3.7.5Additional lessons
Population Growth Curves: Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 86 of 142
Part 1 of 6The Sigmoid Growth Curve
A new or recovering population often follows an S-shaped curve.
1
In the exponential phase resources are plentiful, so the population increases and the rate of growth rises.
2
In the transition phase limiting factors such as competition begin to act, and growth continues but more slowly.
3
In the plateau phase death rate equals birth rate, so the population stops growing at the carrying capacity.
4
The population often fluctuates slightly around the carrying capacity.
Part 2 of 6Interpreting the Graph
Axes
- Time is on the x-axis and population size is on the y-axis.
Steepest part
- The steepest gradient shows the highest rate of population growth.
Plateau
- A flat section shows the population is at the carrying capacity.
Growth rate
- Rate = change in population size ÷ time taken.
Real example
- Antarctic fur seals recovered from hunting by following this pattern, using pup counts as a measure of population.
| Phase | Birth and death rates | Population growth |
|---|---|---|
| Exponential | Births far above deaths | Accelerating |
| Transition | Gap narrowing | Slowing |
| Plateau | Births equal deaths | Zero |
Part 3 of 6Why Growth Slows
Why Growth Slows
Competition
- More individuals compete for the same food and space.
Predation
- Large and dense prey populations attract predators and raise predation.
Disease
- Disease spreads more easily when density is high.
Waste
- Accumulation of waste can harm microorganisms in a culture.
Part 4 of 6Evaluating the Model
Evaluating the Model
Idealised model
- The curve is a simplified model of how populations grow.
Real populations
- Many show fluctuations, overshoots or declines because of environmental change.
Usefulness
- Models allow predictions to be tested, but they are not perfect representations of ecosystems.
Part 5 of 6Worked Example
Question
- A yeast culture has 150 cells per cm³ at 2 hours and 600 cells per cm³ at 5 hours.
- Calculate the mean growth rate.
- Change in population = 600 − 150 = 450 cells per cm³.
- Time interval = 5 − 2 = 3 hours.
- Rate = 450 ÷ 3 = 150.
Answer 150 cells per cm³ per hour
Part 6 of 6Exam Checklist & Tips
3.7.5 – Key Points for the Exam
- Exponential growth Growth in which the rate increases as population size increases.
- Transition phase The phase in which growth slows as limiting factors act.
- Plateau phase The phase in which the population is stable at the carrying capacity.
- Growth rate The change in population size per unit time.
Common Mistakes
- Saying the population stops growing in the exponential phase.
- Taking the plateau to mean that there are no births or deaths, when births and deaths are equal.
- Reading the growth rate from the height of the curve, not from its gradient.
- Treating the sigmoid curve as an exact description of all populations.
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