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 |
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.
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.
Why is less energy available at each trophic level?
What comes up: the exam asks you to explain why a pyramid of biomass narrows towards the top, or why energy is lost between trophic levels. You typically need to give two or more distinct reasons.
Write (two marks — give two separate reasons): (1) energy is released through respiration and lost to the surroundings as heat, especially in mammals and birds that maintain a constant body temperature; (2) some material is not digested and passes out as faeces (egestion), so the energy it contains reaches decomposers rather than the next consumer. Additional creditworthy reasons include energy used in movement, energy lost through excretion of nitrogenous waste such as urea, parts of the organism not eaten (bones, bark, roots), and some individuals dying before they are consumed.
Watch out: simply writing "energy is lost" earns no credit on its own — you must state how or where the energy goes. Do not say energy is "destroyed"; it is transferred to the surroundings as heat (through respiration) or passed to decomposers (through egestion and death). Also note that the largest losses are through respiration and egestion, not just movement.
| 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 |