Such a relationship is called positive feedback and is denoted by a plus sign in the center of the loop. Increases in population lead to increases in both births and deaths , hence the positive signs on the causal connections. But as more births increases the population, more deaths decreases the population. Thus, the arrow from deaths to population has a negative sign to show that the two values vary in opposite directions. The limiting nature of negative feedback can be seen in when the system is run as shown in Figure 47 .
Population grows rapidly, but as it does so, Food decreases.
When Food finally runs out, Population falls. The dynamics are generated not by historical data, but by the internal logic of the model. The lesson is that if a population overshoots the limits of its food supply, then it will eventually collapse. Phrased another way, positive feedback may dominate for a time, but it will eventually be limited by the negative feedbacks. However, this lesson remains controversial and has prompted many critiques and counter-critiques.