Generalist herbivore foraging and its role in competitive interactions

Abstract

Whether herbivores are food limited and compete interspecifically for food has been debated by population/community ecologists. To examine this proposition, a mechanistic approach based upon autecological observations is employed:. 45 foraging studies of 20 species of generalist herbivores indicate that the herbivores behave as energy maximizers in their diet selection, as determined by a linear programming model.The constraints that determine the forager's energy maximizing diet can be used to determine whether a food item should be consumed, based upon a minimum digestibility that determines energy value and a minimum item size or abundance that determines cropping rate.Population densities of a number of herbivore species depend upon the biomass of plants in the environment that satisfy the minimum characteristics sought by the herbivore.Using experimental populations for several herbivore species that are known to be food limited, it can be demonstrated that these species compete with each other. The competitive isoclines are non-linear and arise from the fractions of each forager's food resources that it shares and exclusively uses, which depend upon each species' minimum food characteristics.The results indicate that a mechanistic approach can provide considerable insights into herbivore community structure. The foraging constraints underlying the entire analysis may be body size dependent, providing a more general view. © 1986 by the American Society of Zoologists.