3.7.3Additional lessons
Limiting Population Size — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 84 of 142
Part 1 of 5Carrying Capacity
Definition
- The carrying capacity (K) is the maximum population size that an environment can sustain over time.
At K
- Births plus immigration are balanced by deaths plus emigration, so the population stays roughly constant.
Fluctuation
- Populations often vary around K as conditions change from year to year.
Changing K
- K can rise or fall if resources increase or decline, such as after a drought or habitat loss.
Part 2 of 5Limiting Factors
Light
- It limits photosynthesis in plants, which in turn limits food for consumers.
Temperature
- It affects enzyme activity and the rate of metabolism, so it limits growth and reproduction.
Minerals and water
- Shortage of soil minerals or water reduces plant growth.
Competition
- Individuals compete for limited resources such as food, space and mates.
Predation and disease
- These reduce survival and reproduction, which lowers the carrying capacity.
Part 3 of 5Density-dependent and Density-independent Factors
Density-dependent and Density-independent Factors
Density-dependent
- The effect on the population changes with population density, for example competition, predation and disease.
Why it works
- At high density, competition is stronger, predators find prey easily and disease spreads quickly.
Density-independent
- The effect does not depend on density, for example fire, flood, frost or volcanic eruption.
Effect
- Density-independent factors kill the same proportion of a population whether it is large or small.
| Feature | Density-dependent | Density-independent |
|---|---|---|
| Effect depends on density | Yes | No |
| Examples | Competition, predation, disease | Fire, flood, frost |
| Role | Regulates population around K | Causes sudden changes |
Part 4 of 5Negative Feedback
1
A population grows above the carrying capacity, so resources per individual fall.
2
Competition, disease and predation increase, and birth rate falls while death rate rises.
3
The population declines back towards the carrying capacity.
4
When the population is low, resources are plentiful and population growth resumes.
Part 5 of 5Exam Checklist & Tips
3.7.3 – Key Points for the Exam
- Carrying capacity The maximum population size an environment can support.
- Limiting factor A factor that restricts population growth.
- Density-dependent factor A factor whose effect changes with population density.
- Density-independent factor A factor whose effect does not depend on density.
Common Mistakes
- Saying carrying capacity is a fixed number that never changes.
- Listing fire as a density-dependent factor.
- Writing that populations always stay exactly at K.
- Forgetting that negative feedback is what keeps a population stable.
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