The effects of trapping effort and sources of variability on the estimation of activity-density and diversity of carabids in annual field crops by pitfall trapping; a meta-analysis

Pitfall trapping is widely used for studying the abundance and diversity of ground-dwelling arthropods and small vertebrates. Meta-analysis is a powerful technique to synthesize information across studies, but it requires standardization to make study results comparable. It has never been studied how results of pitfall trapping should be standardized to obtain estimates of activity-density and diversity that are comparable across studies. We analysed samples of Carabidae (Coleoptera) from annual crops, reported in 104 publications from Europe and North-America, spanning a period of 42 years, to find (i) a scaling for pitfall trapping effort to assess activity-density and diversity in pitfall catches across studies; and, (ii) to determine the sources of variability in the catch per unit effort. The total catch was proportional to the number of trap days, with a mean of 1.33 beetles/(trap days). The number of species was allometrically related to the trapping effort defined as the product of the number of traps, their perimeter and the time of exposure in the field. The mean species richness was 7.15 species/(m days)(0.25). The size of the catch and the number of species adjusted per unit effort were higher in crops with narrow as compared to wide rows. Other factors were explored but were not influential. There was no significant change in abundance or diversity of carabids in arable land over the 42 years covered. The results give insight in factors affecting carabid abundance and diversity in field studies and enable standardization of pitfall catches across the literature.