The shape of a Gerbil isocline measured using principles of optimal habitat selection

The isoclines of Gerbillus allenbyi competing with G. pyramidum in Israeli sandy deserts are negatively sloped and at least approximately parallel. 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 2-species, shared-preference system. The isocline field was measured by a new method. 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 2 densities of its competitors. Equilibration requires that the target species distribute its density unequally on the two sides of the fence. -from Authors