3.7.4Additional lessons
Limiting Population Size: Examples — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 85 of 142
Part 1 of 5Predator-Prey Cycles
The Canada lynx and the snowshoe hare are a well-known example.
1
When prey are plentiful, predators have more food and their numbers rise.
2
More predators eat more prey, so the prey population falls.
3
With less prey available, many predators starve or fail to breed, and predator numbers fall.
4
Fewer predators mean less predation, so the prey population recovers and the cycle repeats.
5
The predator peak lags behind the prey peak, which shows on the graph.
Part 2 of 5Top-down and Bottom-up Control
Top-down control
- A population is limited by its predators, as snowshoe hares are limited by lynx.
Bottom-up control
- A population is limited by resources such as food, as lynx are limited by the supply of hares.
Plant example
- Herbivory is a top-down control, while light intensity is a bottom-up control on plants.
Both together
- Both types can act, but usually one is dominant at a given time.
| Type | Limiting factor | Example |
|---|---|---|
| Top-down | Predation or herbivory | Hares limited by lynx |
| Bottom-up | Resources | Lynx limited by hares |
Part 3 of 5Trophic Cascades
Trophic Cascades
Cascade
- A change in one population affects populations at other trophic levels.
Example
- Fewer foxes lead to more rabbits, which graze more grass, so grass growth falls.
Complex webs
- Food chains are linked in food webs, so an effect on one chain can spread to other parts of the web.
Predicting
- Predictions are uncertain because other species and abiotic factors also change.
Part 4 of 5Limits of the Simple Model
Limits of the Simple Model
Single pair
- Real communities contain many predators and prey, so a two-species cycle is an oversimplification.
Other factors
- Weather, disease, and food supply also affect numbers.
Data
- Long-term records are needed to show cycles, as short studies may miss them.
Part 5 of 5Exam Checklist & Tips
3.7.4 – Key Points for the Exam
- Predator An animal that kills and eats other animals.
- Prey An animal that is killed and eaten by a predator.
- Top-down control Limitation of a population by its consumers.
- Bottom-up control Limitation of a population by resources.
- Trophic cascade A series of effects that pass through food-web levels.
Common Mistakes
- Thinking the predator peak comes before the prey peak.
- Mixing up top-down and bottom-up control.
- Assuming predator-prey cycles in nature are caused by only two species.
- Treating a trophic cascade as affecting only one population.
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