The shape of things eaten: the functional response of herbivores foraging adaptively

A major question at the interface of community and behavioural ecology is whether optimal foraging influences patterns of population dynamics. One productive route towards addressing this question has been to treat the solution to an optimal foraging model as an "adaptive" functional response, capable of varying with resource density. Here I integrate resource density into an optimal foraging model by allowing consumption rates, part of the time constraint in the foraging model, to vary in a linear or saturating manner with resource density. Using field estimates to parameterize the model for 11 herbivore mammals and four herbivore Orthoptera, the modeling effort produces a novel set of functional responses that display unique combinations of traditional functional response shapes. These new adaptive functional response shapes arise because of the assumptions about physiology, resource quality and environmental conditions that are embedded in a foraging model. The results of the study indicate that there is 1) resource and body size specific changes in resource intake as resource density varies; 2) a sensitivity of intake to the shape and magnitude of the cropping rate variation; and 3) that herbivores may regulate their resources via the functional response.