Selection of chemostat type an turbidostat type

Although the death rate and the rate of reproduction of organisms in time vary very irregularly and very strongly, from the long-term perspective, the sizes of the populations of the individual species remain constant. This long-term stability can be ensured only by the existence of some kind of negative feedback regulating the size of the population and compensating random effects of the varying intensities of reproduction and death. In principle, there can be only two types of this feedback and technical laboratory models exist for both types (Flegr 1997)(Fig. IV.3). The first of these, “top-down regulation”, can be modeled in the laboratory in continuous cultivation systems of the turbidostattype. In this system a sensor (mostly optical) monitors the size of the population and, when it increases, the instrument increases the flow of nutrient medium through the cultivation vessel and thus increases the rate of flushing organisms out of the vessel. Thus, an increase in the population increases the rate at which individuals are flushed out of the vessel, subsequently leading to reduction in the size of the population to the original value. In nature, negative feedback of the turbidostat type is functional for systems in which an increase in the population leads to an increase in the death rate of its members. This occurs, e.g., in populations in which the size of the population of prey is regulated by the activity of predators. An increase in the number of prey leads to an increase in the number of predators, leading to increased predation and thus to a decrease in the size of the population of prey (and subsequently to a decrease in the size of the population of predators). Similar negative feedback exists in systems in which the size of the population is regulated by the action of an infectious agent (a contagious disease, a parasite). In this case, the effectiveness of spreading of an infectious agent is frequently directly proportional to the number of contacts between members of the population and the frequency of these contacts is directly proportional to the density of the population of hosts or, to be more precise, to the square of the population density.

The second type of negative feedback, “bottom-up-regulation” is modelled in continuous cultivation systems of the chemostat type. In these systems, nutrient medium flows into the cultivation vessel at a constant rate. If the population increases, nutrients are consumed more rapidly from the medium, their concentration decreases, the organisms begin to suffer from a lack of nutrition and the rate of reproduction is reduced. Thus, the natural death rate predominates over the rate of reproduction and the size of the population begins to decrease, the consumption of nutrients also decreases and the rate of reproduction of the population increases again. In nature, this type of negative feedback occurs everywhere where the size of the population is limited (and regulated) by the amount of some resource. For example, the population of predators in the previous case is limited by the size of the population of prey; however, a population can be similarly limited by any scarce resource, such as the number of available hiding places. In case of regulation of the population by lack of hiding places, the “superfluous individuals” are finally eliminated by predators, but the primary reason for their superfluousness is that lack of a resource (hiding places) and thus the growth of the population are regulated by negative feedback of the chemostat type.

The type of negative feedback determines which of the parameters of the organism will decide on the success of the individual and thus which will be the subject of natural selection. It follows from theoretical analysis (Flegr 1997) that the maximum rate of reproduction is the critical parameter in systems of the turbidostat type, e.g., the number of glucose molecules that the organism is capable of converting to biomass per time unit in the presence of an excess of all resources. In contrast, in systems of the chemostat type, the critical parameter is the effectiveness of utilization of a limiting resource, thus, e.g., the number of ATP molecules that a given individual is capable of forming from one glucose molecule (Fig. IV.4). Recalling the properties of r-strategists and K-strategists in the previous part, it can be seen that the properties of r-strategists, i.e. greater rate of reproduction, shorter life cycle, greater number of not very fit progeny, poor ability to compete with other species in stabilized biotopes, can be interpreted as the result of selection for the maximum rate of reproduction under turbidostatic conditions, while the opposite properties of K-strategists can be interpreted as being a result of selection for maximum effectiveness utilizing a limiting resource. This means that the long-known existence of two distinct ecological strategies could be related to the existence of two, and only two, types of negative feedback capable of maintaining a constant size of the population.

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