Density-Dependent Habitat Selection: Evaluation of the Isodar Method

Abstract

We have tested the performance of the isodar method, in detecting density-dependent habitat selection, underlying modes of community organization, and competitive interaction. For that purpose, we used data obtained from intensive manipulation experiments conducted on two granivorous, desert gerbil species, Gerbillus allenbyi and G. pyramidum. The isodar method detected, in both species, density dependent habitat selection. 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 species. The G. pyramidum fitness-density graphs are two parallel straight lines. The G. allenbyi fitness-density graphs are two converging straight lines. Isodars estimated for mixed populations agree with the prediction that they should not differ from that calculated for each species in isolation. This point implies that isodars can be used to detect habitat preference of single species even though they exist in a community of competitors. The isodar method was also able to detect the shared preference community organization pattern of the two gerbil species. We concluded that the isodar method is a useful tool in detecting density-dependent habitat selection and community organization pattern. But we had to conclude that the isodar method is very limited in estimating interspecific competition. The method was not able to detect interference competition between the two gerbil species although results of manipulation experiments showed that it is the key process in the gerbil system. Also, the isodar method assumes that competition is fixed (density independent). This is a major flaw since all theories of optimal foraging, including density dependent habitat selection, predict that competition coefficients are density dependent. Thus, although the isodar method was able to estimate significant exploitation competition for the two species, we are not sure if it means anything. This is because the estimated isoclines of the two species, using field manipulations, revealed that the isoclines are highly nonlinear and that the magnitude of the interaction, for each density combination, depends on the degree of habitat overlap of the two species. Thus, habitat specific constant interaction coefficients estimated by the isodar method probably mean very little, if anything