FIGURE 1. Testing for an ideal free distribution.

a, Evolutionary fitness declines more quickly with increasing population density in habitat 2 than it does in habitat 1. Symbols at the intersections with horizontal lines represent densities where fitness is equal in both habitats (an ideal free distribution). b, The equilibrium densities can be plotted against each other to yield the expected distribution of individuals in the two habitats (the habitat isodar⁷). Haugen et al.² do this for the northern and southern basins of Lake Windermere, and compare these with actual data for pike populations collected since the 1940s. The agreement between the model calculations and the data is astoundingly good.