Two Distinct Types of Natural Selection in Turbidostat-like and Chemostat-like Ecosystems

The size stability of natural populations indicates that natality and mortality rates are on average equal. Fine tuning of the two independent processes suggests the existence of a regulating mechanism. Two types of such mechanisms exist. A decrease in the number of organisms can either speed up the natality rate or slow down the mortality rate. The former mechanism (chemostat-like) acts whenever the population growth is limited by the concentration of a resource. The latter (turbidostat-like) could be suspected whenever the population growth is limited by predators or parasites. Simulation experiments showed that under chemostat-like and turbidostat-like conditions, organisms are selected toward efficiency (grams of biomass produced/grams of resource consumed) and natality rate (grams of biomass/time), respectively. The existence of two types of selection and the nature of parameters that are selected to recall the old idea of r-K strategies. The main difference is that while the old model predicts the existence of r-K continuum, this model shows that the two strategies are exclusive. 7 1997 Academic Press Limited