Energy is lost between trophic levels

1. A trophic level is a feeding position in a food chain, with producers at trophic level $1$.
2. Only energy stored in an organism's new biomass can be transferred to the next trophic level when that biomass is eaten.
3. Some organisms or body parts are not eaten, so the chemical energy in that material is not transferred to the consumer.
4. Some eaten material cannot be digested and is egested in faeces, so its chemical energy is not absorbed into the consumer's body.
5. Some absorbed substances are excreted, including nitrogen-containing waste in urine, so this material cannot become food for the next trophic level.
6. Respiration transfers chemical energy from food for movement, active transport, growth, repair and temperature control, and much of this energy eventually spreads to the surroundings as heat.

Less energy supports fewer organisms

1. Each higher trophic level receives less usable chemical energy, so it can build less new biomass.
2. Less biomass provides food for fewer individuals, which is why populations are usually smaller at higher trophic levels.
3. Top predators need many prey organisms across a large feeding area because so little of the original energy reaches them.
4. After several transfers, too little energy remains to support another stable consumer population, which limits most food chains to a small number of trophic levels.

Biomass pyramids narrow upwards

1. A pyramid of biomass uses horizontal bars whose widths are proportional to the total dry mass of organisms at each trophic level.
2. The producer bar is placed at the bottom, followed by primary, secondary and higher consumers in feeding order.
3. Biomass normally decreases at each transfer because only part of the material at one level becomes new biomass at the next level.
4. The bars therefore become narrower towards the top, showing why less living material and usually fewer organisms can be supported.

Pyramid diagrams must match the data

1. Draw separate horizontal bars rather than a triangle, because each bar must show the biomass of one trophic level.
2. Label the producer at the bottom and arrange every consumer above it in the order shown by the food chain.
3. Use the numerical values to set the relative widths instead of drawing equally spaced steps.

Transfer rates are not fixed

1. The often-used $10\%$ transfer figure is a rough model, not a rule that applies to every ecosystem or every trophic level.
2. Transfer depends on factors such as how much of each organism is eaten, how much is digestible and how much energy the organisms use in respiration.
3. Use values supplied in a question when explaining a particular food chain instead of assuming that every transfer is $10\%$.