Physiological Basis of Ratio-Dependent Predator-Prey Theory: The Metabolic Pool Model as a Paradigm

Ratio-dependent models of predators and prey are approximations of the biology of resource acquisition and allocation and their consequences for population birth and death rates. A demand-driven functional response model that has a physiological basis in mass (energy) dynamics is reviewed (i.e., the metabolic pool model), and its obvious links to the logistic model are outlined. To demonstrate the utility of this approach, a distributed maturation time age-structure model of the dynamics of A. J. Nicholson's classic laboratory population data on the sheep blow fly (Lucilia cuprina Weidman) is developed. The model provides sufficient information on the dynamics of the intermediate life stages to show that the blow fly oscillations were due to the effects of larval competition for food on size, fecundity, and pupation success. These results agree with Nicholson's conclusions. The advantage of this model, in contrast to prior models, is that the dynamics emerge by considering the processes of resource acquisition and allocation as they affect growth, reproduction, and survival. No explicit time delays, which automatically lead to oscillations, were included. Lastly, the notions of the metabolic pool model are found in Nicholson's original model for equilibrium popu- lation density. The metabolic pool paradigm in an age-structure setting is used to model the tri-trophic dynamics of Acyrthosiphon aphids in an alfalfa ecosystem. The model explains the role of the various natural enemies in the regulation of the aphids.