Pyramids and energy transfer

Energy enters a food chain when producers photosynthesise, and it passes up the chain as one organism eats another. At each step, most of the biomass and energy is lost, so only a small part reaches the next trophic level. Pyramids of number and biomass show this, and we can calculate the efficiency of each transfer.

Key facts

• Trophic levels: level 1 producers (plants and algae), level 2 primary consumers (herbivores), level 3 secondary consumers, level 4 tertiary consumers. Apex predators are carnivores with no predators.
• A pyramid of numbers shows how many organisms are at each level. It is not always pyramid-shaped: one oak tree can feed hundreds of caterpillars.
• A pyramid of biomass shows the mass of living material at each level. It is almost always pyramid-shaped, because biomass is lost at every step.
• Only about 10% of the biomass at one level is passed to the next. Some is never eaten (roots, bones), some is egested as faeces, some is lost as waste (carbon dioxide, urea), and much of the glucose is used in respiration, its energy lost as heat.
• Efficiency of transfer = biomass (or energy) transferred to the next level ÷ biomass (or energy) available × 100.
• At A-level: gross primary production (GPP) is the chemical energy stored in plant biomass; net primary production NPP = GPP − R, where R is the energy lost in respiration.
• For consumers, net production N = I − (F + R): I is the energy in the food eaten, F the energy lost in faeces and urine, and R the energy lost in respiration.

Worked examples