DO GERBILS CARE MORE ABOUT COMPETITION OR PREDATION

We used trained barn owls to introduce a controlled predation threat t o two species of gerbil, Gerbillus allenbyi and G. pyramidum in a syst em of 2-ha, sandy-substrate field enclosures in the Negev Desert, Isra el. Using the principles of optimal density-dependent habitat selectio n, we estimated several coefficients of population interaction focusin g on G. allenbyi. G. allenbyi exhibits strong intraspecific competitio n. In the absence of owls, G. pyramidum competes with it (alpha = -0.3 5). We estimated the slope of the G. allenbyi victim isocline to be -0 .60. The competitive effect of G. pyramidum disappeared in the presenc e of owl, although the intraspecific competition remained. Our results indicated that in the presence of owls, the threat of predation overw helms the cost of interspecific competition.