Catches in pitfall traps in relation to mean densities of carabid beetles

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Abstract
Based on continuous pitfall catches summed over the whole activity period for the carabid beetles Pterostichus versicolor Sturm and Calathus melanocephalus L., a satisfactorily linear relationship is presented between the mean densities in several habitats and in different years and the numbers of beetles trapped. This means that with the help of continuous pitfall sampling a reliable relative measure of the sizes of carabid populations can be obtained. Simulation experiments show that for individuals of P. versicolor the distances covered each day should be represented by strips of some width instead of by lines to obtain numbers trapped that equal the field data. The influence on numbers trapped of the high mobility of beetles in unfavorable habitats is probably not significant. Simulation experiments with C. melanocephalus indicate that a much lower capture efficiency than for P. versicolor has to be programmed to fit the simulation results with the field data. The catch from different types of pitfalls is not always proportional to trap perimeter due to different vegetation cover of the immediate trap surroundings and to different kinds of roofs placed over the pitfalls. For the same species the capture efficiency of a pitfall can also change throughout the season. Simulation experiments show that the number and shape of the pitfalls need not influence the catches, provided that the traps keep the same total perimeter and are broadly spaced. Remodelling of some data from the literature reveals that the linear relationship between mean densities and year-catches in pitfalls is probably a general one among carabid species. This leads to the hypothesis that carabid beetles have to work through a fixed reproductive program each year by means of an almost constant total amount of locomotory activity which is realized step by step during spells of suitable weather. Some recommendations are given for intraspecific and interspecific comparisons of pitfall catches between sites and years.