Investigating optimal pit fall trap rim circumference for sampling surface dwelling arthropods in a desert habitat
Salma Elserafi
ABSTRACT
Pitfall trapping is one of the most extensively used methods for sampling surface foraging arthropods in ecological monitoring and biodiversity studies. Despite variations in trap design influencing catch rate and species composition, there is no standardisation protocol associated with pitfall trapping. Larger traps are known to increase catch rate and species richness sampled, but also increase handling and processing time and have been found to enhance the positive body size bias towards larger species. Many studies have investigated how to trap size influences the size bias and trapping efficiency but studies focused on optimal rim circumference are few. This study consisted of a single short sampling period, investigating the role of rim circumference in sampling surface-dwelling arthropods in an Emirati desert environment. Four rim circumferences (22, 27, 35 and 45 cm) were assessed across four sites (4 sizes, 20 replicates, 80 traps) on their diversity estimates and catch mean body length. To assess the strength of any catch body size bias, body length was compared between the pitfall size classes using one-way ANOVAs, and no consistent significant overrepresentation of larger-bodied species in larger traps was found. Three criteria were then considered in choosing optimal rim circumference; handling/processing time, ability to maximise species diversity and ethical consideration. The results suggest that the second-largest trap size was optimal. For the same number of traps and handling time, the 35 cm size class caught the highest number of species (>4) out of all the other traps (<4). Moreover, larger traps may increase the potential for trapping non-target species, further justifying the second-largest circumference as the optimum.
