Energy flow in ecosystems

Where it happens

Energy flow happens along every food chain and food web, starting with sunlight and passing through each trophic level in turn: producers, primary consumers, secondary consumers, and so on.

The entry point is the chloroplast of a producer, a green plant, an alga or a photosynthetic bacterium, where light energy is converted into chemical energy in glucose. From there the energy is passed from organism to organism as one eats another: in a paddy field, rice plant → grasshopper → frog → snake; in a pond, phytoplankton → zooplankton → small fish → heron.

A separate branch runs through the decomposers, bacteria and fungi, which obtain their energy from dead organisms and waste at every trophic level. Whatever the pathway, every organism releases most of the energy it receives as heat through respiration, and that heat leaves the ecosystem for good.

Inputs and outputs

• Input: light energy from the sun, of which producers capture only a small fraction, most sunlight is reflected, passes through leaves or falls on bare ground and water.
• Input at each consumer level: chemical energy in the food eaten, measured as energy per unit area per year (for example kJ m⁻² year⁻¹) in energy-flow diagrams.
• Output: energy stored in new biomass at each level, which is the only part available to the next trophic level.
• Output: heat released during respiration at every level, which cannot be recaptured by any organism.
• Output: energy in faeces, urine and dead remains, which passes to decomposers rather than to the next consumer.
• Net result: a one-way flow, energy enters as light, is transferred along the chain, and leaves as heat, so an ecosystem needs a constant supply of sunlight.

The steps

1. Sunlight reaches the producers; only a small fraction of it is absorbed by chlorophyll and fixed by photosynthesis into glucose, which is then built into the plant's biomass.
2. The producer uses part of that chemical energy for its own respiration, releasing heat; the remainder is stored as new tissue, leaves, stems, roots, seeds.
3. A primary consumer (herbivore) eats the producer, taking in the energy stored in the tissue it consumes; parts it does not eat, such as roots, remain with the producer.
4. Some of the food cannot be digested and leaves the herbivore as faeces; energy in nitrogenous waste is lost in urine.
5. The herbivore respires, using energy for movement, warmth and maintenance and releasing it as heat; only what is left builds herbivore biomass.
6. A secondary consumer (carnivore) eats the herbivore and repeats the same pattern of losses; a tertiary consumer may then eat the carnivore.
7. At each transfer only about 10% of the energy in one level is stored as biomass in the next, so the energy available shrinks rapidly along the chain.
8. Decomposers break down faeces and dead organisms from every level, releasing the last of the stored energy as heat while returning nutrients to the soil.

Why it matters and how it is controlled

Because so much energy is lost at each trophic level, food chains rarely have more than four or five levels, as there is not enough energy left to support a further level of consumers. This is also why a pyramid of energy, unlike a pyramid of numbers, is always upright, with each level holding less energy than the one below.

The amount of energy that reaches each level is governed by the producers' rate of photosynthesis, which depends on light intensity, temperature, water and carbon dioxide, and by the efficiency of each transfer. Warm-blooded consumers lose a larger share of their energy as heat than cold-blooded ones, so a chain ending in a mammal supports a smaller final population than one ending in a reptile of the same size.

Because the losses multiply at each step, the biomass and number of organisms usually fall from producers to top consumers, and the top carnivore in a habitat is always rare compared with the plants that ultimately feed it.

Energy flow contrasts with the cycling of matter. Carbon, nitrogen and water are recycled between organisms and the environment indefinitely, but energy is not: once it has been released as heat it cannot be used again by any organism.

This is why the two topics are examined together, a question on the carbon cycle often asks you to explain why carbon cycles but energy flows. It also explains a practical principle: growing crops for people to eat directly feeds far more people from the same land than feeding those crops to livestock first, because a whole trophic level of loss is removed.

Pyramids of number, biomass and energy

How it is examined

You may be asked to explain why only a small percentage of energy passes to the next trophic level, to explain why food chains are usually short, or to interpret or draw a pyramid of energy.

A typical structured question gives the energy content of each level in a food chain, for example producers 20 000 kJ m⁻² year⁻¹ and primary consumers 2 000 kJ m⁻² year⁻¹, and asks you to calculate the percentage transferred, name where the missing energy went and explain why the chain stops after a certain level. Calculation answers should show working: energy in the higher level divided by energy in the lower level, multiplied by 100.

Data-based items may give a pyramid of numbers for an oak tree, caterpillars and birds and ask why it is inverted, or ask you to draw a pyramid of energy to scale. Essay questions often connect energy flow with food production, asking why a vegetarian diet or fish farming makes more efficient use of energy.

Common misconceptions

Worked exam-style question

Question. In a grassland ecosystem, the energy fixed by grass is 30 000 kJ m⁻² year⁻¹. Grasshoppers that feed on the grass store 3 300 kJ m⁻² year⁻¹ as new biomass, and lizards that feed on the grasshoppers store 280 kJ m⁻² year⁻¹.

(a) Calculate the percentage of energy transferred from grass to grasshoppers. (b) Explain three ways in which energy is lost between the grasshoppers and the lizards.

(c) Suggest why there is no fourth trophic level in this ecosystem. (d) State which type of ecological pyramid would always be upright for this food chain, and give a reason.

Model answer. (a) 3 300 ÷ 30 000 × 100 = 11% (working shown). (b) Energy is lost as heat released during respiration by the grasshoppers; energy remains in parts of the grasshopper not eaten or in material the lizard cannot digest, which leaves as faeces; energy is lost in excretory products such as nitrogenous waste; energy used for movement is also accepted.

(c) Only 280 kJ m⁻² year⁻¹ reaches the lizards, and about 10% of that, roughly 28 kJ m⁻² year⁻¹, would be available to a further consumer, which is too little energy to support a viable population of a larger predator. (d) A pyramid of energy, because energy is lost at every transfer, so each level always contains less energy than the level below; it is measured over a year, so it is not affected by the size or number of the organisms.

Source:SRC-DSKP-EN

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