Food Chains and Food Webs

This lesson covers food chains, food webs, energy transfer and pyramids of biomass as required by the Edexcel GCSE Combined Science specification (1SC0). You need to construct and interpret food chains and webs, understand energy flow through trophic levels and explain the shape of biomass pyramids.

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Food Chains

A food chain shows the flow of energy from one organism to the next in a single pathway. Each organism feeds on the one before it.

Example:

grass → rabbit → fox → eagle

• Grass = producer (trophic level 1)
• Rabbit = primary consumer (trophic level 2)
• Fox = secondary consumer (trophic level 3)
• Eagle = tertiary consumer (trophic level 4)

The arrows in a food chain show the direction of energy transfer — they always point from the organism being eaten to the organism doing the eating.

Exam Tip: The arrow means "is eaten by" or "energy flows to". Never draw the arrows backwards.

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Food Webs

In reality, most organisms eat more than one type of food. A food web shows the interconnected food chains within an ecosystem.

graph TD
    A["Oak tree (producer)"] --> B["Caterpillar"]
    A --> C["Squirrel"]
    A --> D["Aphid"]
    B --> E["Blue tit"]
    D --> E
    C --> F["Fox"]
    E --> F
    E --> G["Sparrowhawk"]
    F --> G
    D --> H["Ladybird"]
    H --> E

Food webs are more realistic than food chains because they show the complexity of feeding relationships and how organisms are interdependent.

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Effect of Removing a Species

If one species is removed from a food web, it has knock-on effects:

Change	Effect
Producer removed	All consumers that feed on it lose a food source; their populations fall
Primary consumer removed	Producers may increase (less grazing); secondary consumers may decrease (less food)
Predator removed	Prey populations may increase (less predation), leading to greater competition and possible overgrazing

Exam Tip: In a food web question, trace every connection to the organism that is removed. Explain effects on species both above and below in the web, and consider competition between species at the same trophic level.

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Energy Transfer Between Trophic Levels

Not all energy is passed from one trophic level to the next. At each stage, energy is lost through:

1. Respiration — energy is released for life processes and lost as heat
2. Excretion — energy is lost in waste products (urine, faeces)
3. Parts not eaten — bones, roots, bark are often not consumed

Typically, only about 10% of the energy at one trophic level is passed to the next.

Trophic Level	Energy Available (kJ)	% Transferred
Producer	100,000	—
Primary consumer	10,000	10%
Secondary consumer	1,000	10%
Tertiary consumer	100	10%

graph LR
    A["Sun: 1,000,000 kJ"] -->|"1% captured"| B["Producer: 10,000 kJ"]
    B -->|"10%"| C["Primary consumer: 1,000 kJ"]
    C -->|"10%"| D["Secondary consumer: 100 kJ"]
    D -->|"10%"| E["Tertiary consumer: 10 kJ"]
    B -->|"90% lost (respiration, waste)"| F["Heat / decomposers"]
    C -->|"90% lost"| F
    D -->|"90% lost"| F

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Why Food Chains Are Short

Because only about 10% of energy passes from one trophic level to the next, there is less and less energy available at higher levels. This is why:

• Food chains rarely have more than four or five trophic levels
• Top predators are always less common than organisms at lower levels
• Large amounts of plant material are needed to support a small number of top predators

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Pyramids of Biomass

A pyramid of biomass represents the total mass of living material (biomass) at each trophic level.

• Biomass is the mass of living organisms, usually measured in grams per square metre (g/m²).
• Each bar represents a trophic level; the producer is always at the bottom.
• Each level is smaller than the one below because energy (and therefore biomass) is lost at every transfer.

Trophic Level	Example	Relative Biomass
Producer	Grass	████████████ (largest)
Primary consumer	Rabbit	████████
Secondary consumer	Fox	████
Tertiary consumer	Eagle	██ (smallest)

Why Pyramids of Biomass Are Always Pyramid-Shaped

Unlike pyramids of numbers (which can be inverted, e.g. when one large tree supports many insects), pyramids of biomass are nearly always pyramid-shaped because:

• Biomass accounts for the size of organisms, not just the number
• Energy is lost at each level, so less biomass can be supported

Exam Tip: If asked to draw a pyramid of biomass, always put the producer at the bottom with the widest bar and make each level progressively narrower.

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Calculating Efficiency of Energy Transfer

You may be asked to calculate the efficiency of energy transfer:

Efficiency (%) = (energy transferred to the next level ÷ energy available at the previous level) × 100

Example: If a producer has 50,000 kJ and the primary consumer receives 5,000 kJ:

Efficiency = (5,000 ÷ 50,000) × 100 = 10%

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Summary

• A food chain shows a single feeding pathway; a food web shows interconnected chains.
• Arrows in food chains point in the direction of energy flow (from prey to predator).
• Removing a species causes knock-on effects throughout the food web.
• Only about 10% of energy passes from one trophic level to the next.
• Energy is lost through respiration, excretion and uneaten parts.
• Pyramids of biomass show total mass at each trophic level — always pyramid-shaped.
• Food chains are short because insufficient energy reaches higher trophic levels.

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Worked Example 1: Biomass Pyramid Calculation

A grassland contains the following measured biomass per square metre: