Ecology and the Environment · 8 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 12% of your exam marks.
Food chains, energy transfer, and ecological definitions are regularly tested, often as short-answer questions.
Energy enters every ecosystem as sunlight captured by producers. From there it passes through each trophic level by feeding.
Only about 10% of the energy at one trophic level is transferred to the next. The other ~90% is lost in various ways. This is the famous "10% rule" of ecology, although the actual figure varies between 5% and 20% in real food chains.
The 90% of energy that does not make it to the next trophic level is lost in several ways:
| Where the energy goes | Why |
|---|---|
| Heat from respiration | Every organism respires constantly, releasing some energy as heat. Warm-blooded animals (mammals, birds) lose a lot here |
| Movement | Energy is used to contract muscles, breathe, pump blood, etc. None of this energy ends up in the consumer that eats them |
| Excretion | Energy in metabolic waste (urea, CO₂) leaves the body in urine and breath |
| Egestion (faeces) | Some food is not digested or absorbed; it passes through the gut unchanged and leaves as faeces. The energy in it goes to decomposers, not to the predator at the next level |
| Uneaten parts | Predators rarely eat everything. Bones, claws, fur, teeth, roots and bark all contain energy that the predator skips |
| Heat lost to surroundings | Especially in mammals and birds that keep a constant body temperature |
Because so much energy is lost at each step, the chain runs out of usable energy after only a few levels. Most food chains end at the third or fourth trophic level. You almost never see a fifth-level predator (something that eats top predators) because there is too little energy left.
The efficiency of energy transfer between two trophic levels is:
percentage efficiency = (energy at higher level ÷ energy at lower level) × 100
The same formula works for biomass, which is closely linked to energy.
Example — In a meadow food chain, the grass holds 150 kJ/m²/year of stored energy. The rabbits that feed on the grass hold 18 kJ/m²/year. Calculate the efficiency of energy transfer from grass to rabbit.
Example — A field of clover has a total dry biomass of 1450 kg. The snails feeding on the clover have a combined dry biomass of 138 kg. Calculate the efficiency of biomass transfer.
The 10% rule has a practical implication. If 1000 kJ of plant energy is grown:
A vegetarian diet uses far less land, water and energy per person than a meat-based diet. This is why feeding the world's growing population is partly about shifting some meat consumption to plants.