AQA GCSE Biology (8461)

4.7.2 Organisation of an ecosystem

Organisation of an ecosystem ties together every food-related idea you've already met — photosynthesis, respiration, feeding relationships, decomposition — and asks you to think about how they fit together as a single system. This page walks through the hierarchy from individual to ecosystem, the four trophic levels, why biomass shrinks at each step, how the carbon and water cycles recycle materials, and what makes a community stable. By the end you'll write the exact phrases AQA examiners want: 'biomass lost in respiration and faeces', 'decomposers release mineral ions to the soil', and 'a change in one population affects the whole community'.

From the 8461 specification

An ecosystem is the interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment. Organisms in a community can be producers, primary consumers, secondary consumers or tertiary consumers. Photosynthetic organisms are the producers of biomass for life on Earth. Feeding relationships within a community can be represented by food chains. All food chains begin with a producer which synthesises molecules. This is usually a green plant or alga which makes glucose by photosynthesis. The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide to be used by plants in photosynthesis. Materials are continually recycled by the action of microorganisms (decomposers) which return mineral ions to the soil.

Why this matters

Ecosystems are everywhere — a rotting log, a pond, a rainforest, the open ocean. What they share is structure: producers harvest energy from sunlight, consumers eat producers (or other consumers), and decomposers break down everything that dies, returning the raw materials to start the cycle again. Energy flows in one direction (sun → producers → consumers → heat) but materials (carbon, water, nitrogen, mineral ions) are recycled endlessly. This means that the same carbon atom in your body has cycled through countless plants, animals and microbes over billions of years. Stable ecosystems balance the gains and losses at each level — they have plenty of producers to support consumers, and plenty of decomposers to recycle nutrients. Disturb that balance — by removing a top predator, cutting down forests, or pumping CO₂ into the atmosphere faster than photosynthesis removes it — and the whole system shifts.

How to learn this topic

Build on what you already know

GCSE 4.4: photosynthesis — plants use CO₂ and water to make glucose using light energy.
GCSE 4.4.2: aerobic respiration — glucose + oxygen → CO₂ + water + ATP. Releases energy in all living things.
GCSE 4.7.1: abiotic and biotic factors that affect organisms in an ecosystem.
KS3: food chains, decomposers, the water cycle, the basics of the carbon cycle.

Build up the hierarchy: individual → population → community → ecosystem.
Producers vs consumers vs decomposers — what each role does and why all three are essential.
Trophic levels — name level 1 to 4 with examples.
Energy and biomass losses — why only about 10% transfers between levels.
Decomposers — bacteria and fungi releasing enzymes outside their cells.
Carbon cycle — photosynthesis, respiration, decomposition, combustion.
Water cycle — evaporation, transpiration, condensation, precipitation.
Interdependence — predator-prey relationships and food web disruption.
Stable communities — what holds them together.

Key terms

ecosystem: The interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment. (AQA wants both halves: living community AND abiotic factors. Don't just say 'all the organisms in an area'.)
community: All the populations of different species living and interacting in a habitat at one time.
population: All the individuals of one species living in the same habitat.
producer: An organism that makes its own biomass from inorganic materials, usually by photosynthesis. On land: plants. In water: algae. (Producers ARE the base of every food chain. Marking phrase: 'producers provide biomass for all living things'.)
trophic level: A feeding position in a food chain or web. Level 1 = producers, Level 2 = primary consumers (herbivores), Level 3 = secondary consumers, Level 4 = tertiary consumers / apex predators. (AQA may number levels — make sure you know which is which. Producers are Level 1, not 0.)
biomass: The mass of living material in an organism, population or trophic level. Measured in grams or kilograms. (Biomass DECREASES up the trophic levels. The four marking phrases are: lost in respiration, lost in faeces, less biomass for next level, lost as heat / not eaten.)
decomposer: A microorganism (bacteria or fungi) that breaks down dead organic material by releasing enzymes onto it and absorbing the small molecules released. Returns CO₂ to the atmosphere and mineral ions to the soil. (Marking phrases: 'release enzymes', 'return mineral ions to the soil', 'release CO₂ for photosynthesis'.)
interdependence: When organisms in a community rely on each other for food, shelter, pollination or other resources. A change in one population affects the whole community. (Marking phrase: 'a change in one population affects the whole community'.)
stable community: A community in which the populations of all species and the abiotic factors remain roughly constant over time. Requires balance of producers, consumers and decomposers. (Examiners reward 'producers + decomposers' specifically. Both are required for stability.)
carbon cycle: The continuous recycling of carbon between the atmosphere, living things and the geosphere through photosynthesis, respiration, decomposition and combustion. (Be ready to name all four processes. 'Photosynthesis removes CO₂; respiration, decomposition and combustion return it.')

