Food chains, food webs and energy transfer

What you'll learn

This revision guide covers how energy flows through ecosystems, from photosynthesis in producers through to top predators. You'll learn to construct and interpret food chains and food webs, understand why energy transfer between trophic levels is inefficient, and explain patterns shown in pyramids of biomass. These concepts form a core part of AQA GCSE Biology Paper 2 (Ecology section).

Key terms and definitions

Producer — an organism that makes its own food through photosynthesis (green plants and algae).

Consumer — an organism that obtains energy by eating other organisms.

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

Biomass — the mass of living material in an organism or population, usually measured as dry mass.

Pyramid of biomass — a diagram showing the relative amount of biomass at each trophic level in a food chain.

Decomposer — an organism that breaks down dead organic material and waste products, returning nutrients to the ecosystem (bacteria and fungi).

Apex predator — a predator at the top of a food chain with no natural predators.

Energy transfer — the movement of energy from one trophic level to the next through feeding relationships.

Core concepts

Food chains and trophic levels

A food chain shows the feeding relationships between organisms in an ecosystem. Energy flows through the food chain from one organism to the next. Each stage in a food chain is called a trophic level.

The basic structure of a food chain always follows this pattern:

• Trophic level 1: Producer (e.g. grass, phytoplankton, mangrove trees)
• Trophic level 2: Primary consumer (herbivore that eats the producer)
• Trophic level 3: Secondary consumer (carnivore that eats the primary consumer)
• Trophic level 4: Tertiary consumer (carnivore that eats the secondary consumer)

Example of a Caribbean marine food chain: Phytoplankton → zooplankton → small fish → barracuda

Arrows in food chains show the direction of energy flow. They always point from the organism being eaten to the organism doing the eating. The arrow means "is eaten by" or "provides energy for".

All food chains begin with a producer that captures light energy through photosynthesis. Producers are the foundation of all ecosystems because they convert light energy into chemical energy stored in glucose.

Food webs

A food web shows multiple interconnected food chains within an ecosystem. Real ecosystems are far more complex than single food chains because:

• Most animals eat more than one type of food
• Most organisms are eaten by more than one type of predator
• Organisms may feed at different trophic levels

For example, in a UK woodland ecosystem:

• Oak trees are eaten by caterpillars, aphids and squirrels
• Blue tits eat caterpillars and aphids
• Sparrowhawks eat blue tits and other small birds

Food webs are more realistic representations of feeding relationships than food chains. They show how changes to one population can affect multiple other species in the ecosystem.

If one species is removed from a food web, the effects ripple through the ecosystem:

• Populations that were eaten by that species may increase
• Populations that fed on that species may decrease
• Competitors may increase as competition is reduced

Energy transfer and efficiency

Only approximately 10% of the energy from one trophic level is transferred to the next level. The remaining 90% is lost from the food chain through:

Respiration — organisms use energy for life processes including movement, keeping warm, and active transport. This energy is eventually transferred to the surroundings as heat.

Waste products — not all of the organism is eaten (e.g. bones, roots, fur). Energy remains in these materials.

Excretion — materials that are eaten but cannot be digested are egested as faeces. Waste products from metabolism are excreted as urine. These contain chemical energy.

This inefficiency explains why:

• Food chains rarely have more than 4-5 trophic levels
• There is less biomass at higher trophic levels
• Top predators are relatively rare compared to prey species

You can calculate the efficiency of energy transfer using:

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

For example, if a trophic level contains 50,000 kJ of energy and the next level contains 5,000 kJ: Efficiency = (5,000 ÷ 50,000) × 100 = 10%

Pyramids of biomass

A pyramid of biomass represents the relative biomass at each trophic level. Each bar represents one trophic level, with producers at the bottom.

Key features of pyramids of biomass:

• Width of each bar is proportional to the dry biomass at that level
• Biomass is measured in g/m² (for a given area) or g (for a whole ecosystem)
• The pyramid shape reflects the loss of biomass at each trophic level
• Typically, each level contains roughly 10% of the biomass of the level below

Biomass pyramids are usually pyramid-shaped because:

• Energy is lost between trophic levels through respiration, waste and incomplete consumption
• Less energy available means less biomass can be supported
• Each trophic level can only sustain a smaller mass of organisms than the level below

Dry biomass is used rather than fresh mass because:

• Water content varies between organisms and over time
• Dry mass gives a more accurate measure of the organic matter present
• It allows fair comparison between different organisms

In aquatic ecosystems, pyramids of biomass can sometimes be inverted (upside-down) at one point in time because phytoplankton reproduce very rapidly. Their biomass at any moment may be less than the zooplankton feeding on them, but their rapid reproduction means they produce enough biomass over time to support the consumers.

Role of decomposers

Decomposers play a crucial role in ecosystems by:

• Breaking down dead organic material (plants and animals)
• Decomposing waste products (faeces and urine)
• Releasing nutrients back into the soil
• Enabling nutrient cycling so materials can be reused by producers

Decomposers (bacteria and fungi) use enzymes to break down complex organic molecules into simpler substances. Through their own respiration, they release carbon dioxide back into the atmosphere.

Without decomposers:

• Dead organisms and waste would accumulate
• Nutrients would remain locked in dead material
• Producers would run out of mineral ions needed for growth
• Ecosystems would collapse

Decomposers are not usually shown in pyramids of biomass but they operate at every trophic level, breaking down dead material from all organisms.

