Linking consumer-resource theory and digestive physiology: Application to diet shifts

We develop mathematical and graphical models of diet selection incorporatin g recent advances in digestive physiology - that is, the adaptive modulatio n of active nutrient transport and gut retention time in response to change s in diet composition - into an explicitly ecological context based on cons umer-resource dynamics and optimal foraging theory. The models indicate tha t gut modulation causes the consumer to treat two resources that are perfec tly substitutable (the benefit derived from consumption of one resource is a constant fraction of the benefit derived from consumption of the second r esource) as if they are antagonistic resources (the benefit derived from co nsumption of either resource alone is greater than the benefit derived from joint consumption of both resources). This will tend to favour diet switch ing and specialization, and also suggests a definitive (laboratory or field ) test of the models. The models also suggest that modulation ultimately le ads to more efficient use of resources, although it incurs an initial cost. We further cast gut modulation in three ecological scenarios. In the first , the consumer species does not deplete its resources and the optimal modul ation strategy is determined by the standing crop of resources. In the seco nd, a fixed population size of consumers results in resource depletion and the standing crop of resources results from a dynamic equilibrium between r esource renewal and resource consumption. Under this scenario, the optimal gut-modulation strategy is determined by this dynamic equilibrium between r enewal and consumption. In the third, we let resource renewal, depletion an d consumer population sizes equilibrate. In this last scenario, the optimal gut modulation strategy emerges from the combined effects of resource rene wal and the intersection of the depletion trajectory with the consumer's ze ro net growth isocline. We conclude that forging tighter links between gut physiology and foraging ecology will lead to greater understanding and pred ictability of diet selection and its ecological consequences.