Notes

Levels of organisation

An ecosystem is the interaction of a community of living organisms (biotic factors) with the non-living (abiotic) parts of their environment. The hierarchy you must know:

Individual — one organism (one oak tree).
Population — all the individuals of one species living in one habitat (every oak tree in one wood).
Community — all the populations of different species living and interacting in one habitat (oaks, squirrels, foxes, fungi, bacteria — every living thing in the wood).
Ecosystem — the community plus the abiotic factors (light, water, temperature, soil, oxygen) that it interacts with.

Producers, consumers, decomposers

Producers are organisms that make their own biomass from inorganic materials. On land they are green plants; in oceans, lakes and rivers they are algae. Both make glucose by photosynthesis:

> carbon dioxide + water → (light) → glucose + oxygen

The glucose is used to build everything else: starch (storage), cellulose (cell walls), proteins, lipids. Producers are the base of every food chain. Without producers, no other organism could live — every animal, every fungus, every consumer gets its biomass and energy ultimately from producers.

Consumers are organisms that get their biomass by eating other organisms:

Primary consumers (herbivores) — eat producers (rabbits, cows, caterpillars).
Secondary consumers (carnivores or omnivores) — eat primary consumers (foxes, frogs).
Tertiary consumers — eat secondary consumers (eagles, lions).
Apex predators — at the top of the chain with no natural predators (eagles, sharks, polar bears).

Decomposers are microorganisms — bacteria and fungi — that break down dead organic material (dead plants, dead animals, faeces). They release enzymes out of their cells onto the dead material; the enzymes digest it, and the small molecules are absorbed by the decomposer. Decomposition releases carbon dioxide (from respiration of the decomposers) and mineral ions (nitrates, phosphates, potassium) back to the soil for producers to absorb.

Food chains, food webs and trophic levels

A food chain shows feeding relationships with arrows pointing from food to feeder:

> grass → rabbit → fox → eagle

The arrows show the direction of biomass and energy transfer. Each step is a trophic level:

Level 1: producers
Level 2: primary consumers
Level 3: secondary consumers
Level 4: tertiary consumers / apex predators

In reality organisms eat (and are eaten by) many species — so we draw food webs with many interlinked chains. Food webs show interdependence: removing one species affects many others.

Biomass and energy decrease up the trophic levels

This is the big AQA marking-phrase topic. Only about 10% of the biomass at one trophic level is transferred to the next. Around 90% is lost. Why?

Respiration — most of the biomass eaten is respired by the consumer to release energy for movement, growth, warmth. The energy ultimately leaves as heat.
Faeces — not all eaten material is digested. The undigested part is egested as faeces (and broken down by decomposers).
Not eaten — many parts of an organism aren't eaten (roots, bones, fur). They die and are decomposed.
Urine — nitrogenous waste excreted from breaking down excess protein.

The four marking phrases AQA wants for this:

Biomass is lost between trophic levels.
Used in respiration.
Lost in faeces.
Less biomass available to support organisms at the next trophic level.

This is why food chains rarely have more than four or five trophic levels — there isn't enough biomass left at the top to support a fifth tier.

Decomposers and material recycling

Decomposers (bacteria + fungi) are essential to a stable community. They:

Release enzymes onto dead organisms and waste.
Respire — releasing carbon dioxide that producers reuse in photosynthesis.
Return mineral ions (nitrates for proteins, phosphates for DNA, potassium) to the soil for producers to absorb through roots.

Without decomposers, dead bodies and waste would pile up and the minerals locked inside them would never return to the soil. Producers would run out of nutrients and die. The whole ecosystem would collapse.

The carbon cycle

Carbon moves between living things and the atmosphere through four key processes:

Photosynthesis — plants and algae take CO₂ from the atmosphere and lock the carbon into glucose, starch, cellulose, proteins.
Respiration — plants, animals and microbes break down glucose to release CO₂ back to the atmosphere.
Decomposition — when organisms die, decomposers respire the carbon back to CO₂.
Combustion — burning fossil fuels (coal, oil, gas) and wood releases stored carbon as CO₂.

The cycle balances when CO₂ added (by respiration, decomposition, combustion) equals CO₂ removed (by photosynthesis). Currently humans burn fossil fuels much faster than photosynthesis removes it — which is why atmospheric CO₂ is rising and the climate is warming.

The water cycle

Evaporation — water from oceans, lakes and rivers turns into water vapour using heat from the sun.
Transpiration — plants lose water vapour from leaves through stomata.
Condensation — water vapour cools high in the atmosphere and condenses into clouds.
Precipitation — rain, snow, hail fall back to the ground.
The water returns to rivers, soil and oceans, completing the cycle.

Interdependence and stable communities

In a stable community the populations of producers, consumers and decomposers stay roughly constant — abiotic factors (temperature, water, soil pH) are also stable.

Organisms in a community are interdependent: they rely on each other for food, shelter, pollination, seed dispersal. Remove one and others suffer. A classic example is predator-prey cycles — when rabbit numbers rise, fox numbers rise (more food), which then drives rabbit numbers down (more predation), which then drives fox numbers down (less food). The populations oscillate around a stable mean, with the predator peak lagging the prey peak.

