B4.1 Ecosystems

Why this matters

Ecosystems are everywhere — a rock pool, a rotting log, a hedgerow, 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. OCR Gateway places particular emphasis on the interaction of biotic and abiotic factors, on positive relationships like mutualism, and on the modern human impact of burning fossil fuels and peat — which adds CO₂ to the atmosphere faster than photosynthesis removes it.

How to learn this topic

Build on what you already know

• OCR B2: photosynthesis — plants use CO₂ and water to make glucose using light energy.
• OCR B2: aerobic respiration — glucose + oxygen → CO₂ + water + ATP.
• OCR B3: microorganisms — bacteria and fungi as decomposers.
• KS3: food chains, decomposers, the water cycle, the basics of the carbon cycle.

1. Build the hierarchy: individual → population → community → ecosystem.
2. Define biotic factors (living) vs abiotic factors (non-living) with worked examples.
3. Producers, consumers, decomposers — what each role does.
4. Interdependence and food-web cascades.
5. Mutualism — clownfish + anemone, nitrogen-fixing bacteria + legume roots.
6. Decomposers — bacteria and fungi releasing enzymes; returning CO₂ and mineral ions.
7. Carbon cycle — photosynthesis, respiration, decomposition, combustion (including modern fossil-fuel + peat burning).

Key terms

ecosystem

The interaction of a community of living organisms (biotic) with the non-living (abiotic) parts of their environment. (OCR 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.

biotic

A living factor that affects the size of a population — for example food availability, predators, pathogens or competitors. (OCR may ask 'name a biotic factor' for 1 mark — any living factor counts.)

abiotic

A non-living physical or chemical factor that affects organisms — for example temperature, light intensity, soil pH, water and oxygen levels, CO₂ concentration. (OCR marking phrase: 'temperature is an abiotic factor'. Don't muddle with biotic — abiotic = non-living.)

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'.)

decomposer

A microorganism (bacteria or fungus) that breaks down dead organic material by releasing enzymes onto it and absorbing the digested products. Returns CO₂ to the atmosphere and mineral ions to the soil. (OCR phrases: 'decomposers break down dead material', 'mineral ions returned to soil', 'CO₂ released into atmosphere'.)

interdependence

When organisms in a community rely on each other for food, shelter, pollination or other resources. A change in one population affects others. (OCR marking phrase: 'organisms depend on each other so a change in one population affects others'.)

mutualism

A relationship between two species in which both benefit. OCR examples: clownfish + sea anemone; nitrogen-fixing bacteria + legume roots. (OCR phrase: 'mutualism — both benefit'. Always name BOTH partners and what each gets.)

carbon cycle

The continuous recycling of carbon between the atmosphere, living things and geological stores through photosynthesis, respiration, decomposition and combustion. (Be ready to name all four processes. 'Photosynthesis removes CO₂; respiration, decomposition and combustion return it.')

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. (OCR rewards 'producers + decomposers' specifically. Both are required for stability.)

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. OCR wants the hierarchy in order:

• Individual — one organism (one rabbit).
• Population — all the individuals of one species in one habitat (every rabbit in one field).
• Community — all the populations of different species living and interacting in one habitat (rabbits, foxes, grass, fungi, bacteria — everything alive there).
• Ecosystem — the community PLUS the abiotic factors (light, water, temperature, soil, oxygen) it interacts with.

Biotic vs abiotic factors

This is the OCR-specific framing — you must be able to sort factors into the two boxes.

Biotic factors are LIVING factors that affect population size:

• Food (and water) availability — more food means more organisms can be supported.
• Predators — predators eat prey, so more predators decreases the prey population.
• Pathogens (disease) — disease can wipe out vulnerable populations.
• Competitors — other organisms competing for the same food, mate or territory reduce the population.

Abiotic factors are NON-LIVING factors:

• Temperature — affects enzyme activity, growth rates and survival.
• Light intensity — affects the rate of photosynthesis, so affects producers (and everything that eats them).
• Soil pH and mineral content — affects which plants can grow.
• Water and oxygen levels — required for respiration and photosynthesis.
• Carbon dioxide concentration — limiting factor for plant growth.

When OCR asks about a rabbit population on grassland, expect to write: predators eat rabbits so the population decreases (biotic); less food availability decreases the population (biotic); temperature is an abiotic factor — lower temperatures slow grass growth, reducing food (abiotic affecting a biotic).

