The pitfalls of pitfalls: a comparison of pitfall trap catches and absolute density estimates of epigeal invertebrate predators in Arable Land

In an agroecosystem, catches of epigeal invertebrate predators obtained by pitfall traps were compared to absolute population densities estimated by ground photoeclectors in two different habitats, a field and an adjacent set-aside land. In general, abundance of Carabidae and Lycosidae were overestimated by pitfalls, while Staphylinidae and Linyphiidae were underestimated, and beetle larvae showed no obvious trend. The overestimation of Carabidae and Lycosidae by pitfalls had its maximum in spring and summer. Numbers collected by pitfalls did represent actual population densities better in the field than in the set-aside. This is probably caused by the higher vegetation diversity and complexity of the set-aside land which additionally influenced the catches. Pitfalls recovered higher numbers of animals and species, and species composition differed significantly between pitfalls and photoeclectors, in the field as well as in the set-aside. Body size seemed to be the main factor in determining the catch, the relatively larger species being more frequently caught by pitfall traps. In sum, the relationship between pitfall trap catches and actual population densities appeared to be either absent, weak or highly variable among taxa, habitat and time of the season. Thus numbers caught in pitfall traps cannot be considered as reliable indices of “real” abundance nor do they reflect the relative abundance within a given predator community correctly. Therefore, in most cases absolute density estimates should be preferred for the study of epigeal invertebrates in arable land. However, I suggest a combination of both pitfall traps and standardised area samples, because the two approaches possibly provide information of different qualities equally important for the evaluation of epigeal predator species, namely searching activity and population density.