The theory of population dynamics-II. Physiological delays

Traditional population models describe changes in population size as a function of changes in the resources. Such first-order models cannot describe certain properties of population dynamics. General models with delays can account for all the observed dynamic complexities but are judged overgeneralized. It is proposed that the simplest model of intermediate complexity that explains such dynamic properties is a second-order model, which describes population dynamics as a function of a physiological variable, the dynamics of which in turn depends on resources. Data on accelerated decline of populations in the absence of food from experiments with brown and green hydra as well as literature data support the arguments.