1.1.6 Positive and Negative
• Living systems display the following 3
characteristics: functionality, sustainability,
• 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
Examples of Forrester’s “Causal Loop
• This one illustrates a Balancing Loop or a
negative feedback loop.
• 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
• “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
• 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
• 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 predatorincrease 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.