Pitfall traps are widely used for investigating ground-dwelling arthropods, but have been heavily criticized due to their species-, habitat- and attractant-specific trapping radius which produces unreliable estimation of species diversity and density.

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.

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.