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
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- The initial growth is slow when the population is small
2. Exponential growth phase
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- With low or reduced limiting factors, the population expands exponentially into the habitat
3. Transitional phase
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- 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
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- 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
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- The initial growth is slow when the population is small
2. Exponential growth phase
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- 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
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- 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
