Growth curve

A graph in which the number of organisms in a population is plotted against time. Such curves are amazingly similar for populations of almost all organisms from bacteria to human beings and are considered characteristic of populations.

Growth curves typically have a sigmoid or S-shaped curve. When a few individuals enter a previously unoccupied area, growth is at first slow during the positive acceleration phase. The growth then becomes rapid and increases exponentially, called the logarithmic phase. The growth rate eventually slows down as environmental resistance gradually increases; this phase is called the negative acceleration phase. It finally reaches an equilibrium or saturation level. The final stage of the growth curve is termed the carrying capacity of the environment .

A good example of a species' growth curve is demonstrated by the sheep population in Tasmania. Sheep were introduced into Tasmania in 1800. Careful records of their numbers were kept, and by 1850 the sheep population had reached 1.7 million. The population remained more or less constant at this carrying capacity for nearly a century.

The figures used to plot a growth curve—time and the total number in the population—vary from one species to another, but the shape of the growth curve is similar for all populations. Once a population has become established in a certain region and has reached the equilibrium level, the numbers of individuals will vary from year to year depending on various environmental factors. Comparing these variations for different species living in the same region is helpful to scientists who manage wildlife areas or who track factors that affect populations.

For example, a study of the population variations of the snowshoe hare and the lynx (Lynx canadensis ) in Canada is a classic example of species interaction and interdependence. The peak of the hare population comes about a year before the peak of the lynx population. Since the lynx feeds on the hare, it is obvious that the lynx cycle is related to the hare cycle. This leads to a decline in the population of hares and secondarily to a decline in the lynx population. This permits the plants to recover from the overharvesting by the hares, and the cycle can begin again.

Growth curves are just one of the characteristics of populations. Other characteristics that are a function of the whole group and not of the individual members include population density, birth rate, death rate, age distribution, biotic potential, and rate of dispersion.

See also Population growth

[Linda Rehkopf ]