1.1.6 Positive and Negative Feedback

Living systems display the following 3 characteristics: functionality, sustainability, evolution.

The way that living systems and even inanimate systems self-regulate or maintain homeostasis (the maintenance of a steady state in an organism, ecosystem or biosphere) is through feedback systems

Negative feedback systems Negative feedback systems include a sequence of events that will cause an effect that is in the opposite direction to the original stimulus and thereby brings the system back to its equilibrium position.

Examples of Forrester’s “Causal Loop Diagrams” This one illustrates a Balancing Loop or a negative feedback loop. http://www.systems-thinking.org

Positive feedback Positive feedback includes a sequence of events that will cause a change in the same direction as the stimulus and thereby augments the change, moving the state of the system even further from the equilibrium point.

A Reinforcing Loop or a positive feedback loop

James Lovelock “James Lovelock argues that such things as the level of oxygen, the formation of clouds, and the saltiness of the oceans may be controlled by interacting physical, chemical and biological processes. He believes that "the self-regulation of climate and chemical composition is a process that emerges from the rightly coupled evolution of rocks, air and the ocean - in addition to that of organisms. Such interlocking self-regulation, while rarely optimal - consider the cold and hot places of the earth, the wet and the dry - nevertheless keeps the Earth a place fit for life." The New York Times Book Review has called his arguments in favor of Gaia "plausible and above all illuminating."

The following diagram depicts negative feedback when the body temperature goes up above the set point

an example of the homeostatic control of blood glucose levels through negative feedback using insulin and glucagon.

Negative feedback mechanisms in the nervous system are depicted below.

Predator/prey relationships are usually controlled by negative feedback where: The increase in prey-increase in predator- decrease in prey decrease in predatorincrease in prey---and so on in a cyclical manner. The classic study in Northern Canada between the Lynx and the hare populations is famous for its regular 11 year cycle of rising and falling populations.