Ecosystems: energy, nutrients and disturbance
Explain feeding relationships, follow nutrient transfers and test an ecological explanation.
Read a food web
Producers convert light energy into chemical energy through photosynthesis. Consumers feed on organisms; decomposers break down organic material and release nutrients.
Food-web arrows conventionally show the direction of energy transfer from food to feeder. Removing one species can affect several connected populations.
Worked example
In a fictional web grass feeds rabbits, and rabbits feed foxes. A reduction in grass could reduce rabbit food and then fox food; the outcome also depends on alternative prey.
Common mistake: An arrow from grass to rabbit means energy moves to the rabbit.
Distinguish nutrient cycling from energy flow
Nutrients move among organisms, litter, soil and water. Decomposition and uptake connect these stores. Energy passes through the system and is dissipated as heat; it does not cycle in the same way.
Climate affects rates of growth and decay. A large biomass store does not automatically imply a fertile soil store.
Worked example
A fictional litter store receives 12 units and loses 9 units in a month. Its net change is +3 units; the opening store is needed to calculate its final size.
Common mistake: A transfer rate is not the size of a store.
Test a disturbance explanation
Disturbance can alter habitat, feeding relationships and nutrient pathways. A plausible cascade gives a mechanism, but observations must test its timing and alternatives.
Compare spatial coverage, climate, other species and management alongside the focal change. Use Yellowstone as a bounded example of interpreting ecological evidence.
Worked example
Fictional plant cover rises from 20% to 35% after a predator returns: +15 percentage points. A simultaneous wetter period gives an alternative influence that must be investigated.
Common mistake: A before-and-after association cannot identify a sole cause.
Try a different resource
Scatter plots
Six locations were observed once. Several unmeasured factors could influence the number of trees.
Scroll the diagram sideways to see the full resource.
| Distance (km) | Trees within 50 m |
|---|---|
| 1 | 21 |
| 2 | 12 |
| 3 | 3 |
| 4 | 3 |
| 5 | 12 |
| 6 | 21 |
Resource description
1 km: 21 trees; 2 km: 12 trees; 3 km: 3 trees; 4 km: 3 trees; 5 km: 12 trees; 6 km: 21 trees
Write an explanation
Check your answer
- Build connected mechanisms across feeding and nutrient relationships.
- Apply a defined example.
- Consider buffering or alternative pathways.
- Separate mechanism from proof of its size.
Practise and review a written answer
Case studies
Wolves and ecosystem management in Yellowstone · Desert springs in Death Valley
These pages provide selected evidence. Check the whole case requirement with your teacher.
Read further
- www.aqa.org.uk — source or course reference
- www.aqa.org.uk — source or course reference
- qualifications.pearson.com — source or course reference
- www.sqa.org.uk — source or course reference
- qualifications.pearson.com — source or course reference
- www.ocr.org.uk — source or course reference
- www.ocr.org.uk — source or course reference
- www.eduqas.co.uk — source or course reference
- www.eduqas.co.uk — source or course reference
- www.eduqas.co.uk — source or course reference
Ecosystem interdependence · Association and geographical explanations
Course links
AQA GCSE Geography · Pearson Edexcel GCSE Geography B · National 5 Geography · Pearson Edexcel GCSE Geography A · OCR GCSE Geography A · OCR GCSE Geography B · Eduqas GCSE Geography A · Eduqas GCSE Geography B · Eduqas A-level Geography
Next topics
Tropical rainforests: relationships and land-use change · Carbon cycles: stocks, fluxes and changing feedbacks
Selected teaching sequence with fictional worked data. The linked case records supply bounded evidence; they do not satisfy every course case requirement.
Scottish links identify reusable parts of the topic. Scottish landforms, soil, weather and disease requirements need additional course-specific teaching.