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.