investigations performed in two large ecosystem projects in Schleswig-Holstein. The data were obtained from determination of density and from pitfall trapping, and the results of the two methods were compared. According to the present study, results from measures of density and from pitfall trapping were weakly related only for intermediate-large species. For small species no significant relationships were found. In general, small species were under-estimated and large species were over-estimated when the pitfall trap method was used. In particular, there were major differences in the composition of the carabid assemblages between the two methods. Results of pitfall trap studies mostly showed a dominance of large species, while in reality small species usually dominate in ecosystems. Concerning biomass estimation this fact is mainly responsible for the different results of the two methods. In the present study no relationship was found between biomass derived from the density measuring method and pitfall traps. In contrast, biomass derived from pitfall trapping mainly represented the biomass of the largest dominant species, which, however, has only a small effect on the area-related biomass.