5.2.1 Positive and negative feedback

‘Feedback’ is the term used to describe the situation where the output from a process affects the input to that process. You may have encountered the ‘howl’ that can occur when a microphone is placed too near a loudspeaker; the sound from the loudspeaker feeds back to the microphone, gets amplified and is fed back again so that the volume of sound keeps on increasing until the amplifier overloads. This is an example of positive feedback.

Populations of organisms can exhibit the same effect. If one generation produces more than one surviving offspring per adult, there are more organisms to produce young in the next generation, who produce more young in the next, and so on. This leads to a population explosion. Economic growth is supposed to work the same way – increased wealth this year allows us to spend and invest to produce more wealth next year, with this continuing year after year.

Of course, the sound from the loudspeaker cannot get louder and louder forever, populations of organisms don’t actually go on expanding forever, and whatever economists may say, economic growth is unlikely to continue unchecked. The sound from the speaker is limited by the power available to the amplifier, and populations can be limited by their food supply. These limits can either have an effect like running into a brick wall, or be more subtle.

The subtler version is the phenomenon of negative feedback, where an increase in the output from the process causes the process itself to ‘slow down’, so that output returns to a lower level. A room thermostat is a classic example. If the room warms too much, the thermostat reduces the central heating output to let the room cool to the correct temperature. Populations offer another negative feedback situation. When there are more organisms present, there is likely to be less food available per individual (or the increased population may attract more predators), so that the rate of production of young decreases (or the rate of mortality increases) and the population tends to stabilise. Negative feedback is a fundamental concept in the control of machinery and electronic devices, and there are many other examples from ecosystems.

Note that the popular uses of ‘positive feedback’ and ‘negative feedback’ are praise and criticism, but the scientific meanings are not inherently ‘good’ or ‘bad’.