Food chains and food webs

What you'll learn

This revision guide covers how energy flows through ecosystems via feeding relationships. You'll learn to construct and interpret food chains and webs, understand trophic levels, and analyse pyramids of numbers and biomass. These concepts are essential for Paper 2 and appear regularly in both multiple-choice and structured questions.

Key terms and definitions

Producer — an organism that makes its own organic nutrients, usually using energy from sunlight through photosynthesis (e.g. green plants, algae)

Consumer — an organism that gets its energy by feeding on other organisms

Herbivore — an animal that feeds only on plant material (primary consumer)

Carnivore — an animal that feeds on other animals (secondary or tertiary consumer)

Decomposer — an organism that breaks down dead organic material and waste products, releasing nutrients back into the environment (e.g. bacteria, fungi)

Trophic level — the position of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass

Food chain — a diagram showing the transfer of energy from one organism to the next, beginning with a producer

Food web — a network of interconnected food chains showing the feeding relationships within an ecosystem

Core concepts

Producers and the start of food chains

All food chains begin with a producer. In most ecosystems, producers are green plants or algae that carry out photosynthesis. They convert light energy into chemical energy stored in glucose and other organic compounds.

Producers are described as autotrophic because they synthesise their own food from simple inorganic substances (carbon dioxide and water). This distinguishes them from consumers, which are heterotrophic and must obtain organic nutrients by feeding on other organisms.

The energy captured by producers forms the foundation of all feeding relationships in an ecosystem. Without producers, no other organisms in that ecosystem could survive.

Consumers and trophic levels

Organisms that feed on producers or other consumers occupy different trophic levels:

Primary consumers (first-level consumers):

• Feed directly on producers
• Always herbivores
• Examples: caterpillars feeding on leaves, rabbits eating grass, zooplankton consuming phytoplankton

Secondary consumers (second-level consumers):

• Feed on primary consumers
• Usually carnivores, though some are omnivores
• Examples: small birds eating caterpillars, foxes eating rabbits, small fish eating zooplankton

Tertiary consumers (third-level consumers):

• Feed on secondary consumers
• Top carnivores in many food chains
• Examples: hawks eating small birds, sharks eating smaller fish

Some food chains extend to quaternary consumers (fourth-level consumers), but energy limitations typically restrict food chains to four or five trophic levels maximum.

Constructing and interpreting food chains

Food chains use arrows to show the direction of energy flow. The arrow means "is eaten by" or "provides energy for."

Basic structure:

grass → rabbit → fox

This reads as: "grass is eaten by rabbit, which is eaten by fox" or "energy flows from grass to rabbit to fox."

Important conventions:

• Always start with a producer (green plant or algae)
• Use arrows pointing from food source to consumer
• Name specific organisms, not general groups (write "cabbage" not "plant")
• Typically show 3-4 organisms in a chain

Food webs and interconnected feeding relationships

Real ecosystems contain multiple, interconnected food chains. A food web shows these complex relationships more accurately than individual food chains.

Key features of food webs:

• Multiple producers and consumers shown
• Most organisms eat more than one type of food
• Most organisms are eaten by more than one predator
• Demonstrates the interdependence of organisms
• Shows how changes in one population can affect many others

Example of a simple food web:

Grass → Grasshopper → Frog → Snake
  ↓         ↓            ↓
Rabbit → Fox ← Mouse ← Owl

The interconnected nature of food webs means that removing one organism affects multiple others, not just the organism that directly eats it.

Energy transfer and why food chains are limited

Energy is lost at each trophic level, which limits the length of food chains. Approximately 10% of the energy at each trophic level is transferred to the next level. The remaining 90% is lost through:

Metabolic processes:

• Respiration (producing heat energy)
• Movement
• Maintaining body temperature in warm-blooded animals
• Chemical reactions in cells

Waste materials:

• Faeces (undigested material)
• Urine (excretion products)

Not all material consumed:

• Parts not eaten (bones, roots, fur)
• Dead organisms not consumed before decomposition

This inefficient energy transfer means:

• Less energy is available at each successive trophic level
• Fewer organisms can be supported at higher trophic levels
• Food chains rarely exceed 4-5 trophic levels
• Top carnivores have relatively small populations

Pyramids of numbers

A pyramid of numbers shows the number of organisms at each trophic level in a food chain at a particular time. Each bar represents one trophic level, with producers at the bottom.

Typical pyramid shape:

        [Tertiary consumers - 1]
      [Secondary consumers - 10]
    [Primary consumers - 100]
  [Producers - 1000]

Advantages:

• Easy to count and construct
• Gives snapshot of population sizes
• Quick to collect data

Limitations:

• Doesn't account for organism size (one tree vs. one aphid both count as "1")
• Can produce inverted pyramids (e.g., one oak tree supporting thousands of insects)
• Doesn't show biomass or energy transfer
• Population numbers fluctuate seasonally

Pyramids of biomass

A pyramid of biomass shows the total dry mass of all organisms at each trophic level at a particular time. This is typically measured in g/m² or kg/m².

