C4.2.12—Energy reductions
Energy reductions explains how energy flow, matter cycling, trophic transfer or carbon movement is represented, measured or limited within ecosystems.
- Syllabus
- First assessment 2025
- Objective
- C4.2.12
- Level
- HL
Exam analysis
Chance of appearing6%of analysed past papers
Latest appearanceMay 2023
Most common paperPaper3
Typical marks1–3
Common command terms
- Identify
- Explain
- Outline
- Calculate
- State
- Deduce
- Distinguish
- Suggest
- Discuss
Scoring notes
- Common mistake
- Reversing food-web arrows instead of showing energy transfer direction.
Recent exam appearances
May 2023Paper3 ["HL"] · TZ115(d)[ 2 ]C4.2.12—Energy reductions
November 2019Paper3 ["HL"] · TZ014(a)[ 2 ]C4.2.12—Energy reductions
May 2019Paper2 ["HL"] · TZ25(a)[ 2 ]C4.2.12—Energy reductions
May 2018Paper3 ["HL"] · TZ214(b)[ 3 ]C4.2.12—Energy reductions
May 2016Paper2 ["HL"] · TZ07(b)[ 3 ]C4.2.12—Energy reductions
Practice this objective
Coverage 2012–2023 · Updated 16 Jul 2026
Energy transfer between trophic levels is incomplete
Energy availability decreases at successive trophic levels because only part of one level's chemical energy becomes biomass eaten and assimilated by the next.
Losses include uneaten material, indigestible material egested as faeces, carbon compounds used in respiration, and heat from metabolism. Movement, maintenance and other work reduce energy stored as new biomass.
Decomposers and detritus feeders are not usually drawn as one step in a food chain, but they receive uneaten, egested and dead organic matter and transform its chemical energy through feeding and respiration.
Plant roots left uneaten and herbivore faeces can supply detrital pathways, while only new herbivore biomass remains available to a predator.
Transfer efficiency is not a universal ten-percent rule. Species, tissues, temperature and ecological conditions alter the size of each loss.
Energy reductions
Assessment in practice
1–2 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Explain / Outline.
Command terms
Identify / Explain / Outline / Calculate / State / Deduce / Distinguish / Suggest / Discuss
What earns marks
Build the answer around this relationship: Energy reductions must be linked to the correct source, store, transfer or loss process.
Watch for
Reversing food-web arrows instead of showing energy transfer direction.
Representative question
Question 1
The efficiency of energy transfer along the various food chains in a food web varies. Suggest reasons for the differences.
Energy and Matter
- Ecosystems are open systems: energy flows through them and leaves as heat, while matter is recycled and may enter or leave.
- Photoautotrophs capture light; chemoautotrophs oxidize inorganic substances. Both build biomass from inorganic carbon. Heterotrophs obtain organic carbon from other organisms.
- Food-web arrows show energy and biomass transfer. Energy decreases between trophic levels through respiration, heat, egestion, excretion and uneaten material, limiting chain length.
- Gross primary production minus producer respiration gives net primary production; secondary production is heterotroph biomass gain.
- Decomposers obtain energy from detritus and return inorganic nutrients to producers.
- Carbon-cycle diagrams distinguish stores and fluxes. Photosynthesis removes CO2; respiration, decomposition and combustion release it.
- A sink absorbs more carbon than it releases; a source does the reverse. The Keeling Curve shows a long-term atmospheric CO2 rise with seasonal oscillation.
Concept essentials
- Energy reductions must be linked to the correct source, store, transfer or loss process.
- The evidence for energy reductions depends on distinguishing energy flow from matter cycling.
- Energy reductions is clearer when arrows, units and trophic positions match the biological process.
- Data or diagrams for energy reductions need interpretation as ecosystem transfer evidence.