Pyramid of energy and the limits of ecological pyramids
The pyramid of energy is always upright, because energy is lost as heat at every transfer. Pyramids have limitations: they ignore food webs, multi-level species and saprophytes.
Pyramid of energy
Each bar in a pyramid of energy shows the energy at a trophic level in a given time, or per year per unit area. It is always upright and can never be inverted. When energy flows from one trophic level to the next, some energy is always lost as heat. So each level always has less energy than the one below it. Number and biomass are snapshots of what is present at one moment, but energy is the total flowing through a level over time. That is why only the energy pyramid can never be inverted.
Worked example: building a pyramid of energy
Sunlight of 2,000,000 J reaches the plants of a field. The plants turn 1% of it into NPP, and each later level gets 10% of the level below. Find the energy at the producer, herbivore, carnivore and top carnivore levels.
- Producers: 1% of 2,000,000 J = 20,000 J.
- Primary consumers (herbivores): 10% of 20,000 J = 2,000 J.
- Secondary consumers (carnivores): 10% of 2,000 J = 200 J.
- Tertiary consumers (top carnivores): 10% of 200 J = 20 J.
20,000 J, 2,000 J, 200 J and 20 J. Each bar is a tenth of the one below, so the pyramid is upright.
How to draw a pyramid of energy
- Draw four stacked bars, each clearly shorter than the one below it.
- Label them from the bottom: primary producers (plants), primary consumers (herbivores), secondary consumers (carnivores) and tertiary consumers (top carnivores).
- Write the energy value in each bar, with its unit (for example J m⁻² yr⁻¹).
- Draw a small arrow from each bar to the side, labelled heat loss.
- Write the title: Pyramid of energy (always upright).
Limitations of ecological pyramids
- They do not account for a species at two or more trophic levels, such as a sparrow eating both seeds and insects.
- They assume a simple food chain, which almost never exists in nature; they cannot show a food web.
- Saprophytes (decomposers) are given no place, though they play a vital role in the ecosystem.
Comparing the three pyramids
| Pyramid | Can it be inverted? | Example |
|---|---|---|
| Number | Yes | One tree with thousands of insects |
| Biomass | Yes | Sea: phytoplankton and fish |
| Energy | Never | Always upright, because heat is lost at every step |
A field of millet feeds far more people if they eat the millet themselves than if it is fed to goats that people then eat. About 90% of the energy is lost at each step. Eating lower on the food chain saves land and water.
1 marks: “Name the ecological pyramid that can never be inverted.” The pyramid of energy.
2 marks: “Why is the pyramid of energy always upright?” When energy flows from one trophic level to the next, some is always lost as heat, so each higher level has less energy than the one below.
3 marks: “State three limitations of ecological pyramids.” They do not account for the same species at two or more trophic levels; they assume a simple food chain and cannot show a food web; they give no place to saprophytes.
Energy only runs downhill: the energy pyramid always stands upright.