An isodar, or habitat isodar, is a theory in evolutionary ecology developed in the late 1980s by Douglas W. Morris.[2][3] Isodars model density-dependent habitat selection for one or two species in two habitats according to the ideal free and ideal despotic distributions. Isodar is a two-part word: "iso" meaning equal in Latin; "dar" for Darwinian evolution, and is defined as all combinations of population densities in habitats A and B such that both habitats offer the same fitness reward.
Animals displaying an ideal free distribution distribute themselves among patches (sources of a resource) in such a way that each individual get the same amount of the resource. For example, if food is twice as abundant in habitat A compared to habitat B, the ideal free distribution-based model would predict that there will accordingly be twice as many animals competing for food in habitat A compared to habitat B. If the total number of animals is considered to be variable, there are two ways this can be plotted on a two-dimensional Cartesian graph. One way is to plot two lines on a graph of fitness vs. density of individuals. This graph can be used to predict the density of individuals at any given level of fitness. The second is to plot the density of individuals in habitat A vs. the density of individuals in habitat B when the fitness of all individuals is equal. The line in this second graph is an isodar line.
Ideal free isodars predict that a species density in habitat A will increase linearly with its density in habitat B so that each individual in the species has the same fitness. If habitat A has higher quality resources than habitat B, then proportionately more individuals would be in habitat A then in habitat B. This can be shown on either a Fitness-Density graph (Figure 1) or a graph of density in two habitats (Figure 2).
