2.3 Negative feedback and stability
If positive feedback results in change, then another mechanism must exist that creates stability. This is negative feedback.
What stops water hyacinth from taking over the world? Clearly, it is the lack of tropical freshwater. As the number of water hyacinth reaches the limits of their water body, there is a sudden increase in the death rate as offspring compete for the ever decreasing levels of sunlight. The sudden overcrowding allows the establishment of a negative feedback loop, where reaching the full surface coverage of the water body results in a decrease in plant viability. Thus, the water hyacinth population has now reached an equilibrium with the water body surface area: in years of drought, the water body area will decrease and so will the water hyacinth population, while in years of plentiful water, the increasing water body area will allow the water hyacinth to expand again.
A second example of a negative feedback loop is a simple model of human sleep. Here we can connect the ‘amount of sleep’, the number of hours slept each night, with the ‘amount of tiredness felt’ (see Figure 4.3). This variable is perhaps difficult to define but it is easy to understand what is meant, since you understand your own perception of tiredness. It may be the case that the more you sleep the less you may feel tired, and the less tired you feel the less you may sleep, so again you have a reversal of direction. Each of us has our own particular pattern of sleep, a particular number of hours per night, but for everyone if that pattern is disturbed the tendency is always to revert back to the normal pattern after a few nights.
Negative feedback loops therefore result in stability – so if you want stability you must use this model. One of the first uses in engineering was the famous example of Watt’s Conical Pendulum Governor (Ewing, 1899) the function of which was to stabilise the speed of the steam engine. Any tendency for the speed of the engine to rise was corrected by cutting off the steam, and any decrease by allowing in more steam, thus keeping the steam engine stable, working at constant speed.