Pitfall traps have been the standard and most frequently used approach for surveying ground-dwelling arthropods because they are easy to handle and can collect high numbers of individuals and species efficiently. However, trapping efficiency is often sensitive to the population density of a species, its locomotion and olfaction, and the habitat specificity. Arthropod species also respond differently to the choice of liquid attractant and detergent, and the arrangement of traps. Due to these complications, density estimation and community assemblage structures portrayed from pitfall trapping are often biased.

We developed a two-circle method (TCM) for simultaneously estimating densities of ground-dwelling arthropods and the effective trapping radius. Multiple pairs of traps are located different distances apart, and the intersection of trapping areas can be calculated using the inverse trigonometric function. The density and effective trapping radius can be estimated from a nonlinear regression of the change in the total number of individuals caught with the distance between the paired pitfall traps. We compared the performance of TCM with the estimator based on the nested-cross array (NCA) for arranging pitfall traps, by comparing predicted densities from these two methods with the real density obtained from the suction sampling method (SSM). Simulations with known arthropod densities and effective trapping radius suggested that TCM produced accurate density estimation while NCA significantly underestimated the known density. Pitfall trapping of ground-dwelling arthropods on two habitats (crop field and desert steppe) confirmed this conclusion when comparing estimation from TCM and NCA with densities obtained from the SSM.

In summary, TCM is a promising technique for the density estimation of ground-dwelling arthropods, especially for traps with liquid attractant, and areas with relatively homogenous habitat and away from habitat edges. This paper has been published in Methods in Ecology and Evolution (Zi-Hua Zhao, Pei-Jian Shi, Cang Hui, Fang Ouyang, Feng Ge , Bai-Lian Li, Solving the pitfalls of pitfall trapping: a two-circle method for density estimation of ground-dwelling arthropods，2013, 4, 865–871). This project is supported by the State Key Program of National Natural Science of China (No. 31030012), the National Key Technology R & D Program (2012BAD19B05).