Food Chains & Nutrient Cycling

Energy and nutrients are the lifeblood of every ecosystem. Understanding how energy flows through food chains and how nutrients are recycled is essential for the AQA GCSE Living World topic. This lesson explains food chains, food webs, trophic levels, and the nutrient cycle — and shows how these processes differ between biomes.

---

Energy Flow in Ecosystems

All energy in an ecosystem ultimately comes from the Sun. Producers (green plants) capture solar energy through photosynthesis and convert it into chemical energy stored in glucose. This energy is then passed along the food chain when organisms eat each other.

The Basic Food Chain

A food chain shows the transfer of energy from one organism to the next in a linear sequence:

Sun → Producer → Primary Consumer → Secondary Consumer → Tertiary Consumer

Example (UK woodland):

Sun → Oak tree (leaves) → Caterpillar → Blue tit → Sparrowhawk

The following diagram shows how energy flows through a typical ecosystem, including the role of decomposers in recycling nutrients:

graph LR
    A[Sun] --> B[Producers — Plants]
    B --> C[Primary Consumers — Herbivores]
    C --> D[Secondary Consumers — Carnivores]
    D --> E[Tertiary Consumers — Top Predators]
    B --> F[Decomposers]
    C --> F
    D --> F
    E --> F
    F --> G[Nutrients returned to soil]
    G --> B

Each stage of the food chain is called a trophic level:

Trophic Level	Name	Role	Example
1	Producer	Makes food via photosynthesis	Oak tree, grass, algae
2	Primary consumer	Herbivore — eats producers	Caterpillar, rabbit
3	Secondary consumer	Carnivore — eats primary consumers	Blue tit, frog
4	Tertiary consumer	Top predator — eats secondary consumers	Sparrowhawk, fox

---

Energy Loss Between Trophic Levels

A critical concept is that energy is lost at every trophic level. Only about 10% of the energy at one level is passed on to the next. The rest is lost through:

• Respiration — organisms use energy for life processes (movement, keeping warm)
• Excretion — energy is lost in waste products (urine, faeces)
• Heat loss — energy escapes as heat from the organism's body
• Uneaten parts — not all of an organism is consumed (bones, roots, bark)

This is why food chains rarely have more than four or five trophic levels — there is simply not enough energy left to support another level.

Exam Tip: If asked why food chains are usually short, explain that energy is lost at each trophic level through respiration, excretion, and heat. By the fourth or fifth level, there is not enough energy remaining to sustain another population of consumers.

---

Food Webs

In reality, organisms do not exist in simple, single food chains. Most animals eat more than one type of food, and most organisms are eaten by more than one predator. A food web shows these complex, interconnected feeding relationships.

Why Food Webs Matter

Food webs illustrate interdependence more clearly than food chains. If one species is removed from a food web, it can affect many other species in unpredictable ways.

Example: If a disease kills all the rabbits in a grassland ecosystem:

• Foxes lose a major food source and may decline or switch to eating more voles
• Vole populations then decline due to increased predation
• Grass grows taller because there are no rabbits grazing it
• Ground-nesting birds may benefit from the taller grass providing more cover
• The entire balance of the ecosystem shifts

Exam Tip: When discussing food webs, always explain the knock-on effects of removing or adding a species. Use phrases like "this would lead to...", "as a result...", "consequently..." to show connected reasoning.

---

Decomposers and Their Role

Decomposers (bacteria and fungi) are often overlooked but are absolutely essential to ecosystems. They break down dead organic matter (dead plants, dead animals, faeces) and release the nutrients locked inside back into the soil.

Without decomposers:

• Dead material would pile up
• Nutrients would be locked away and unavailable
• Producers would eventually run out of the minerals they need to grow
• The entire ecosystem would grind to a halt

---

The Nutrient Cycle

Nutrients (such as nitrogen, phosphorus, and carbon) are constantly being recycled within an ecosystem. Unlike energy, which flows through and is eventually lost as heat, nutrients are cycled — used, returned to the soil, and used again.

The nutrient cycle has three main stores and three main flows:

Stores

Store	Description
Biomass	Nutrients stored in living organisms (plants, animals)
Litter	Nutrients stored in dead organic matter on the ground (fallen leaves, dead animals)
Soil	Nutrients stored in the soil as minerals dissolved in water

Flows (Transfers)

Flow	Description
Uptake	Plants absorb nutrients from the soil through their roots
Fallout / Death	Nutrients transfer from biomass to litter when leaves fall or organisms die
Decomposition	Decomposers break down litter, releasing nutrients back into the soil

There are also inputs (weathering of rock, rainfall adding dissolved minerals) and outputs (leaching, surface run-off removing nutrients from the system).

---

Nutrient Cycling in Different Biomes

The speed and efficiency of nutrient cycling varies dramatically between biomes. This is one of the most important comparisons for your exam:

Tropical Rainforest

Feature	Detail
Biomass store	Very large — dense vegetation stores huge amounts of nutrients
Litter store	Very small — dead material decomposes rapidly in the hot, humid conditions
Soil store	Small — nutrients are quickly taken up by plants or washed away by heavy rain
Rate of cycling	Very fast — decomposition happens quickly, and plants absorb nutrients rapidly

The nutrient cycle in a tropical rainforest is rapid and efficient. However, most nutrients are stored in the biomass, not the soil. This is why deforestation is so devastating — when trees are removed, the nutrients are lost and the thin, infertile soil is exposed.

Hot Desert

Feature	Detail
Biomass store	Very small — sparse vegetation due to extreme aridity
Litter store	Very small — few plants means little dead organic matter
Soil store	Small — thin, sandy soils with few nutrients
Rate of cycling	Very slow — lack of moisture slows decomposition; few organisms contribute to the cycle

Temperate Deciduous Forest (UK)

Feature	Detail
Biomass store	Moderate — deciduous trees shed leaves seasonally
Litter store	Moderate-large — leaf fall in autumn creates a thick litter layer
Soil store	Large — nutrients accumulate in the fertile brown earth soils
Rate of cycling	Moderate — decomposition is slower than in tropical forests due to cooler temperatures

Exam Tip: You may be asked to compare the nutrient cycle of two biomes. The key comparison is between the tropical rainforest (fast cycling, nutrients stored in biomass) and the hot desert or temperate forest (slower cycling, nutrients stored in soil or litter). Always explain why the rate differs — it comes down to temperature and moisture affecting the rate of decomposition.

---

Gersmehl Diagrams

The Gersmehl model is a simple diagram used to represent nutrient cycling. It shows the three stores (biomass, litter, soil) as circles of different sizes, with arrows showing the flows between them.