C4.2.14—Restrictions on trophic levels
Restrictions on trophic levels explains how energy flow, matter cycling or carbon transfer is represented, measured or limited within ecosystems.
- Syllabus
- First assessment 2025
- Objective
- C4.2.14
- Level
- HL
Few trophic levels can be supported when transfer is low
Repeated energy losses restrict the number of trophic levels that an ecosystem can support.
Each higher level receives less total chemical energy, so less total biomass can be maintained. Successive levels therefore tend to contain fewer organisms or organisms with smaller combined body size.
Lower-level energy → incomplete transfer → smaller upper-level energy budget → lower total biomass → fewer/smaller organisms → an additional trophic level eventually becomes unsustainable.
A food web with low primary production or large transfer losses may support herbivores but too little predator biomass to support another persistent consumer level.
Total biomass decreases upward, but the energy content per unit mass is not assumed to decrease. The limit comes from less total energy/biomass, not poorer-quality joules.
Restrictions on trophic levels
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Outline.
Command terms
Identify / Outline
What earns marks
Build the answer around this relationship: Restrictions on trophic levels 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
[Maximum number: 1]
Outline why the number of trophic levels is limited in a food chain.
energy losses between trophic levels;
OR only part of the energy in one trophic level will become part of the next trophic level
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
- Restrictions on trophic levels must be linked to the correct source, store, transfer or loss process.
- The evidence for restrictions on trophic levels depends on distinguishing energy flow from matter cycling.
- Restrictions on trophic levels is clearer when arrows, units and trophic positions match the biological process.
- Data or diagrams for restrictions on trophic levels need interpretation as ecosystem transfer evidence.