Increase in population size is known as population growth. It depends upon number of persons added to the population and number of persons lost from the population. Addition in population occurs by birth or immigration. Loss in population occurs by death or emigration. If more persons are added than lost, the population will show positive growth. If more persons are lost than added, the population will show negative growth. If addition and loss are balanced, the population will become stationary or show zero growth.

Population growth curve:

            These are curves which show growth (positive or negative) in population. A graph is made by plotting number of individuals against time.

            These are of two types:

(i)                 J-shaped growth curve

(ii)               S-shaped growth curve

(i)     J-shaped growth curve:

It has two phases, lag phases and exponential phase. In lag phase, the growth curve shows little or no increase in population, where as in exponential phase population multiply rapidly in an unrestricted manner. This type of growth is only possible in laboratory where conditions for growth are provided.

(ii)         S-shaped or sigmoid growth curve:

It has three phases-lag phase, exponential phase and stationary phase. It shows little growth in lag phases, rapid in exponential and stable or balanced growth in stationary phases. The upper level beyond which no major increase in growth occurs is also called as carrying capacity (K). Carrying capacity is the environment's ability to provide necessary resources to support population. It is also known as logistic pattern of growth.