Numbers matter: Predatory ability increases with forager group size in omnivorous ant species with similar predatory traits

Abstract

Predation is an important force structuring ecological communities. However, it is still controversial whether larger predator groups are more efficient at exploiting abundant resources. Here, we explored the association between the number of foragers and predation ability in generalist ant species that differ in forager numbers when exploiting resources. We conducted a field experiment by increasing caterpillar density around nests of two abundant Dorymyrmex ant species in the semiarid Patagonian steppe, where D. tener allocates a higher number of foragers to resource exploitation than D. antarcticus. We (1) compared the association between predation effectiveness (success to complete a task) and efficiency (speed of task performance and economy of foragers) with the number of foragers involved between species, and (2) studied how they responded numerically to increasing prey densities, by sequentially adding 3, 6 and 12 larvae in the same foraging arena. Finally, (3) we compared behavioural and morphological traits related to predation between these ant species. Although D. tener discovered a similar number of arenas with larvae than D. antarcticus, it was more effective as it recruited more and removed more larvae. This species was also more efficient than D. antarcticus in all predation subtasks, and the time used to remove one larva depended on prey density, being faster for the high-larvae density. Besides the number of foragers, ant species did not differ in other behavioural traits, and D. tener's foragers were slightly larger than those of D. antarcticus. This study illustrates how, in social predators, the predator group size and individual behavioural characteristics may act in conjunction, with relevant consequences at ecological, evolutionary, and applied levels, including potential implications for pest control.