Typical pyramid shape:

        [Tertiary consumers - 1 kg/m²]
      [Secondary consumers - 10 kg/m²]
    [Primary consumers - 100 kg/m²]
  [Producers - 1000 kg/m²]

Advantages:

• More accurate representation of energy stored at each level
• Usually produces proper pyramid shape
• Accounts for organism size differences
• Better reflects energy transfer efficiency

Why biomass pyramids are usually pyramid-shaped:

• Energy loss between trophic levels means less biomass can be supported at higher levels
• Approximately 10% energy transfer efficiency between levels
• Biomass accumulation requires energy, which becomes increasingly scarce at higher levels

Measuring biomass:

• Organisms are dried to remove all water (dry mass)
• Water content varies significantly between organisms
• Dry mass provides consistent, comparable measurement
• Process kills organisms, so samples must be carefully selected

Worked examples

Example 1: Constructing a food chain

Question: The following organisms are found in a pond ecosystem: water beetles, algae, small fish, pike (large predatory fish). Construct a food chain showing these four organisms. (2 marks)

Answer: algae → water beetles → small fish → pike

Mark scheme guidance:

• 1 mark for correct organisms in logical order
• 1 mark for arrows in correct direction (showing energy flow from food source to consumer)
• Must start with producer (algae)
• Common error: arrows pointing wrong direction

Example 2: Interpreting a food web

Question: Study the food web below:

Grass → Rabbit → Fox
  ↓       ↓
Lettuce → Snail → Thrush → Hawk
            ↓
          Slug

(a) Name one producer in this food web. (1 mark) (b) Name one organism that is both a secondary and tertiary consumer. (1 mark) (c) Explain what would likely happen to the slug population if all the snails died from disease. (2 marks)

Answers: (a) Grass OR Lettuce (1 mark)

(b) Hawk (1 mark) Explanation: Hawk eats thrush (making it a tertiary consumer) and could also eat other secondary consumers

(c) The slug population would likely increase (1 mark) because there would be less competition for food (lettuce) between slugs and snails (1 mark).

Mark scheme notes:

• Part (a): Must name a specific plant shown, not just write "producer"
• Part (b): Fox is only a secondary consumer; Hawk feeds at multiple levels
• Part (c): Need both the effect (increase) and the explanation (reduced competition) for full marks

Example 3: Pyramids of biomass

Question: A pyramid of biomass for an ocean food chain is shown below:

        [Shark - 8 kg/m²]
      [Small fish - 120 kg/m²]
    [Zooplankton - 450 kg/m²]
  [Phytoplankton - 800 kg/m²]

(a) Calculate the percentage of biomass transferred from phytoplankton to zooplankton. (2 marks) (b) Suggest two reasons why not all the biomass in small fish is transferred to sharks. (2 marks)

Answers: (a) (450 ÷ 800) × 100 (1 mark) = 56.25% or 56% (1 mark)

(b) Any two from:

• Small fish use energy for respiration/movement/maintaining body temperature (1 mark)
• Energy lost as heat (1 mark)
• Not all of the fish is eaten (e.g., bones left behind) (1 mark)
• Energy lost in waste products (faeces/urine) (1 mark)
• Some small fish die and are decomposed before being eaten by sharks (1 mark)

Mark scheme notes:

• Part (a): Must show working for first mark; correct answer for second mark
• Part (b): Must give specific biological processes, not vague statements

Common mistakes and how to avoid them

Arrows pointing the wrong direction in food chains

• Remember: arrows show energy flow from food source to consumer ("is eaten by")
• Check: the arrow should point toward the organism that is eating
• Incorrect: rabbit → grass ✗
• Correct: grass → rabbit ✓

Confusing trophic levels

• A carnivore isn't always a secondary consumer
• Identify what the organism eats: if it eats a herbivore, it's secondary; if it eats another carnivore, it's tertiary
• Some organisms (like hawks) can be at different trophic levels in different food chains

Forgetting that food chains must start with a producer

• All food chains begin with an organism that photosynthesises
• Never start a food chain with a herbivore or carnivore
• In aquatic ecosystems, remember producers include algae and phytoplankton

Misunderstanding energy loss between trophic levels

• Don't say energy is "destroyed" — it's transformed (mainly to heat) or remains in unconsumed/undigested material
• Remember energy loss is approximately 90% at each level, not 10%
• Only about 10% is successfully transferred to the next trophic level

Drawing pyramids of numbers with gaps or overlapping sections

• Each bar should be directly above the one below
• Bars should be centred and aligned vertically
• Use a ruler for neat, straight-edged bars
• Label each level clearly with organism name and numbers/biomass

Confusing biomass with numbers

• Pyramids of numbers count individuals regardless of size
• Pyramids of biomass measure total dry mass
• A pyramid of numbers can be inverted; pyramids of biomass rarely are

Exam technique for "Food chains and food webs"

Command words to watch for:

• State/Name: Give the fact with no explanation (1 mark each) — e.g., "Name one producer"
• Explain: Give reasons why something happens — need cause and effect for full marks
• Describe: Give characteristics or observations without explanation — e.g., "Describe what happens to energy at each trophic level"
• Construct/Draw: Create a food chain or pyramid — marks for correct order and arrows

Answering food web questions:

• Always trace the complete feeding pathway when identifying trophic levels
• Consider all connections, not just the most obvious ones
• If asked about population changes, consider both direct effects (being eaten) and indirect effects (competition, alternative food sources)

Calculation questions:

• Always show your working, even for simple calculations
• Include units in your final answer
• For percentage transfer: (biomass at higher level ÷ biomass at lower level) × 100
• Don't round intermediate steps; only round the final answer

Drawing pyramids:

• Use a ruler and draw neat rectangles
• Largest bar (producers) at the bottom
• Label each trophic level clearly
• Include numbers or biomass values with correct units
• Keep bars centred and proportional to the values given

Quick revision summary

Food chains show linear energy transfer starting with producers (photosynthetic organisms), through primary consumers (herbivores), to secondary and tertiary consumers (carnivores). Food webs show interconnected feeding relationships more realistically. Energy is lost at each trophic level (approximately 90%) through respiration, heat, movement, and waste, limiting chains to 4-5 levels. Pyramids of numbers show organism counts at each level but can be inverted. Pyramids of biomass show dry mass at each level and more accurately represent energy storage, typically forming proper pyramid shapes. Decomposers recycle nutrients from dead material throughout ecosystems.