Feedback Loops

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Feedback loops are one way to illustrate complexity in a specific system. Feedback describes a sequence of events or interactions that results in a change in the very conditions that started the process.

Figure 1: Example of a Feedback Loop

A feedback loop occurs when a signal leaves one component (output) and returns to the original component (input) after passing through one or more other components in the system. If the signs of the output and input signals are the same (either both positive or both negative), then it is a positive feedback loop. Another way to describe positive feedback is  that it sends a signal to increase the effect in a particular direction. It thus can be destabilizing. Competition (-/-) and mutualism (+/+) can result in positive feedback, as May 1973, 1974 pointed out, because of this NET positive feedback qualitative relation. May 1973 also neatly observed that this may appear to be counter-intuitive.

Negative feedback sends a signal to decrease the effect of the process, thus creating conditions of stability. Eventually, the outcome is so small, it no longer effects the process; the process will then increase in magnitude again until negative feedback is initiated again, causing it to decrease. If the signs of the output and input signals are opposite, then it is negative feedback. Thus, Fig. 1 depicts a negative feedback loop, since the link from predator to prey is negative while the link returning to the predator from the prey is positive. An increase in predators results in a decrease in prey, the decrease in prey then causes a decrease in predators, the decrease in predators then causes an increase in prey, etc. In general, through this moderating effect negative feedback loops promote stability in systems. A positive feedback loops would be depicted by the output and input signals having the same sign, and these type of interactions tend to be destabilizing.

Figure 2: Illustration of a self effect (self-limiting) loop symbol.

Self limiting, self regulating, or density dependent components, such as plants, should include a symbol indicating a “self effect” loop, as shown in Fig. 2. This is required because in nearly all situations the plants are competing for limiting water or nutrients in the soil; any nutrients that go to one individual are thus denied to the others in its species or component type.

Self limiting feedback loops are generally shown on the components most regulated by self limitations (e.g. plants and top predators), even though all components have some self limitations.

 

Figure 3: Feedback within a three species system 

 

Any circuit of interactions produces feedback, as shown in figure 3. As mentioned above, a feedback is negative when the product of each of the signs in the loop is negative and when there is an odd number of links. A negative feedback loop is a very important concept in producing stability in a system. A negative feedback loop returns a signal to a component that is opposite in sign to the signal that left the component, thereby stabilizing the system. The most familiar example of a negative feedback system is the thermostat . If the room cools down below a set temperature, the thermostat responds by turning on the furnace that will heat up the room. When the room temperature increases to a level above the temperature set on the thermostat, the thermostat will shut down the furnace.