In logistic growth, the population approaches an equilibrium

Some populations appear to reach a relatively stable population size, or equilibrium, that changes little over time. When this occurs, the number of individuals first increases in size, then fluctuates by a relatively small amount around what appears to be the maximum sustainable population size.

With few exceptions, in nature, population growth does not precisely match the classic logistic growth curve shape. For example, the graph of sheep abundance in Tasmania over time (FIGURE 10.5) is roughly similar to the characteristic S- shaped logistic curve but includes more fluctuation around a carrying capacity population size.

FIGURE 10.5 Logistic Growth Rises First, Then Levels Off Population growth rarely matches a classic logistic curve (see Figure 11.13). For sheep introduced to the island of Tasmania, the population increased rapidly and then leveled off with fluctuation above and below a carrying capacity or maximum sustainable population size. (After J. Davidson. 1938. Trans R Soc S Aust 62: 342-346. cc BY-NC-SA 3.0.) View larger image