Combined effects of density and predation on the foraging behaviour of gerbils

Intraspecific competition and predation are core interests of ecology due to their ability to influence population dynamics. While these interactions have been investigated at length, non-lethal effects of predation and their interaction with intraspecific competition are still poorly understood. In this study, we placed two species of gerbils (Gerbillus andersoni allenbyi and Gerbillus pyramidum) in a vivarium where we could manipulate predator (barn owl, Tyto alba) presence and gerbil density, while keeping both species isolated. In that setting, we quantified gerbil foraging behavior by measuring the time they spent in food patches and the vigilance they employed in those patches. Food patches varied in risk (probability of being preyed upon), depending on moonlight and presence of owls. We expected that increased competition would decrease gerbil vigilance and lead them to use more dangerous patches. We found support for that expectation. Intraspecific competition suppressed the response to predators in contrast to a predator-mediated competition, where predation suppressed competition. Both species of gerbils behaved differently at high-density with no predators, with G. a. allenbyi decreasing vigilance in the face of predators while G. pyramidum increased vigilance in predators absence. This difference may be caused by G. a. allenbyi facing stronger intraspecific competition than G. pyramidum. If it were subjected to high degrees of intraspecific competition, G. a. allenbyi may experience less interruptions in foraging bout when the owl is present as it might reduce the number of confrontations with competitors. Thus, the interaction between predation risk and competition can be diverse even among species that are in the same genus. Clearly, there is a wealth of knowledge to be studied in this area.