The Shape of a Gerbil 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 theory of Fretwell. Using short term experiments (3-4 wk), they tested it by measuring the isocline of Gerbillus allenbyi competing with G. pyramidum. In the present study, we measured the isocline again using a one year experiment. In this experiment we fixed the numbers of G. pyramidum in six 1-ha enclosures but allowed individuals of G. allenbyi to move between adjacent enclosures and the surrounding environment. The resulting isocline of the present study was very similar to that obtained during the short term study. The isocline was nonlinear and agreed with the unique predictions of the theory of optimal density-dependent habitat selection in two-species, shared-preference systems.