Explain ecosystem energy, productivity and cycles
Connect productivity, food chains, energy transfer, ecological pyramids and the carbon cycle to environmental management.
Section 4.1 · Ecosystems
Exam scope and assessment notes
Original StudyForA teaching and practice, not Cambridge-authored questions, mark schemes or official grades. Cambridge expects candidates to study all eight topics, carry out investigative work and learn teacher-selected case studies. This course does not replace a centre's named case-study examples, current local environmental data, supervised fieldwork or the candidate's own investigation. Practice datasets are illustrative, not observations from a real place. No official assessment or certification is provided. The examples and practice datasets in these lessons are fictional and formative; learners must add their centre's verified case studies and current local evidence.
Before you begin
- Cambridge recommends prior environmental/science study equivalent to IGCSE or O Level. This is an AS-only qualification; use the exam-year syllabus and confirm centre entry details.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What makes environmental evidence useful?
What you will learn
- Distinguish gross and net primary productivity
- Explain trophic transfer and interpret ecological pyramids
- Trace carbon through photosynthesis, respiration, feeding, decomposition, fossilisation and combustion
Energy transfer is incomplete
Gross primary productivity is total energy fixed by producers; net primary productivity subtracts energy used in respiration. NPP is energy stored as new producer biomass over the stated period.
Energy is lost between trophic levels through respiration, waste, death and uneaten material. Pyramids of numbers, biomass and energy represent different quantities; identify the measure before interpreting shape.
Carbon moves through stores and processes
Photosynthesis transfers carbon dioxide into organic material. Feeding moves carbon between organisms; respiration and decomposition return carbon dioxide, while burial and fossilisation store some carbon. Combustion releases stored carbon.
A cycle diagram needs labelled arrows and processes. Do not imply every pathway is equally large or fast.
Sources for this lesson
Pause and explain
How is net primary productivity calculated?
Worked example
Producers fix 1,200 energy units; respiration uses 360. Calculate NPP and explain why herbivores receive less.
Show the worked solution
- Subtract respiration from gross productivity.
- State what NPP represents.
- Name two reasons energy is not transferred to herbivores.
Answer NPP = GPP − respiration = 1,200 − 360 = 840 units. This is energy stored as producer biomass over the stated period. Herbivores receive less because some material is uneaten or not assimilated and energy is lost through respiration and waste.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Producer energy is 800 kJ m⁻² yr⁻¹ and herbivore energy is 96 kJ m⁻² yr⁻¹. Calculate transfer efficiency.
Give me a hint
Divide energy at the next level by energy at the previous level.
Compare my reasoning
- Identify the relevant data, place or process in the prompt.
- Apply distinguish gross and net primary productivity to the stated context.
- Explain the inference and one limitation or trade-off.
Transfer efficiency = 96÷800×100 = 12%. The rest was not transferred as measured herbivore biomass; it may have been respired, excreted, uneaten or not captured.
Look for these in your work
- Distinguish gross and net primary productivity
- Uses relevant evidence and explains the environmental mechanism
- Distinguishes observation, inference and limitation
A top predator is removed from a food web. Predict a possible indirect effect and one reason it is uncertain.
Give me a hint
Trace at least two feeding relationships.
Compare my reasoning
- Identify the relevant data, place or process in the prompt.
- Apply explain trophic transfer and interpret ecological pyramids to the stated context.
- Explain the inference and one limitation or trade-off.
A prey population could increase and consume more of its food source. However, competition, disease, alternative predators and habitat conditions can change the result, so a simple cascade is not guaranteed.
Look for these in your work
- Explain trophic transfer and interpret ecological pyramids
- Uses relevant evidence and explains the environmental mechanism
- Distinguishes observation, inference and limitation
A carbon-cycle diagram shows combustion but omits decomposition. What pathway should be added?
Give me a hint
Trace carbon from dead material through decomposers.
Compare my reasoning
- Identify the relevant data, place or process in the prompt.
- Apply trace carbon through photosynthesis, respiration, feeding, decomposition, fossilisation and combustion to the stated context.
- Explain the inference and one limitation or trade-off.
Add an arrow from dead organic material to decomposers, then show carbon returning to the atmosphere as carbon dioxide through respiration. Some carbon may enter soil or sediment storage; label the processes.
Look for these in your work
- Trace carbon through photosynthesis, respiration, feeding, decomposition, fossilisation and combustion
- Uses relevant evidence and explains the environmental mechanism
- Distinguishes observation, inference and limitation
Common mistakes
- Using GPP and NPP interchangeably
- Assuming all biomass transfers to the next trophic level
- Drawing a carbon cycle without naming processes
What can you explain now?
How is net primary productivity calculated?
Compare with the explanation
NPP = GPP − energy used in producer respiration
NPP is energy stored as producer biomass after respiration.
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Make it stick
Why is NPP more directly relevant than GPP when estimating biomass available to consumers?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.