Feedback Loops

In ecosystems, most parameters stay within a narrow range under particular environmental conditions. Biological systems use a varitety of regulatory circuits. Feedback describes a sequence of events or interactions that loops back to change the very conditions that started the process.

Figure 1: Example of a Feedback Loop. As lynx population consumes hare (positive signal to lynx), the number of hare decrease (negative signal to hare). Then, the lynx decrease (negative feedback loop), causing the hare population to increase again.


 


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 sigbnals fro input and output are opposite, as in figure 1, it results in negative feedback. If the signs of the output and input signals are the same (either both positive or both negative),  it results in a positive feedback loop. Negative feedback sends a signal to decrease the effect of the process. 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. 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. 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.

Another way to describe positive feedback is  that it sends a signal to increase the effect in a particular direction. ICompetition (-/-) 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. 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. Positiver feedback loops tend to accelerate or amplify a process. Positive and negative does not imply that the consequences of the feedback are either good or bad.

 

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

Self limiting, self regulating, or density dependent components, such as plants and top predators, have their own "shut down" mechanisms. This can be shown using a symbol indicating a “self effect” loop, as shown in Fig. 2. In nearly all situations,  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, 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. The climate system is characterized by strong positive and negative faadback loops between processes that affect the state of the atmosphere, ocean and land. One example is the ice-albedo feedback loop; melting snow exposes more dark ground or ocean surface, which in trun, absorbe hear and causes more snow to melt.