Why does so little of the Sun's energy reach a top predator?
The transfer of biomass and energy through trophic levels in food chains and food webs; producers, primary, secondary and tertiary consumers, decomposers and saprobionts; the reasons why biomass and energy decrease at successive trophic levels; the calculation of the efficiency of energy transfer between trophic levels.
A focused answer to the AQA 3.5 dot point on energy and ecosystems. Explains trophic levels and food webs, why biomass and energy fall between levels, the role of decomposers, and how to calculate the percentage efficiency of energy transfer.
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What this dot point is asking
AQA wants you to describe how energy and biomass move through trophic levels, identify each named loss between levels, explain the role of decomposers, and calculate percentage efficiency of energy transfer.
The answer
An ecosystem is all the living organisms in an area and the non-living components they interact with. Energy enters as light, is fixed by producers, and flows through the community along food chains and food webs.
Trophic levels
- Producers (autotrophs) such as plants and algae fix light energy by photosynthesis to make organic molecules. They form the first trophic level.
- Primary consumers (herbivores) eat producers.
- Secondary consumers eat primary consumers.
- Tertiary consumers eat secondary consumers.
- Decomposers (saprobionts), mainly bacteria and fungi, feed on dead organisms and waste by extracellular digestion, releasing nutrients back into the ecosystem.
A food web shows the many interconnected food chains in a community, which is more realistic because most organisms have several food sources.
Why energy and biomass decrease
Only a small fraction of the energy at one level reaches the next, because energy is lost at every step.
- Not all of an organism is eaten. Roots, bones and other parts may be left.
- Not all that is eaten is digested. Indigestible material is lost as faeces (egestion).
- Respiration. A large proportion of the energy assimilated is released as heat during respiration and lost to the surroundings.
- Excretion. Energy is lost in nitrogenous excretory products such as urea.
Because of these losses, food chains rarely have more than four or five trophic levels: there is not enough energy left to support another level. The same reasoning explains the pyramid of biomass and energy, which narrows towards the top.
Calculating efficiency
The efficiency of energy transfer between two trophic levels is:
Transfer is roughly 10 percent between most levels, but the value varies. It is usually higher for transfers involving warm-blooded animals only at low rates because endotherms lose much energy maintaining body temperature.
Examples in context
Example 1. North Sea cod fishery. A simplified marine chain runs from phytoplankton to zooplankton to small fish to cod. Because only about a tenth of the energy passes between each of the four levels, the biomass of cod that the sea can support is a tiny fraction of the phytoplankton biomass beneath it. This is why overfishing of a top predator has such large effects: each cod represents a very large amount of producer energy.
Example 2. Saprobionts in a compost heap. In a garden compost heap, fungi and bacteria respire and digest dead plant material extracellularly, releasing heat (which warms the heap), carbon dioxide and mineral ions. This both demonstrates respiratory energy loss and shows how decomposers return locked-up nutrients to a form plants can reuse.
Try this
Q1. Define producer, primary consumer and decomposer. [3 marks]
- Cue. Producer: autotroph that fixes light energy by photosynthesis (e.g. a plant). Primary consumer: herbivore that eats producers. Decomposer (saprobiont): organism that feeds on dead and waste material by extracellular digestion, releasing nutrients.
Q2. A producer level contains 50 000 kJ per m squared per year; the primary consumers contain 4000 kJ. Calculate the percentage efficiency of energy transfer. [2 marks]
- Cue. (4000 / 50000) x 100 = 8 percent.
Q3. Explain why food chains rarely have more than five trophic levels. [3 marks]
- Cue. Energy is lost at each transfer through respiration, egestion and excretion, so only about 10 percent passes on; after several transfers too little energy remains to support a further level.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2019 AQA4 marksExplain why the amount of energy available decreases at each successive trophic level in a food chain.Show worked answer →
A 4-mark answer needs the named energy losses.
- Not all of an organism is eaten or digested, so some energy is lost in faeces (egested) or in indigestible parts (roots, bones).
- Energy is lost as heat from respiration at each level.
- Some energy is lost in excretory products such as urea.
- Therefore only a small percentage (about 10 percent) is passed on as biomass to the next level, so less energy is available higher up the chain.
Markers reward respiration (heat), egestion (faeces) and excretion as distinct losses.
Related dot points
- Photosynthesis as a two-stage process: the light-dependent reactions in the thylakoid membranes (photoionisation of chlorophyll, photolysis of water, the production of ATP by photophosphorylation, the production of reduced NADP, and the role of the electron transport chain); the light-independent reactions in the stroma (the Calvin cycle: fixation of carbon dioxide by RuBP to form GP, reduction of GP to TP using reduced NADP and ATP, and regeneration of RuBP); the effect of light intensity, carbon dioxide concentration and temperature as limiting factors.
A focused answer to the AQA 3.5 dot point on photosynthesis. Covers the light-dependent reactions (photoionisation, photolysis, the electron transport chain and photophosphorylation), the Calvin cycle in the stroma, and how light, carbon dioxide and temperature act as limiting factors.
- Aerobic respiration as four stages: glycolysis in the cytoplasm (phosphorylation of glucose, oxidation to pyruvate, net yield of ATP and reduced NAD); the link reaction and the Krebs cycle in the mitochondrial matrix (decarboxylation, dehydrogenation, production of reduced NAD, reduced FAD, ATP and carbon dioxide); oxidative phosphorylation on the inner mitochondrial membrane (the electron transport chain, chemiosmosis, ATP synthase and the role of oxygen as the final electron acceptor); anaerobic respiration in animals (lactate) and in microorganisms and plants (ethanol).
A focused answer to the AQA 3.5 dot point on respiration. Covers glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation by chemiosmosis, the role of oxygen as the final electron acceptor, and anaerobic respiration producing lactate or ethanol.
- Biomass as the mass of living material, measured as dry mass or as the chemical energy stored in dry biomass using calorimetry; gross primary production (GPP) as the chemical energy store in plant biomass; net primary production (NPP) as GPP minus respiratory losses; the calculation and units of GPP, NPP and net production of consumers; the ways in which farming practices increase the efficiency of energy transfer in food production.
A focused answer to the AQA 3.5 dot point on productivity and biomass. Defines biomass and dry mass, explains how calorimetry measures energy content, sets out GPP, NPP and net production of consumers with their units, and reviews farming practices that raise energy-transfer efficiency.
- The nitrogen cycle and the roles of saprobionts, nitrogen-fixing, nitrifying and denitrifying bacteria; the phosphorus cycle and the role of mycorrhizae in phosphorus uptake; the role of microorganisms in recycling nutrients; the use of natural and artificial fertilisers and the environmental consequences of using nitrogen-containing and phosphorus-containing fertilisers, including leaching and eutrophication.
A focused answer to the AQA 3.5 dot point on nutrient cycles. Sets out the nitrogen cycle and its four bacterial groups, the phosphorus cycle and mycorrhizae, the role of microorganisms in recycling, and how fertilisers cause leaching and eutrophication.