Food Chains & Food Webs

Food Chains

A food chain shows the feeding relationships between organisms in an ecosystem and the direction in which energy flows between them.

Example: Grass → Rabbit → Fox → Decomposers

Key rules:

• Arrows show the direction of energy transfer (from the organism being eaten to the one eating it)
• All food chains start with a producer — an organism that makes its own food
• Food chains end with a top predator (apex predator) or with decomposers
• Food chains show only a simplified, linear feeding relationship

Trophic Levels

Each position in a food chain is called a trophic level:

Trophic level	Role	Example
1st — Producer	Makes food via photosynthesis	Grass, oak tree, algae, phytoplankton
2nd — Primary consumer	Eats the producer; herbivore	Rabbit, caterpillar, deer, zooplankton
3rd — Secondary consumer	Eats the primary consumer	Fox, thrush, small fish
4th — Tertiary consumer	Eats the secondary consumer	Eagle, orca, large fish
Decomposers	Break down dead matter	Bacteria, fungi, earthworms

Food Webs

A food web is a network of interconnected food chains within an ecosystem. It is a more realistic representation than a single food chain because:

• Most animals eat more than one type of food (they are omnivores or eat multiple prey species)
• Many organisms are eaten by more than one predator
• The web shows how changes to one species can have knock-on effects throughout the ecosystem

Example in a woodland:

• Grass → Rabbit → Fox
• Grass → Rabbit → Hawk
• Grass → Vole → Fox
• Grass → Vole → Owl
• Leaves → Caterpillar → Thrush → Sparrowhawk

A food web shows these relationships simultaneously, revealing how removing or adding a species affects others.

Energy Flow and the 10% Rule

Energy enters ecosystems through photosynthesis by producers, which convert sunlight, water, and CO2 into glucose (chemical energy).

As energy passes from one trophic level to the next, most is lost:

• As heat from respiration (organisms release energy as heat to stay warm and move)
• In movement and bodily functions
• In waste (urine, faeces — energy in undigested food)
• In parts of the organism not eaten (bones, roots, shells)

On average, only about 10% of the energy at one trophic level is stored in the bodies of organisms at the next level. This means:

• 10,000 kJ of grass energy → 1,000 kJ of rabbit energy → 100 kJ of fox energy
• After 4–5 links, so little energy remains that there is not enough to support another level

This is why food chains are short (rarely more than 4–5 links) and why there are always far more producers than top predators.

Pyramids of Numbers, Biomass, and Energy

Scientists use pyramid diagrams to represent food chains:

Pyramid of Numbers

Shows the count of organisms at each trophic level. Usually widest at the base (many producers), narrowing toward the top. Can be unusual — a single oak tree (1 producer) might support thousands of caterpillars.

Pyramid of Biomass

Shows the total dry mass of organisms at each trophic level. Almost always a true pyramid shape — wider at the bottom because there is more total mass of producers than consumers.

Pyramid of Energy

Shows the total energy available at each trophic level. Always a true pyramid shape. The most accurate representation of energy flow.

Decomposers and Nutrient Cycling

Decomposers (mainly bacteria and fungi) are essential for ecosystem function:

• They break down (decompose) dead organisms, fallen leaves, animal waste, and other organic matter
• They release the nutrients (minerals like nitrates, phosphates) locked up in dead matter back into the soil
• Plants absorb these nutrients, feeding producers and restarting the cycle
• Without decomposers, nutrients would stay locked in dead bodies; the ecosystem would eventually run out of nutrients for plants

Detritivores — organisms like earthworms, woodlice, and millipedes — also help decomposition by breaking large pieces of dead matter into smaller pieces, giving decomposers (bacteria and fungi) more surface area to work on.

Predator-Prey Relationships

Predator and prey populations are linked — changes in one affect the other in a cyclical pattern:

1. Prey population increases (abundant food, few predators)
2. With more food, predator population increases
3. More predators eat more prey → prey population decreases
4. With less food, predator population decreases
5. With fewer predators, prey population recovers
6. Cycle repeats

This creates natural oscillations in both populations — a key feature of stable ecosystems. The lynx-snowshoe hare cycle in Canada is a classic example.

Humans and Food Chains

Understanding food chains and energy loss has practical applications:

• Eating lower in the food chain is more energy-efficient (e.g., eating plants directly rather than feeding them to animals then eating the animals)
• Farming efficiency — intensive livestock farming loses more energy than arable (crop) farming
• Overfishing — removing top predators (e.g., sharks) disrupts entire marine food webs
• Bioaccumulation — some pollutants (e.g., mercury, DDT) increase in concentration at higher trophic levels because they are not broken down and accumulate in body fat

Key Terms

Term	Definition
Producer	Organism that makes its own food via photosynthesis
Consumer	Organism that obtains energy by eating other organisms
Herbivore	Animal that eats only plants (primary consumer)
Carnivore	Animal that eats other animals
Omnivore	Animal that eats both plants and animals
Apex predator	Top predator with no natural predators
Decomposer	Organism (bacteria or fungi) that breaks down dead matter
Trophic level	Position of an organism in a food chain
Biomass	Total dry mass of organisms at a trophic level
Bioaccumulation	Build-up of toxins in organisms at higher trophic levels

Worked Example

Q: In the food web below, foxes eat rabbits and voles. Rabbits and voles both eat grass. Owls eat voles. What would happen to the owl population if all foxes were removed?

Step 1: Foxes eat voles → removing foxes means fewer predators of voles.

Step 2: With fewer predators, vole population increases.

Step 3: More voles means more food for owls → owl population increases.

Step 4 (longer term): More owls eat more voles → vole population may decrease again; owl numbers stabilise.

Answer: In the short term, the owl population would increase because vole numbers rise when foxes (which compete with owls for voles) are removed.

Common Mistakes

• Drawing arrows in the wrong direction — arrows in food chains point in the direction of energy flow (from prey to predator), not "who eats whom" pointing at the prey.
• Saying decomposers are "at the bottom" of a food chain — decomposers are not part of the main chain; they act on organisms at all levels.
• Thinking only 10% of organisms survive — the 10% rule refers to energy, not individuals. 10% of the energy at one level is stored in organisms at the next.
• Forgetting that food webs show more realistic relationships than food chains — always use a food web answer when discussing ecosystem effects.

Tips and Tricks

• Arrow direction: "energy flows into the predator" — draw arrows pointing toward the thing that is eating.
• The 10% rule means every 10 kg of grass supports about 1 kg of rabbit, which supports 0.1 kg of fox. Energy decreases dramatically at each level.
• When predicting effects of removing a species, trace through the food web — going both up (less food for predators) and down (less predation of prey).
• Decomposers recycle nutrients but are not in the main energy flow pyramid — they process energy from all trophic levels.