Distribution of a naturally fluctuating ungulate population among heterogeneous plant communities: ideal and free?

1. Herbivore distribution is often assumed to follow the ideal free distribution (IFD) model. This assumes that organisms are omniscient about forage quality and availability within the area available to them and are free to move, with negligible cost, throughout this environment. If this were the case we would expect that, at lowest densities, all animals would be found in the best habitat patches, with less desirable habitats being occupied stepwise as population density increases. We test this using data from a naturally fluctuating population of feral Soay sheep. 2. We show that, although the distribution of individuals is correlated positively with food quality, in line with patterns reported for hill sheep in Scotland, their distribution does not conform to the predictions of the IFD model. We argue that it is the dynamic nature of their food resource that causes this departure from the predictions of the IFD model and make the case that the IFD model, in its unmodified form, is inappropriate for use in modelling distribution among patches containing dynamic resources.