DENSITY-DEPENDENT HABITAT SELECTION - EVALUATION OF THE ISODAR METHOD

We have tested the performance of the isodar method, in detecting dens ity-dependent habitat selection, underlying modes of community organiz ation, and competitive interaction. For that purpose, we used data obt ained from intensive manipulation experiments conducted on two granivo rous, desert gerbil species, Gerbillus allenbyi and G. pyramidum. The isodar method detected, in both species, density dependent habitat sel ection. The isodar method is a good tool to assess differences between habitats, shape and general location of fitness-density graphs. Using the isodar method we discovered new details about the two gerbil spec ies. The G. pyramidum fitness-density graphs are two parallel straight lines. The G. allenbyi fitness-density graphs are two converging stra ight lines. Isodars estimated for mixed populations agree with the pre diction that they should not differ from that calculated for each spec ies in isolation. This point implies that isodars can be used to detec t habitat preference of single species even though they exist in a com munity of competitors. The isodar method was also able to detect the s hared preference community organization pattern of the two gerbil spec ies. We concluded that the isodar method is a useful tool in detecting density-dependent habitat selection and community organization patter n. But we had to conclude that the isodar method is very limited in es timating interspecific competition. The method was not able to detect interference competition between the two gerbil species although resul ts of manipulation experiments showed that it is the key process in th e gerbil system. Also, the isodar method assumes that competition is f ixed (density independent). This is a major flaw since all theories of optimal foraging, including density dependent habitat selection, pred ict that competition coefficients are density dependent. Thus, althoug h the isodar method was able to estimate significant exploitation comp etition for the two species, we are not sure if it means anything. Thi s is because the estimated isoclines of the two species, using field m anipulations, revealed that the isoclines are highly nonlinear and tha t the magnitude of the interaction, for each density combination, depe nds on the degree of habitat overlap of the two species. Thus, habitat specific constant interaction coefficients estimated by the isodar me thod probably mean very little, if anything.