Ground-dwelling arthropods are highly diverse and abundant in crop fields and in semi-natural habitats. They have important functions in agro-ecosystems such as
controlling pests and maintaining food chain robustness. Pitfall trapping has been a popular approach for surveying ground-dwelling arthropods because pitfall traps are easy to
handle and can collect large numbers of individuals and species. However, trapping efficiency is often sensitive to the population density of a species, its movement and olfaction,
and the habitat specificity of a species. Arthropod species also react differently to the choice of liquid attractant and the arrangement of the pitfall traps. Due to these
complications, density estimation and community assemblage structures described from pitfall trapping are often biased.

C·I·B researcher, Cang Hui, and co-workers developed the Two-Circle Method for simultaneously predicting densities of ground-dwelling arthropods and
the effective trapping radius, contrasting the conventional methods that require the effective trapping radius to estimate densities. In the Two-Circle Method, multiple pairs of
traps are located different distances apart, and the intersection of trapping areas can be calculated using a mathematical model. The density of animals and the effective trapping
radius can then be estimated from the change in the total number of individuals caught with distance between the paired pitfall traps. Using both simulated and real data, the
authors show that the Two-Circle Method is a promising technique for density estimation of ground-dwelling arthropods, especially if the traps contain liquid, such as detergent;
it is also effective when used in areas with relatively homogenous habitat and away from habitat edges.

An illustration of the spatial arrangement of pitfall traps for the two-circle method (a) and the nested-cross array (b).

**Read the paper**

Zhao ZH, Shi PJ, Hui C, Ouyang F, Ge F & Li BL (2013) Solving the
pitfalls of pitfall trapping: a two-circle method for density estimation of ground-dwelling arthropods. *Methods in Ecology and Evolution* **4**:865-871.

For more information, contact Cang Hui at chui@sun.ac.za