Resolving the departures of observed results from the Ideal Free Distribution with simple random movements

1. The Ideal Free Distribution (IFD) is one of the most widely applied theoretical concepts in ecology. Many experimental tests of the IFD incorporating unequal competitors have revealed three consistent departures of observed results from those predicted by the theory. Specifically these are: (1) the full range of predicted equilibria are rarely realized; (2) under-matching of individuals to resource inputs; and (3) continued movement of individuals after equilibrium is reached. 2. It has been suggested previously that a model including simple random movements can explain simultaneously all three of these departures. The acceptance of this theory has been hindered by arguments about model construction relating to the timing of events and whether an analytical or individual-based model is more appropriate. 3. Here we present the first internally consistent and biologically realistic model of the IFD incorporating random non-IFD movements (movements made at a rate independent of intake rates). We use this model to confirm that random movements alone can explain all three of the observed departures from the IFD. Furthermore, it is not necessary to have any detailed a priori knowledge about the motivation behind the non-IFD movements, in order to predict their consequences. We expect non-IFD movements to be widespread in animal systems, and so warrant further experimental consideration and investigation