Group-living folivores and herbivores typically feed on continuously distributed and seemingly abundant resources and are thus thought to experience low levels of feeding competition. While low rates of contest competition observed in folivorous primate species are consistent with this presumed lack of competition, their generally small group sizes are not. Even continuously distributed resources are not uniform and have intrinsic heterogeneity. Animal foraging patterns can increase such heterogeneity. We hypothesize that these two sources of heterogeneity – intrinsic and induced – influence patterns of feeding competition in herbivores. Using a spatial agent-based model of animal foraging on a continuously distributed food resource of variable quality (where intrinsic heterogeneity is modeled using semivariogram functions), we explore the impacts of food resource quality and spatial heterogeneity on individual energy intake, travel expenditure and contact patterns.
We find that feeding competition always occurs and significantly reduces folivores’ energetic balance, even when food resources are abundant. As expected, intrinsic and induced fine-scale heterogeneity drives contest competition. Scramble competition, the indirect loss of resources to other foraging individuals, always occurs, and can have a strong impact on animal’s energetic balance: groups deplete their immediate surroundings, thereby decreasing the amount of food per capita in this area. Although this form of competition is difficult to measure in the field, our model shows that it is likely pervasive and may explain the small average group sizes observed in many folivorous primates.