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The isoclines of Gerbillus allenbyi competing with G. pyramidum are negatively sloped and at least approximately parallel. They are not linear, however; they are steeper over both low and high G. pyramidum densities and only very slightly negative at intermediate densities. This nonlinearity agrees with the predictions of the theory of optimal density-dependent habitat selection in a two-species, shared-preference system, the system that obtains here.

The isocline field was measured by a new method based on the single-species habitat selection theory of Fretwell. The species whose isoclines are being measured (the target species) is given a chance to equilibrate its per capita reproductive rate across a fence that separates two densities of its competitors. Equilibration requires that the target species distribute its density unequally on the two sides of the fence. The general features of the method may be useful in other animal studies where densities can be experimentally manipulated.