THE SHAPE OF A GERBILLUS-PYRAMIDUM (RODENTIA, GERBILLINAE) ISOCLINE -AN EXPERIMENTAL FIELD-STUDY

Abramsky et al. developed a new technique to measure isoclines in the field. The method is based on the single-species habitat selection the ory of Fretwell. Using short term (3-4 wk) and long term (1-yr) experi ments, they tested it by measuring the isocline of Gerbillus allenbyi competing with G. pyramidum. In the present study, we measured the con verse isocline of G. pyramidum competing with G. allenbyi. Like the is ocline of G. allenbyi, the isocline of G. pyramidum is nonlinear and a grees with the predictions of the theory of optimal density-dependent habitat selection in a two-species, shared-preference system. This is the first natural system in which both sets of isolegs and isoclines h ave been measured, in the field, for two competing species. A stabilit y analysis suggested that the gerbil species can coexist under most ob served density combinations.