Habitat Selection: An Experimental Field Test with Two Gerbil Species

We provide experimental evidence for the isoleg theory of habitat selection in a pair of psammophilic gerbil species. Gerbillus allenbyi (mean mass: 26 g) and G. pyramidum (mean mass: 40 g) coexist in Israel's Negev desert in areas that may contain three distinct sandy habitats: stabilized sand fields, semistabilized dunes, and drifting dunes. When all three habitat types are available, coexistence between the two species has been explained by a centrifugal model of community organization that has been untested until now. To begin testing it, we conducted a field experiment in six 1 ha enclosures, each containing similar proportions of two of the sandy—habitat types (stabilized sand and semi—stabilized dune). This experiment tested the following hypotheses concerning the coexistence of the two species: (1) both species prefer the same primary habitat type; (2) G. allenbyi and G. pyramidum exhibit intraspecific density—dependent habitat selection; (3) habitat preference of both G. allenbyi and G. pyramidum is affected by the interspecific density of the other species; and (4) in the presence of the two habits of our experiment, the theory predicts that the habitat preferences of the two gerbil species should collapse from a centrifugal to a shared—preference model of habitat selection. We tested these hypotheses during two summers by measuring activity of the two species after introducing predetermined and different densities of gerbils into the enclosures during eight 3—4 wk long temporal replicates. During each temporal replicate, we further manipulated the density of G. allenbyi. Activity and habitat utilization of the rodents was measured by tracks left in the sand. Results of the experiments supported all four of our hypotheses and allowed the construction of their isoleg graph.