Feedback is a process in which outputs from a system affect the inputs. Predator-prey systems are examples of negative feedback. For instance, any increase in the size of a predator population means that more prey are eaten, so the prey population decreases, which in turn leads to a decrease in the predator population. Reproduction is an example of positive feedback: births increase population size, which in turn increases the rate of reproduction, which leads to yet more births. Feedback loops arise when a sequence of interactions form a closed loop, for example, A-B-C-A. Feedback loops play an important role in food webs and ecosystem stability. Time delays in the response within a feedback loop often lead to cyclic behavior (e.g., in predator-prey systems).

Both positive and negative feedback are important in self-organization. By dampening changes, negative feedback acts as a stabilizing force. It is one of the principal

Figure 2 The role of feedback in self-organization of a food web. In this diagram, circles represent populations and arrows indicate the influence (positive or negative) of one population on another. In a food web, circular chains of interaction between populations form feedback loops, as in the example shown here. (a) The initial food web contains both positive and negative feedback loops. Internal dynamics within the positive feedback loops leads to the local extinction of several populations. (b) The resulting food web contains only negative feedback loops, which stabilize the community.

mechanisms of homeostasis, the maintenance of dynamic equilibrium by internal regulation. In contrast, positive feedback magnifies minor deviations. An example is competitive exclusion: any small decrease in size of a competing population is likely to lead to further decreases, until it dies out (Figure 2).

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