Exponential population growth produces a J-shaped curve because the population grows at an accelerating rate without effective resource limits. Logistic population growth produces an S-shaped curve because growth slows as the population approaches the environment's carrying capacity.
This is core SL and HL content in IB ESS subtopic 2.1, Individuals, populations, communities and ecosystems.
The Reasoning
| Feature | Exponential growth (J-curve) | Logistic growth (S-curve) |
|---|---|---|
| Environmental conditions | Resources are effectively unlimited | Resources become increasingly limited |
| Pattern | Population rises at an accelerating rate | Rapid growth is followed by slowing growth |
| Limiting factors | Have little immediate effect | Increasingly restrict population growth |
| Carrying capacity | Not represented | Population approaches carrying capacity, |
| Feedback | No strong stabilizing feedback is shown | Negative feedback stabilizes population size |
| Long-term realism | Usually temporary | More realistic for populations in resource-limited systems |
During exponential growth, a constant proportion of the population reproduces during each time interval. For example, a population doubling from to , then , then , increases by progressively larger numerical amounts.
In logistic growth, the population initially grows rapidly. As population density rises, density-dependent limiting factors such as competition, disease, predation, and food scarcity increase mortality or reduce reproduction. Growth therefore slows as population size approaches , the maximum population determined by competition for limited resources.
A logistic curve is a model, so real populations may fluctuate around carrying capacity or overshoot it. Gause's laboratory populations of Paramecium aurelia showed approximately sigmoid growth as food and space became limiting.
A common misconception is that every population permanently follows either a perfect J-curve or S-curve. Both curves are simplified models, and human carrying capacity is especially difficult to assess because technology, consumption, trade, and unequal access to resources can alter effective limits.
Exam Technique
For an explain question, identify the curve shape and connect slowing logistic growth to density-dependent factors, negative feedback, and carrying capacity. Do not merely state that the S-curve “stops”; populations may fluctuate around , and environmental change can shift carrying capacity.