Feedback is said to occur when a change in state of a dynamic variable is affected by past values of that variable. In other words, if the variable of interest is population density, N, then a feedback process is defined by
where 
represents the change in population density over a finite unit of time, and 
represents an undefined function of population density at some time, d, in the past. For example, the 1st principle of population dynamics fits this definition of feedback because when the time step is unity (i.e., 
) we have
This equation describes a +feedback process because a change in N is a direct (positive) linear function of N, and this meets the feedback criterion above.
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