Energy Flow and Productivity in Ecosystems: Every organism in an ecosystem ultimately depends on one thing: the ability of plants to capture energy from…
Energy Flow and Productivity in Ecosystems is part of Energy and ecosystems (A-level only) in A Level AQA Biology.
7 min
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12
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A Level AQA
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Energy Flow and Productivity in Ecosystems: Every organism in an ecosystem ultimately depends on one thing: the ability of plants to capture energy from…
Use this lesson to stay inside Energy and ecosystems (A-level only) while connecting the detail back to the wider Energy transfers in and between organisms (A-level only) topic.
Read the core explanations from Energy Flow and Productivity in Ecosystems before testing yourself from memory.
Every organism in an ecosystem ultimately depends on one thing: the ability of plants to capture energy from sunlight and lock it into chemical bonds. Without this, no herbivore feeds, no predator hunts, and no decomposer breaks anything down.
Plants synthesise organic compounds by fixing atmospheric or aquatic carbon dioxide during photosynthesis. The carbon from $CO_2$ becomes the backbone of glucose, which is then used to build all other biological molecules: carbohydrates, lipids, proteins, and nucleic acids.
Here is the key split that drives everything else in this topic. Most of the sugars a plant synthesises are used immediately as respiratory substrates to power the plant's own metabolic processes. The remainder are converted into other biological molecules, and these form the plant's biomass — the physical, chemical-energy-containing material of the plant body.
Dry mass is used rather than fresh mass because water content varies enormously between samples and between times of day — it would make comparisons meaningless. Calorimetry gives a direct measure of the energy store, which is what ecologists actually need when tracking energy flow through an ecosystem.
Now that we know biomass represents stored chemical energy, we can ask a more precise question: how much energy is actually available in a plant community at any given time? This is where the distinction between gross and net primary production becomes essential.
Gross Primary Production (GPP) is the total chemical energy store in plant biomass in a given area or volume. Think of it as the full amount of energy fixed by photosynthesis — the plant's total income before any bills are paid.
Plants, like all living organisms, must respire to survive. Respiration releases energy from organic molecules, which is initially used to power metabolic processes (such as biosynthesis, transport, and cellular maintenance). Ultimately, much of this energy is dissipated as heat to the environment and is lost from the ecosystem, meaning it cannot be passed on to the next trophic level. The energy remaining after these respiratory losses is the Net Primary Production (NPP).
NPP is the figure that really matters for the rest of the ecosystem. It represents the energy available for plant growth and reproduction, and it is also the energy available to all other trophic levels — herbivores that eat the plants, and decomposers that break down dead plant material.
Exam questions on this topic almost always give you two of the three values and ask you to find the third. The equation NPP = GPP - R can be rearranged, so it is worth practising all three forms. Here is a full worked example to model the method examiners expect.
Worked example: Calculating NPP and Interpreting the Result
A woodland ecosystem has a GPP of 32,000 kJ ha⁻¹ year⁻¹. The plants lose 9,600 kJ ha⁻¹ year⁻¹ through respiration. (a) Calculate the NPP. (b) State what this NPP value represents for the ecosystem.
Answer: (a) NPP = 22,400 kJ ha⁻¹ year⁻¹. (b) This energy is available for plant growth and reproduction, and is also available to other trophic levels such as herbivores and decomposers.
Notice that part (b) is where many students drop marks. They state that NPP is available for plant growth but forget to mention that it is also the energy source for herbivores and decomposers. Both uses must appear in your answer.
So far we have been discussing production as a quantity — a total amount of energy stored. Productivity shifts the focus to rate: how quickly is that biomass being produced?
Secondary productivity follows the same logic but applies to consumers. Secondary productivity is the rate at which consumers (such as herbivores or carnivores) convert the chemical energy in their food into new biomass of their own. It is also measured as biomass produced per unit area per unit time.
The distinction matters because energy is lost at every trophic level — through respiration, excretion, and material that is not digested. Secondary productivity is therefore always lower than the primary productivity of the level below it. This is why food chains rarely extend beyond four or five trophic levels: there is too little energy left to support another consumer population.
| Feature | Primary Productivity | Secondary Productivity |
|---|---|---|
| Who produces it? | Plants (producers) | Consumers (herbivores, carnivores) |
| Energy source | Light energy via photosynthesis | Chemical energy in food consumed |
| Gross measure | GPP — total energy fixed | Total energy assimilated from food |
| Net measure | NPP = GPP - R | Net secondary production = assimilation - R |
| Typical units | kJ ha⁻¹ year⁻¹ | kJ ha⁻¹ year⁻¹ |
Understanding the theory is one thing; knowing how ecologists actually measure these values is another — and AQA does ask about measurement methods. Biomass is measured as either the mass of carbon per unit area or the dry mass of tissue per unit area.
To obtain dry mass, a sample of tissue is dried in an oven at around 80°C until its mass is constant, removing all water. The chemical energy store in that dried biomass can then be estimated using calorimetry: the dried sample is combusted in a bomb calorimeter, and the temperature rise of a known mass of water is used to calculate the energy released per gram of biomass.
Productivity is then expressed as the rate of biomass production: for example, kJ ha⁻¹ year⁻¹ tells you how many kilojoules of energy are stored per hectare of ecosystem per year. This unit combines an energy measure, an area measure, and a time measure — all three are required for a complete productivity value.
With NPP defined and productivity understood, we can now trace what happens to energy as it moves through the ecosystem. The NPP of plants is the starting point for all consumer trophic levels — it is the total energy available to herbivores and decomposers.
Herbivores do not assimilate all of the plant material they consume. Some passes through the gut undigested and is lost in faeces. Of the energy that is assimilated, a large proportion is used in respiration. Only what remains is incorporated into new herbivore biomass — this is the secondary production that becomes available to carnivores at the next trophic level.
Decomposers — bacteria and fungi — occupy a parallel role. They obtain energy from dead organic matter at every trophic level, breaking down complex molecules and releasing inorganic nutrients back into the ecosystem. The NPP of plants feeds decomposers just as it feeds herbivores, which is why decomposers are considered part of the same energy flow network.
The pattern in the chart above illustrates a consistent principle: energy availability decreases dramatically at each successive trophic level. The values shown are illustrative, but the proportional drop — roughly 80–90% of energy lost between levels — reflects real ecosystem data and explains why large predators are always rare relative to their prey.
This topic rewards precision. The calculations are straightforward, but the language around them is where marks are won or lost. Here is what the examiners are looking for and where students most often go wrong.
For extended-response questions asking you to evaluate energy flow through an ecosystem, structure your answer as: state the energy value at each trophic level, identify the losses (respiration, excretion, undigested material), and conclude with a judgement about why energy availability limits the number of trophic levels. This pattern directly matches the AQA mark scheme structure for 6-mark evaluate questions.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Here is the key split that drives everything else in this topic. Most of the sugars a plant synthesises are used immediately as respiratory substrates to power the plant's own metabolic processes. The remainder are converted into other biological molecules, and these form the plant's biomass — the physical, chemical-energy-containing material of the plant body.
By the end of this lesson, you should be able to Explain how plants synthesise organic compounds and how these form biomass; Define gross primary production (GPP) and net primary production (NPP) using correct AQA terminology; Calculate NPP using the equation NPP = GPP - R; Compare primary and secondary productivity and describe how productivity is measured.
Read each section, then close the notes and explain the idea from memory. Check your explanation against the page and finish with an exam question from Energy transfers in and between organisms (A-level only).