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Question: Why does the habitat distribution of some populations not vary with density?

Hypotheses: Two theories predict that each habitat will contain a fixed proportion of a varying population: (1) habitats differ only qualitatively but are identical quantitatively, at constant carrying capacities (prediction of Morris’s isodar theory), and (2) habitats differ only quantitatively, carrying capacities are variable, and densities fluctuate around carrying capacities (prediction of our paraisodar model and modification of isodar theory).

Organisms: Wagner gerbil, Gerbillus dasyurus.

Field sites: Two areas in the central Negev Desert (Israel), 13 km apart. In one the substrate is loess, in the other it is gravel.

Methods: We estimated gerbil density with mark-recapture techniques on eight sampling grids in each area. We estimated fitness as the ratio of the number of young individuals at the end of the breeding season to the number of females at the beginning of the breeding season. We estimated resource abundance as the amount of seeds in the seed soil bank. We estimated habitat quality as a short-term population reaction to experimental food additions.

Conclusions: Resource abundance differed significantly between habitats, and varied among years in accordance with rainfall. The density of G dasyurus was positively correlated with resource abundance. Per capita fitness was equal between habitats within years, varied among years, and was positively correlated with the amount of rainfall. Food addition, habitat, and season all influenced local density. But there was no synergistic effect between habitat and supplemental food (a quantitative difference). Constant niche breadth in G dasyurus is caused by density-vague dynamics around varying carrying capacities. Habitat distribution thereby appears to be independent of density, even though the gerbils appeared to select habitats in a density-dependent manner.