A of negative feedback governing the system,

A cybernetic system, which can also be an open system, has some sort of feedback system to make it self-regulating. It main­tains itself by some forms of information feedback into the system. To function in such a manner the cybernetic system has an ideal state or set pohrt about which it operates. In a purely mechanical system that set point can be fixed specifically. For example, a de- humidifier which is set for a humidity level of 50 per cent repre­sents a cybernetic system.

When the humidity of the air in a room exceeds 50 per cent, the switch on the dehumidifier turns on and the fans start to pull air over the refrigerated coils on which the water condenses, to be carried away by a hose. When sufficient water has been wrung out of the air, the dehumidifier shuts off.

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The feedback of information on humidity that causes the dehumi­difier to shut off or turn on is called negative feedback, because it halts or reverses a movement away from a set point.

Most living systems possess cybernetic systems that can func­tion at various levels, from the cell to the community, but always functioning through organisms —the difference being that in living systems the set point is not fixed.

Rather, organisms have a limited range of tolerances, called homeostatic plateaus, within which conditions must be maintained. If environmental conditions exceed the operating limits of the system, it goes out of control. Instead of negative feedback governing the system, positive feed­back takes over.

Positive feedback is a continual movement away a system’s set point or homeostatic plateau that ultimately can destroy the system. A feedback loop can be defined as the relationship in which a change in some original state of being ‘feed­back” onto that original state to alter the rate or direction of further change.

An example of a cybernetic control in a living system is temperature regulation in homeothermal animals, such as human (Fig. 7.1). The normal temperature for humans is 98.6 ?F. If the temperature of the environment rises, sensory mechanisms detect it, send the message to the brain which acts (involuntarily) on the information and relays the message to the effectors mechanism that increase blood flow to the skin and induce sweating. Water excreted through the skin evaporates, cooling the body.

If the environmental temperature falls below a certain point, a similar in the system takes place, this time reducing blood flow and causing shivering, an involuntary muscular exercise producing more heat. If environmental temperature becomes extreme, the cyber­netic system breaks down. If the temperature becomes too hot, the body is unable to lose heat fast enough to hold the tempe­rature at normal.

Body metabolism speeds up further increasing body temperature and eventually ending in heat stroke. If the environmental temperature drops too low, metabolic proces­ses slow down, further decreasing body temperature and eventually resulting in death by freezing.

Cybernetic systems produce stable systems. A stable system is one that responds to stimuli by developing forces to restore it to its original condition. In natural ecosystems this stability may mean that the system can survive many changes but still preserve a similar structure. Stability also means persistence, i.e., the system remains much the way it was.

Fig. 7.2 shows an ecological example of positive and negative feedbacks. In both, we start with a given number of animals. In the positive feedback loop, production of a large number of offspring leads to a larger population than the original. This then becomes the new reproducing population, which can produce even more offsprings.

The result is a self-sustaining population increase, Driven by the offspring feeding back into the breeding population in the same way, if more breeders die than are replaced each year (which happens in some populations), this loss will be reflected in a smaller breeding population and will result in self-sustaining decline in population number by the same type of the positive- feedback loop.

In the negative feedback-loop, the population is controlled by external factors, such as food supply. If the popu­lation increase in size, there will not be sufficient food to go around, and more individuals than usual will die from starvation ; conversely, if the population decrease in size, there will be more than enough food to go around, and the animals will be well- nourished, leading to a smaller than normal die off.

In either case, the equilibrium population size is regained4 Most interactions between different species of organisms, and many interactions bet­ween a given species and its physical environment, depend on either positive or negative feedback. All organisms in an ecosystem are part of several different feedback loops at any point in time. Some of these relationships are negative, while others are positive.