Isodar theory was applied to the analysis of habitat selection by a diurnal folivorous gerbil species, Psammomys obesus, in the Central Negev Desert, Israel. In this site, the species demonstrated seasonal habitat shifts, from densely vegetated and well protected dry river beds (wadi) in winter and spring to open and less protected flat terraces in summer and autumn. These shifts were associated with different phenologies of the main foraging plants in the two microhabitats (winter vegetation of Atriplex halimus in the first and summer vegetation of Anabasis articulata in the second). The life history of P. obesus, thus, suggests that in winter and spring the wadi is better both quantitatively and qualitatively, whereas in summer and autumn, river beds are quantitatively poorer but qualitatively better than the flat terraces. Thus, the winter distribution of P. obesus should fit the divergent model of population regulation for quantitatively different habitats while the summer distribution should fit the convergent or crossover model. The analysis of long-term data supports the predictions for summers and winters following dry autumn conditions but supported the divergent model of population regulation for winters following autumn seasons with high precipitation. The result is consistent with foraging by P. obesus in winters after wet autumns not only on Atriplex and Anabasis, but also on fresh annual plants that are abundant during such winters in both habitats. This assumption was tested and supported by the analysis of annual data on micro-habitat distribution and diet composition. Thus, the isodar analysis can be used not only as a descriptive technique for habitat selection patterns, but also as a powerful tool for natural history studies. The tight connection between food plants and habitat use is clear evidence that foraging and habitat selection are linked through their joint interactions with the ultimate mechanism maximizing fitness.