Organisation of an Ecosystem

This lesson explores how energy flows through ecosystems and how matter is cycled. You will learn about trophic levels, producers and consumers, food chains and food webs, pyramids of biomass, and the role of decomposers in recycling nutrients. These concepts are central to AQA GCSE Biology Topic 7: Ecology.

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Producers, Consumers and Decomposers

Every organism in an ecosystem can be classified by how it obtains its energy:

Role	Definition	Examples
Producer	An organism that makes its own food by photosynthesis (autotroph)	Grass, trees, algae, phytoplankton
Primary consumer	A herbivore that eats producers	Rabbit, caterpillar, zooplankton
Secondary consumer	A carnivore or omnivore that eats primary consumers	Fox, blue tit, frog
Tertiary consumer	A top predator that eats secondary consumers	Eagle, shark, lion
Decomposer	An organism that breaks down dead organic matter and waste, returning nutrients to the soil	Bacteria, fungi, earthworms

Exam Tip: Remember that producers are always at the start of a food chain because they are the only organisms that can convert light energy into chemical energy (glucose) through photosynthesis. Without producers, no food chain could exist.

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Trophic Levels

A trophic level is the position an organism occupies in a food chain. Each step in the chain is a new trophic level:

Trophic Level	Organism Type	Example in a Grassland
1	Producer	Grass
2	Primary consumer	Rabbit
3	Secondary consumer	Stoat
4	Tertiary consumer	Buzzard

graph LR
    A["Trophic Level 1<br/>Producer<br/>(Grass)"] -->|"Energy"| B["Trophic Level 2<br/>Primary Consumer<br/>(Rabbit)"]
    B -->|"Energy"| C["Trophic Level 3<br/>Secondary Consumer<br/>(Stoat)"]
    C -->|"Energy"| D["Trophic Level 4<br/>Tertiary Consumer<br/>(Buzzard)"]
    style A fill:#c8e6c9,stroke:#2e7d32
    style B fill:#fff9c4,stroke:#f9a825
    style C fill:#ffccbc,stroke:#d84315
    style D fill:#e1bee7,stroke:#6a1b9a

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Energy Transfer in Food Chains

The Sun is the ultimate source of energy for almost all ecosystems on Earth. Producers absorb a small fraction of the light energy from the Sun and convert it into glucose through photosynthesis. This energy is then transferred to consumers when they eat.

However, energy is lost at every trophic level. Not all the energy from one level is passed on to the next. The losses occur because:

Energy Loss Pathway	Explanation
Respiration	All organisms respire, releasing energy as heat to the surroundings
Excretion (urine and faeces)	Not all food eaten is digested; undigested material passes out as faeces
Movement	Energy used for movement is transferred to the environment as heat
Maintaining body temperature	Endothermic (warm-blooded) animals use a significant proportion of energy to stay warm
Parts not eaten	Predators may not eat bones, fur, feathers or roots

Typically, only about 10% of the energy at one trophic level is passed on to the next. This means food chains rarely have more than four or five trophic levels — there simply is not enough energy left to support another level.

graph TD
    SUN["Sun<br/>Light Energy"] --> P["Producers<br/>100% of absorbed energy"]
    P -->|"~10%"| PC["Primary Consumers"]
    P -->|"~90% lost"| L1["Respiration, heat,<br/>uneaten material"]
    PC -->|"~10%"| SC["Secondary Consumers"]
    PC -->|"~90% lost"| L2["Respiration, heat,<br/>faeces, urine"]
    SC -->|"~10%"| TC["Tertiary Consumers"]
    SC -->|"~90% lost"| L3["Respiration, heat,<br/>faeces, urine"]
    style SUN fill:#fff9c4,stroke:#f9a825
    style P fill:#c8e6c9,stroke:#2e7d32
    style PC fill:#bbdefb,stroke:#1565c0
    style SC fill:#ffccbc,stroke:#d84315
    style TC fill:#e1bee7,stroke:#6a1b9a
    style L1 fill:#f5f5f5,stroke:#9e9e9e
    style L2 fill:#f5f5f5,stroke:#9e9e9e
    style L3 fill:#f5f5f5,stroke:#9e9e9e

Exam Tip: If asked to calculate the efficiency of energy transfer, use the formula: Efficiency = (energy transferred to next level / energy available at previous level) x 100. Make sure you can use this formula with data from tables or diagrams.

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Calculating Energy Transfer Efficiency

The efficiency of energy transfer between trophic levels can be calculated:

Efficiency = (Energy transferred to next level / Total energy at previous level) x 100

Worked Example

A grassland ecosystem has the following energy values:

Trophic Level	Energy (kJ per m squared per year)
Producers	20,000
Primary consumers	2,000
Secondary consumers	300

• Efficiency from producers to primary consumers: (2,000 / 20,000) x 100 = 10%
• Efficiency from primary consumers to secondary consumers: (300 / 2,000) x 100 = 15%

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Pyramids of Biomass

Biomass is the total mass of living material in an organism or at each trophic level. It is usually measured in grams per metre squared (g/m2).

A pyramid of biomass represents the biomass at each trophic level. In almost all ecosystems, each trophic level has less biomass than the one below it, creating a pyramid shape:

Trophic Level	Typical Relative Biomass	Why
Producers	Largest bar	Most energy captured from sunlight
Primary consumers	Smaller bar	Energy lost through respiration and waste
Secondary consumers	Even smaller bar	Further energy losses
Tertiary consumers	Smallest bar	Very little energy remains at this level

Why Pyramids of Biomass Are Better Than Pyramids of Numbers

Pyramids of numbers show the number of organisms at each trophic level, but they can give misleading shapes. For example:

• One large oak tree (producer) may support thousands of caterpillars (primary consumers) — this gives an inverted pyramid of numbers
• But the pyramid of biomass would still be the correct pyramid shape because the oak tree has far more biomass than all the caterpillars combined

Pyramids of biomass almost always give a true pyramid shape and are therefore a more reliable representation of energy flow.

Exam Tip: In the exam you may be asked to draw or interpret a pyramid of biomass. Make sure the bars are drawn to scale, centred on a vertical axis, with the producer at the bottom and the top consumer at the top. Label each bar with the organism name and trophic level.

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Food Webs and Their Complexity

A food web is a network of interconnected food chains within an ecosystem. Food webs give a more realistic picture of feeding relationships because most organisms eat more than one type of food and are eaten by more than one predator.

Interpreting Changes in Food Webs

When one population in a food web changes, it can have multiple effects: