4.11.1Additional lessons
Stability in Ecosystems — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 92 of 101
Part 1 of 5Features of Stable Ecosystems
A stable ecosystem stays similar over time and can absorb disturbance.
Nutrient cycling
- Efficient recycling of nutrients by decomposers keeps the system self-supporting without large inputs from outside.
High biodiversity
- Many species give many food web links, so the loss of one species is less likely to cause a collapse.
Steady populations
- Consumer numbers do not change greatly, so resources such as food and shelter are not over-used.
Productivity
- Continuing high rates of photosynthesis supply the energy that supports all the trophic levels.
| Term | Meaning | Example |
|---|---|---|
| Resistance | Ability to avoid change when disturbed | Large old trees surviving a drought |
| Resilience | Ability to recover after change | Grassland regrowing after a fire |
| Tipping point | Threshold beyond which change is rapid and hard to reverse | Lake turning from clear water to algal bloom |
Part 2 of 5From Disturbance to Recovery or Collapse
1
A disturbance such as fire, drought, disease or pollution alters conditions in the ecosystem.
2
A resistant ecosystem shows little change, because many species and large stores of nutrients buffer the effect.
3
If change occurs, a resilient system recovers as surviving species and seeds rebuild the community.
4
If the disturbance is too large or too frequent, a tipping point may be crossed and the system moves to a different stable state.
5
Reversing a shift after a tipping point often needs much more effort than preventing it.
Part 3 of 5Factors That Support Stability
Factors That Support Stability
Diversity
- A varied community has species that respond differently to stress, so some always keep the ecosystem functioning.
Keystone species
- Protecting species with large effects on the food web helps maintain community structure.
Size and connection
- Large habitat areas joined by corridors allow recolonisation after local loss.
Low pollution
- Clean air, water and soil keep the processes of cycling and growth working normally.
Part 4 of 5Worked Example
Question
- A grassland population of 500 voles falls to 200 after a drought and later recovers to 480.
- Calculate the percentage fall and the percentage recovery relative to the original.
- Fall = 500 − 200 = 300, so 300 ÷ 500 × 100 = 60%.
- Recovered level relative to original = 480 ÷ 500 × 100 = 96%.
Answer A 60% fall followed by recovery to 96% of the original, showing good resilience.
Part 5 of 5Exam Checklist & Tips
4.11.1 – Key Points for the Exam
- Stability The tendency of an ecosystem to remain similar over time.
- Resistance Capacity to withstand disturbance without changing.
- Resilience Capacity to recover after being changed.
- Tipping point A threshold beyond which an ecosystem changes to a new state.
- Biodiversity The variety of living organisms in an ecosystem.
Common Mistakes
- Using resistance and resilience as if they mean the same thing.
- Stating that stable ecosystems never change; they change within limits.
- Assuming a damaged ecosystem will always return to its old state.
- Forgetting that nutrient cycling and energy flow are both needed for stability.
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