Changes in Foraging Effort in Two Gerbil Species Correlate with Habitat Type and Intra- and Interspecific Activity

The effect of intraand interspecific competition on the foraging effort of two gerbil species, Gerbillus allenbyi and G. pyramidum, was studied in six 1-ha enclosures located in an area containing semistabilized dunes and stabilized sand fields in the Negev desert, Israel. It was found in both species that per capita foraging and non-foraging activity was negatively and significantly correlated with its own population density. When G. pyramidum was common, G. allenbyi limited most of its activity to the stabilized sand while most G. pyramidum activity occurred in the semistabilized dunes. However, with none or one G. pyramidum present, most G. allenbyi activity was shifted to the semistabilized dune. Per capita activity of both species declined significantly as a function of the increasing activity of the other. However, the decline in per capita activity of G. allenbyi in the presence of G. pyramidum was significantly greater than the decline in per capita activity of G. pyramidum in the presence of G. allenbyi. In the stabilized sand, per capita activity of each of the two species was also significantly correlated with the density of the other species. The density of G. allenbyi was positively correlated with per capita activity of G. pyramidum and the density of G. pyramidum was negatively correlated with per capita activity of G. allenbyi. Several possible mechanisms that might explain these interactions are discussed. We concluded that the asymmetric interspecific competition between G. allenbyi and G. pyramidum probably fits the shared preference habitat selection model.