The increase in a population that occurs when the birth rate is higher than the death rate, or when immigration exceeds emigration, or when a combination of these factors is present. The per capita growth rate (r) is determined by the difference between the birth rate and the death rate. Under ideal conditions the population will achieve its maximum possible value of r, called the intrinsic rate of increase (r_max). A growth curve, obtained by plotting population size against time, is typically S-shaped (sigmoid) or J-shaped (see graphs). A sigmoid growth curve shows an initial phase of exponential growth. The curve levels off when the environment has reached its carrying capacity, i.e. when food, space, and other conditions can support a given number of individuals without an increase in population numbers. A J-shaped growth curve shows an initial phase of exponential growth that ceases abruptly, with a sudden decrease in population numbers. This decrease may be caused by a number of factors, such as the end of the life cycle of the prey or any other factor contributing to environmental resistance that may suddenly take effect. See also bacterial growth curve; fecundity; reproduction rate; survivorship curve.

Population growth curves