The value of b is arbitrary and depends on the size of the population at the designated time 0. Figure 16-4 illustrates the classic logistic growth curve. Figure 16-5 illustrates the logistic curve fitted to the data for the U.S. population from 1790 through 1910. The calculations of Pearl and Reed predicted a leveling off at about 197 million people in the U.S. Subsequent censuses show the population growing in a more exponential fashion. Current estimates are that the population will level off in the mid-200 millions, but time will tell.
Most ecologists view the logistic model as too simplistic to represent real populations.
The fact that the U.S. population growth between 1790 and 1910 fit so well is unusual. The logistic model is most useful as a qualitative model. Populations with a density below their equilibrium K value will grow toward K . Populations with a density above K , will decrease toward K . This model suggests that populations may come under the influence of multiple equilibrium points, each with a range of population sizes within which the population tends toward the specific equilibrium.