A model for territory and group formation in a heterogeneous habitat

Abstract

We present a simple model for the formation of territories in habitats containing discrete food patches and for the formation of social groups based on these territories. Territories are determined by the distribution of food resources; they are assumed to be as small as is possible while providing adequate food resources for their primary occupants, allowing for the discrete distribution of food, and its variability over time. We show that such territories may often contain sufficient resources to support additional animals at no cost to the primary occupants, especially if individual food patches have a high yield, the variation between patches is large, or the requirements of secondary animals are smaller than those of primary animals. The model thus gives a rigorous basis for the resource dispersion hypothesis of Macdonald (1983, Nature, Lond. 301 , 379–384.) The model predicts that, provided the variation between patches is not too small, the correlation between group size and territory size within such a habitat is likely to be low. The correlation between the amount of resources a territory contains and the size of the group it can support, on the other hand, is likely to be high. In addition, the model predicts that, counter-intuitively, group size does not necessarily increase as the variability of yield over time increases: in some cases, group size may decrease with increasing variability of yield. The main predictions of the model are shown to be unaffected by small changes in parameters, and by changes in some of the model's assumptions.