4.10.2Additional lessons
Ecosystem Stability: Skills — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 136 of 142
Part 1 of 5Amazon Deforestation
The Amazon is a large, diverse and very productive ecosystem, but its stability depends partly on the forest itself.
1
Large areas of the Amazon rainforest are cleared for farming, grazing and timber.
2
Fewer trees means less transpiration, so less water vapour enters the air above the forest.
3
Reduced water vapour alters local rainfall and temperature patterns.
4
Rainfall and temperature affect photosynthesis, decomposition and nutrient cycling, so the ecosystem is less self-sufficient.
5
Further forest loss pushes the system towards conditions it cannot recover from.
6
Satellite maps showing annual loss are combined with rainfall and temperature records to test whether forest loss changes climate.
Part 2 of 5Tipping Points
Positive feedback makes change self-reinforcing once a threshold is crossed, so recovery is difficult.
Tipping Points
Definition
- A tipping point is the threshold beyond which an ecosystem changes rapidly to a new state that may be hard to reverse.
Cause
- Positive feedback, such as drying forest causing more tree death, amplifies change once a threshold is passed.
Uncertainty
- Scientists do not know exactly how much forest must be lost to reach the tipping point.
Consequence
- The forest could change into drier savanna-like vegetation with lower biodiversity and less carbon storage.
Early warning
- Monitoring forest cover, rainfall and tree death helps scientists judge how close a system is to a threshold.
Part 3 of 5Interpreting Data
Questions on this topic usually give a map, graph or table of forest area and ask for calculations or interpretation.
Percentage change
- Divide the change in area by the original area and multiply by 100 to compare loss between regions.
Rate of loss
- Dividing the area lost by the number of years gives an average annual loss for comparisons.
Maps and graphs
- Read axes and units carefully and compare like with like, such as annual loss with annual loss.
Evidence limits
- Correlation between deforestation and rainfall change is supported by mechanism but does not by itself show cause.
| Year | Forest area (million km²) | Change from start |
|---|---|---|
| Start | 5.2 | – |
| End | 4.1 | −1.1 (−21%) |
Part 4 of 5Worked Example
Question
- The forest area in a region falls from 5.2 million km² to 4.1 million km².
- Calculate the percentage loss.
- Change in area = 5.2 − 4.1 = 1.1 million km².
- Percentage loss = 1.1 ÷ 5.2 × 100.
- 1.1 ÷ 5.2 = 0.2115.
Answer The percentage loss is 21%.
Part 5 of 5Exam Checklist & Tips
4.10.2 – Key Points for the Exam
- Deforestation The large-scale removal of trees from an area.
- Transpiration Loss of water vapour from the leaves of plants.
- Tipping point A threshold beyond which an ecosystem shifts rapidly to a new state.
- Percentage loss Change in area divided by original area, multiplied by 100.
Common Mistakes
- Mixing up final and original values when calculating percentage loss; always divide by the original value.
- Forgetting units such as km² or hectares in the answer.
- Saying a tipping point is a gradual change; it is a threshold of rapid change.
- Ignoring the role of transpiration in local climate.
Last updated:
\n