Human impact on food chains

Humans affect food chains and energy transfer through:

Agriculture — farmers try to maximise energy transfer to humans by:

• Reducing the number of trophic levels (eating crops directly rather than feeding them to animals)
• Limiting energy loss in livestock through restricted movement and controlled temperature
• Reducing competition from pests and weeds using pesticides and herbicides

Bioaccumulation — toxic substances (e.g. pesticides, heavy metals) accumulate in organisms and become more concentrated at higher trophic levels. This is called biomagnification and can harm apex predators even when the original concentration was low.

Overfishing — removing large numbers of fish disrupts marine food webs, affecting predators and prey populations.

Habitat destruction — removing producers (e.g. deforestation of rainforest, destruction of Caribbean coral reefs) eliminates the base of food chains, causing ecosystem collapse.

Worked examples

Example 1: Interpreting a food chain

Question: The diagram shows a food chain from a Caribbean mangrove ecosystem.

Mangrove leaves → crab → heron

(a) Name the producer in this food chain. [1 mark] (b) Explain why there are rarely more than five organisms in a food chain. [3 marks]

Mark scheme answers:

(a) Mangrove leaves / mangrove tree [1 mark]

(b) Award 1 mark for each of the following points, up to 3 marks:

• Energy is lost at each trophic level / between each organism [1]
• Energy is lost through respiration / as heat / in waste / in movement / keeping warm [1]
• Only approximately 10% of energy is transferred to the next level [1]
• Eventually there is insufficient energy to support another trophic level [1] (Maximum 3 marks from these points)

Example 2: Calculating efficiency

Question: A food chain in a UK grassland ecosystem shows energy values at each trophic level:

Grass (200,000 kJ) → rabbit (20,000 kJ) → fox (1,500 kJ)

Calculate the efficiency of energy transfer from rabbit to fox. Show your working. [3 marks]

Mark scheme answer:

Efficiency = (energy in fox ÷ energy in rabbit) × 100 [1 mark] = (1,500 ÷ 20,000) × 100 [1 mark] = 7.5% [1 mark]

Example 3: Explaining pyramids of biomass

Question: A student measured the biomass at each trophic level in a pond ecosystem:

• Producers (algae): 5,000 g/m²
• Primary consumers (water fleas): 500 g/m²
• Secondary consumers (small fish): 50 g/m²
• Tertiary consumers (pike): 5 g/m²

(a) Draw a pyramid of biomass for this ecosystem. [2 marks] (b) Explain why the pyramid has this shape. [2 marks]

Mark scheme answers:

(a) Award marks for:

• Four correctly labelled horizontal bars with producers at the base [1]
• Bars drawn proportional to the values given (10:1 ratio between levels) [1]

(b) Award 1 mark for each of the following:

• Energy / biomass is lost between trophic levels [1]
• Due to respiration / waste / not all organism eaten / heat loss [1]

Common mistakes and how to avoid them

• Drawing arrows the wrong way in food chains — Remember arrows show energy flow, not "who eats who". The arrow points FROM the organism being eaten TO the organism eating it. Think of it as "provides energy for".

• Confusing pyramids of biomass with pyramids of numbers — Pyramids of biomass show the mass of organisms, not how many there are. A pyramid of numbers can be irregular (e.g. one oak tree supporting thousands of caterpillars), but pyramids of biomass are almost always pyramid-shaped.

• Forgetting that energy is lost, not destroyed — Energy isn't destroyed between trophic levels; it's transferred to the surroundings, mainly as heat during respiration. Use precise language: "energy is lost from the food chain" not "energy is lost completely".

• Stating percentages without context — When asked to calculate efficiency, always show your working and include units. Write the formula first, substitute values, then calculate.

• Calling all organisms "animals" — Use specific terms: producers (plants/algae), consumers (animals), decomposers (bacteria/fungi). Not all organisms are animals.

• Forgetting decomposers — Although not shown in pyramids of biomass, decomposers are essential for nutrient cycling. Include them when discussing complete ecosystems.

Exam technique for "Food chains, food webs and energy transfer"

• Command word "explain" requires you to give reasons WHY something happens, not just describe WHAT happens. For energy transfer questions, state that energy is lost AND give examples of how (respiration, waste, heat, incomplete consumption). Aim for 2-3 developed points for 3-4 marks.

• Calculations always require working. Write the formula, substitute values with units, calculate the answer and include the unit in your final answer. For efficiency calculations, remember to multiply by 100 to get a percentage.

• Pyramid questions — if asked to draw a pyramid of biomass, use a ruler, make bars proportional to given values, and always put producers at the base. Label each trophic level clearly.

• 6-mark questions on this topic often ask you to explain the effect of removing a species from a food web. Structure your answer to discuss impacts on prey (increase), predators (decrease), and competitors (may increase), using named organisms from the food web provided.

Quick revision summary

Food chains show linear feeding relationships starting with producers. Food webs show interconnected chains in an ecosystem. Energy transfer between trophic levels is approximately 10% efficient because energy is lost through respiration, waste and incomplete consumption. Pyramids of biomass represent the mass of organisms at each level, forming a pyramid shape due to energy loss. Decomposers break down dead material and waste, recycling nutrients. Humans affect energy transfer through agriculture, bioaccumulation and ecosystem disruption.