3.8.4Additional lessons
Limiting Population Size: Examples — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 13 of 101
Section 1 of 5Predator and Prey
Predation is a density-dependent factor, and predator and prey numbers often fluctuate together.
Snowshoe hare and lynx
- Records from fur trapping show a roughly ten-year cycle in which the numbers of hares and lynx rise and fall, with the lynx peaks following the hare peaks.
Why the cycle occurs
- When hares are abundant, lynx have plenty of food and increase, which then increases predation until hares decline.
Predator crash
- As hares become scarce, many lynx starve, predation falls and the hare population begins to recover.
Other influences
- Food supply for the hares and other predators also affect the cycle, so predation is not the only factor.
Section 2 of 5Food, Space and Disease
Food, Space and Disease
Food supply
- In winter the amount of food falls, so the population of many herbivores shrinks until food becomes plentiful again.
Territory
- Birds and mammals defend areas containing food and nest sites, so only a limited number of breeding pairs can occupy a habitat.
Disease
- Myxomatosis spread through European rabbit populations and killed a high proportion of them, especially where animals lived close together.
Parasites
- Heavy parasite loads in crowded populations reduce growth and breeding success.
Section 3 of 5Abiotic Limits
1
Temperature, rainfall and sunlight set the range of conditions in which a species can survive and breed.
2
A harsh winter, flood or drought can kill many individuals regardless of the size of the population.
3
Populations recover afterwards at a rate that depends on how quickly the survivors can reproduce.
4
Over many years the population fluctuates around the carrying capacity set by long-term conditions.
| Example | Limiting factor | Type |
|---|---|---|
| Lynx numbers | Availability of hares as prey | Density-dependent |
| Rabbits and myxomatosis | Disease | Density-dependent |
| Insects in a hard winter | Cold temperature | Density-independent |
| Territorial birds | Space and nest sites | Density-dependent |
Section 4 of 5Interpreting Population Data
Lag between peaks
- If the peak in predators comes after the peak in prey, this supports the idea that prey abundance drives predator numbers.
Correlation
- Two graphs that rise and fall together do not by themselves prove that one factor causes the other.
Data source
- Long-term records, such as fur-trading data, are valuable but may be affected by trapping effort.
Several factors
- Real populations are limited by more than one factor at once, so explanations should refer to all that apply.
Section 5 of 5Exam Checklist & Tips
3.8.4 – Key Points for the Exam
- Predation One organism killing and eating another.
- Territory An area defended by an animal for food or breeding.
- Fluctuation A rise and fall in population size over time.
- Parasite An organism living on or in a host and harming it.
- Abiotic factor A non-living influence such as temperature or rainfall.
Common Mistakes
- Saying the predator population controls the prey with no feedback, when each limits the other.
- Quoting the hare and lynx cycle without noting that the predator peak lags behind the prey peak.
- Treating disease as density-independent, although it spreads more quickly in dense populations.
- Assuming that populations remain constant, when most fluctuate around the carrying capacity.
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