Syllabus Edition
First teaching 2015
Last exams 2025
Population Growth
Written by: Alistair Marjot
Reviewed by: Bridgette Barrett
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
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