Food-web disruption — for example removing a top predator — can cascade through the whole community. If foxes are removed, rabbit numbers explode, the grass is over-grazed, other herbivores starve, soil quality drops. A small change in one species ripples outward. This is why we worry about extinctions: ecosystems are tangled webs, not chains.

Exam tips

Memorise the hierarchy in order: individual → population → community → ecosystem. AQA loves single-mark definition questions on these.
For 'biomass decreases up the levels' (4 marks) the four phrases are: biomass lost between levels / used in respiration / lost in faeces / less biomass available for the next level.
Decomposers are BACTERIA and FUNGI — not worms, not woodlice. They release ENZYMES outside their cells (extracellular digestion).
Always name the TWO things decomposers return: CARBON DIOXIDE (to the atmosphere, used in photosynthesis) and MINERAL IONS (to the soil, absorbed by producer roots).
When asked about stable communities, say 'producers AND decomposers' explicitly — both are required.
Predator-prey graphs: the predator peak LAGS behind the prey peak. The cycle is: prey ↑ → predator ↑ → prey ↓ → predator ↓ → prey ↑.
Plants both PHOTOSYNTHESISE AND RESPIRE. Don't write 'plants only do photosynthesis' — that's wrong. Plants take in CO₂ for photosynthesis but also release CO₂ in respiration; the net effect is uptake during the day.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What's the difference between a community and an ecosystem?

A COMMUNITY is just the living organisms — all the populations of different species in one habitat interacting with each other. An ECOSYSTEM is the community PLUS the abiotic (non-living) factors — light intensity, temperature, water availability, soil pH, oxygen levels. So a pond ecosystem includes the fish, plants, insects and microbes (the community) AND the water, sunlight, dissolved oxygen and minerals (the abiotic factors). One ecosystem can contain many communities.

Why does biomass decrease up the trophic levels?

Because not all the biomass eaten by a consumer becomes part of that consumer. About 90% is lost at each level. The losses are: (1) RESPIRATION — most of the food is respired to release energy for movement, growth and warmth; the energy ultimately escapes as heat. (2) FAECES — not all the food is digested, so some is egested as faeces. (3) NOT EATEN — roots, bones, fur and so on are usually not consumed and end up decomposed instead. (4) URINE — excretion of nitrogenous waste. Only about 10% of the biomass at one level becomes biomass at the next — which is why food chains rarely have more than four or five trophic levels.

What do decomposers actually do?

Decomposers (bacteria and fungi) feed on dead organic material — dead plants, dead animals, faeces. They use EXTRACELLULAR DIGESTION: they release enzymes out of their cells onto the dead material, the enzymes break it down into small soluble molecules, and the decomposer absorbs those molecules. As they respire, they release CARBON DIOXIDE back to the atmosphere (which producers reuse in photosynthesis). They also release MINERAL IONS like nitrates and phosphates back to the soil, where producer roots absorb them. Without decomposers, dead matter would pile up and minerals would stay locked away — ecosystems would grind to a halt.

How does the carbon cycle work?

Carbon moves between the atmosphere, living things, and stores like fossil fuels in a continuous cycle. PHOTOSYNTHESIS removes CO₂ from the atmosphere — plants and algae lock the carbon into glucose, starch, cellulose and proteins. RESPIRATION (in plants, animals and microbes) breaks glucose back down and releases CO₂. DECOMPOSITION returns the carbon in dead organisms to the atmosphere via decomposer respiration. COMBUSTION — burning fossil fuels, wood or other organic material — releases stored carbon as CO₂. In a balanced cycle, removal by photosynthesis matches return by respiration, decomposition and combustion. Burning fossil fuels at the current rate is adding CO₂ faster than photosynthesis can remove it — which is why atmospheric CO₂ keeps rising.

What is interdependence and why does it matter?

Interdependence means organisms in a community depend on each other — for food, for pollination, for seed dispersal, for shelter. Because of these links, a change in one population affects the whole community. Classic example: if foxes are killed off by disease, rabbit numbers rise, the rabbits over-graze the grass, other herbivores (like deer) have less food, and the soil quality drops. The effect ripples outward. This is why protecting biodiversity matters — removing one species can have knock-on effects no one predicts.

Why does the fox population peak after the rabbit population?

Because foxes depend on rabbits for food, so their numbers respond to changes in prey numbers with a delay. When rabbits become plentiful, foxes have more food, fox cubs survive and the fox population GROWS — but this takes time (gestation, growing up). By the time fox numbers are high, the foxes have already started eating down the rabbits, so the rabbit population falls. Then the foxes start running out of food and their numbers fall too — letting the rabbits recover. The result is two oscillating curves with the predator (fox) peak always lagging behind the prey (rabbit) peak by about a quarter of a cycle.