A spatial model of producing and scrounging

When foraging in groups, foragers in producer mode search for food independently and foragers in scrounger mode search for opportunities to join the food discoveries of other companions. Producer–scrounger (PS) models predict that at equilibrium scrounging will increase with group size and with patch richness but will be independent of food patch encounter rate. Earlier PS models assume that foragers arrive instantaneously at scrounged food patches, which is not realistic at the habitat level, at which food patches can be far apart. Here, I develop a spatially explicit model of producing and scrounging in which individuals pay a time cost when travelling to scrounged food patches and may join patches with little or no food left when they arrive. Using a genetic-algorithm approach, I confirm that scrounging increases with patch richness and add several novel predictions. In particular, scrounging is expected to increase with a decrease in food patch encounter rate and can actually decrease when forager density is high. In habitats in which food intake rate increases with the number of foragers at food patches, scrounging at low forager densities is actually predicted to increase success. The spatial model also highlights the self-organizing nature of scrounging. Because scrounging reduces the average distance between foragers, thus reducing the costs of joining, attempts to scrounge make further scrounging more likely to evolve. The spatial model of producing and scrounging provides novel insights into the evolution of group foraging.