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The following points highlight the two main types of population growth curves. The types are: 1. J – Shaped Curve 2. S – Shaped or Sigmoid Curve.
Type # 1. J – Shaped Curve:
In the case of J-shaped growth form, the population grows exponentially, and after attaining the peak value, the population may abruptly crash. This increase in population is continued till large amount of food materials exist in the habitat.
After some time, due to increase in population size, food supply in the habitat becomes limited which ultimately results in decrease in population size. For example, many insect populations show explosive increase in numbers during the rainy season, followed by their disappearance at the end of the season.
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The following equation exhibits J-shaped growth:
dN/dt = rN
Here dN/dt represents rate of change in population size, r is biotic potential and N stands for population size.
Type # 2. S – Shaped or Sigmoid Curve:
When a few organisms are introduced in an area, the population increase is very slow in the beginning, i.e., positive acceleration phase or lag phase, in the middle phase, the population increase becomes very rapid, i.e., logarithmic phase, and finally in the last phase the population increase is slowed down, i.e., negative acceleration phase, until an equilibrium is attained around which the population size fluctuates according to variability of environment.
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The level beyond which no major increase can occur is referred to as saturation level or carrying capacity (K). In the last phase the new organisms are almost equal to the number of dying individuals and thus there is no more increase in population size.
The S-shaped sigmoid growth form is represented by the following equation:
dN/dt = rN (K – N/K) = rN (1 – N/K)
where, dN/dt is the rate of change in population size,
r is biotic potential
N is population size,
K – N/K or 1 – (N/K) is for environmental resistance.
