The term 'feedback' refers to the output controlling or influencing the input. The cyberneticists, who recognised two kinds of feedback, introduced the concept of the 'feedback loop'. And they called self-balancing feedback 'negative feedback', and self-reinforcing feedback 'positive feedback'.

Negative feedback is illustrated by all homeostatic mechanisms. For example, the automatic steering of a ship exhibits a phenomenon that engineers call 'hunting'. That is, the ship tends to veer slightly away from its proper course, and the homeostatic steering mechanism brings it back again. But it may then veer slightly off course in the other direction, and the steering mechanism brings it back again. This control, this homeostasis, which returns the ship to its optimum, is negative feedback which, obviously, is self-balancing and valuable.

The resilience, flexibility, and overall stability of a complex adaptive system, such as an ecosystem, are a consequence of its many negative feedback loops. For example, if a parasite becomes excessively damaging, the host population accumulates resistance to it, and the parasitism returns to its normal level. It is this feedback which brings the system back to its optimum whenever there is a deviation from the norm.

Positive feedback, being self-reinforcing, is often destructive. In common usage, it is often called a vicious spiral. With positive feedback, there is an exponential increase. That is, the rate of increase is itself increasing.

An example of positive feedback is the self-fulfilling prophecy. If false rumours circulate that an airline is about to go bankrupt, passengers will avoid it for fear of cancelled flights and valueless tickets. There is then a rapid loss of business, which leads to increasingly strong rumours of bankruptcy. These increasing rumours are positive feedback, and the previously solvent airline loses so much business that it may well go bankrupt. A similar positive feedback can lead to a stock market crash. Another example is the ear-splitting howl that often develops in a public address system. The microphone picks up the hum of the speakers. The hum is then amplified, and the hum of the speakers increases. Positive feedback makes the hum rapidly louder and louder.

Perhaps the most important example of positive feedback in biology is the population explosion. The total reproduction of a population depends on numbers of individuals. As reproduction increases, so numbers increase, and the rate of increase itself increases, in a vicious spiral. Species that are capable of very rapid population explosions are called r-strategists (see 1.13) and they are of special significance in crop pathosystems.

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