Fighting for resources: The economics of defense and appropriation

The study of the ecological factors that govern fighting over resources has been analyzed by resource defense theory and evolutionary hawk-dove games. In essence, the models predict that increasing economic defendability of a resource should lead to increased frequency of aggression among competitors. We question this prediction because it is based on the assumption that individuals that do not possess a resource and must decide whether to attempt aggressive appropriation of another individual's resource face the same economic decision as those that already have possession of a resource and must decide whether to defend against an appropriator's attack. Using an evolutionary game analysis, we explore how changes in group size, clump density, patch richness, and predation hazards affect both the decision to appropriate and defend a resource and interact to predict effects on the frequency of aggression and aggressiveness of individuals. Our model predicts that increasing group size should result in a dome-shaped response of frequency of aggression and a slight decrease in aggressiveness. Increasing clump density is predicted to increase both the frequency of aggression and aggressiveness while increases in clump richness result in reduced aggressiveness at high richness values and a decelerated increase in the frequency of aggression. Finally, predation risk is predicted to increase the frequency of aggression only slightly but to lead to increases in aggressiveness. We compare these predictions to earlier resource defense models. We conclude that most experimental findings agree with our model's predictions but call for explicit experimental tests of our predictions concerning the frequency of aggression and aggressiveness.