Population Growth (DP IB Environmental Systems & Societies (ESS)) : Revision Note

Population Growth

• S and J population curves describe a generalised response of populations to a particular set of conditions (abiotic and biotic)

S-population Curves

• For most populations, when population growth is plotted against time, an S-population curve is produced

• An S-population curve describes the growth pattern of a population in a resource-limited environment

• The S-population curve has four distinct phases:

   1. Lag phase

• The initial growth is slow when the population is small

   2. Exponential growth phase

• With low or reduced limiting factors, the population expands exponentially into the habitat

   3. Transitional phase

• As the population grows, there is increased competition between individuals for the same limiting factors or resources - this competition results in a lower rate of population increase

   4. Plateau phase

• The population reaches its carrying capacity and fluctuates around a set point determined by the limiting factors - changes in limiting factors cause the population size to increase and decrease (these increases and decreases around the carrying capacity are controlled by negative feedback mechanisms)

An S-population curve describes the growth pattern of a population in a resource-limited environment

J-population Curves

• For some populations, when population growth is plotted against time, a J-population curve is produced

• A J-population curve describes the growth pattern of a population in an environment with unlimited resources

• The J-population curve has three distinct phases:

   1. Lag phase

• The initial growth is slow when the population is small

   2. Exponential growth phase

• Population growth accelerates exponentially as the number of individuals increases

• The curve takes a J-shape due to exponential growth, as resources are not limiting the growth of the population

• The population will continue to grow until a limiting factor such as disease or predation occurs

   3. Crash phase

• At this point, if there has been a significant population overshoot (the population has increased far beyond the natural carrying capacity), there may be a sudden decrease in the population, known as a population crash

A J-population curve describes the growth pattern of a population in an environment with unlimited resources

Comparing S-population Curves and J-population Curves

• Both curves show how populations grow and reach a carrying capacity

• The S-curve occurs in a limited environment, while the J-curve occurs in an unlimited environment

• The S-curve represents logistic growth, while the J-curve represents exponential growth

• The S-curve eventually levels off at the carrying capacity, while the J-curve continues to increase until a limiting factor is encountered

• The S-curve is more common in nature, while the J-curve is less common and is often observed in laboratory conditions