Producers, consumers, decomposers

Producers 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

Producers are the base of every food chain. Without producers, no other organism could live.

Consumers get their biomass by eating other organisms — primary consumers eat producers, secondary consumers eat primary, tertiary eat secondary, apex predators sit at the top.

Decomposers are bacteria and fungi that break down dead organic material. They release enzymes outside their cells (extracellular digestion), the enzymes digest the dead material, and the decomposer absorbs the small molecules. Decomposition releases carbon dioxide (from decomposer respiration) and mineral ions (nitrates, phosphates, potassium) back to the soil for producers to absorb.

Interdependence — food-web cascades

Organisms in a community are interdependent: they rely on each other for food, shelter, pollination, seed dispersal. A change in one population affects others — sometimes far down the food web.

A classic OCR woodland example: oak → caterpillar → blue tit → sparrowhawk. Suppose sparrowhawk numbers RISE. Then:

1. More sparrowhawks eat more blue tits, so the blue tit population FALLS.
2. Fewer blue tits means fewer caterpillars eaten, so the caterpillar population RISES.
3. More caterpillars eat more oak leaves, so the oaks are DAMAGED.

The phrase OCR rewards is: organisms depend on each other, so a change in one population affects others.

The same logic explains classic predator-prey cycles: prey numbers rise → predator numbers rise (more food) → prey numbers fall (more predation) → predator numbers fall (less food) → prey recover. The predator peak always LAGS behind the prey peak.

Mutualism — a positive interdependent relationship

Mutualism is a relationship between two species where both benefit. OCR uses two named examples:

Clownfish and sea anemones. The anemone has stinging tentacles that deter predators; the clownfish lives among the tentacles, protected from predators. In return, the clownfish chases away butterflyfish and other anemone-eating fish, and its waste provides nutrients for the anemone. If the clownfish were removed, anemone-eating fish would not be driven away and the anemone population would decrease. Both benefit — that is mutualism.

Nitrogen-fixing bacteria and legume roots. Legumes (peas, beans, clover) have root nodules containing nitrogen-fixing bacteria (e.g. Rhizobium). The bacteria convert atmospheric N₂ into ammonia / nitrates that the plant can use to make proteins. In return, the plant provides the bacteria with sugars (from photosynthesis) and a protected home. Both species benefit — again, mutualism.

Decomposers and material recycling

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

1. Release enzymes onto dead organisms and waste.
2. Respire — releasing CO₂ back into the atmosphere, where producers reuse it in photosynthesis.
3. Return mineral ions (nitrates, phosphates, potassium) to the soil, where plants / producers absorb mineral ions from soil to grow.

Without decomposers, dead bodies and waste would pile up and minerals would never return to the soil — producers would run out of nutrients and the ecosystem would collapse.

The carbon cycle

Carbon moves between living things, the atmosphere and geological stores through four key processes. OCR rewards naming all four explicitly:

• Photosynthesis — plants and algae take CO₂ from the atmosphere and lock the carbon into glucose, starch, cellulose, proteins. Photosynthesis removes / absorbs CO₂.
• Respiration — plants, animals and microbes break down glucose to release CO₂. Respiration releases CO₂.
• Decomposition — when organisms die, decomposers break down the dead material; their respiration releases CO₂. Decomposers break down dead material releasing CO₂.
• Combustion (burning) — burning fossil fuels (coal, oil, gas), wood and peat releases stored carbon as CO₂. Burning / combustion releases CO₂.

In a balanced cycle, removal by photosynthesis matches return by respiration, decomposition and combustion. Modern combustion is the key disturbance: humans burn fossil fuels and peatlands much faster than photosynthesis removes the CO₂, so atmospheric CO₂ keeps rising and the climate warms.

Stable communities

In a stable community the populations of producers, consumers and decomposers stay roughly constant, and the abiotic factors (temperature, water, pH) are also stable. Both producers AND decomposers are required: producers provide biomass for everything else, decomposers recycle the mineral ions so producers can keep growing. Disturb either side and the ecosystem can collapse.

Exam tips

• Memorise the OCR hierarchy in order: individual → population → community → ecosystem. Single-mark definition questions are common.
• Have a list ready of FOUR biotic factors (food availability, predators, pathogens, competitors) and FOUR abiotic factors (temperature, light intensity, soil pH/nutrients, water/oxygen/CO₂ levels) — OCR will ask for examples of each.
• For mutualism: ALWAYS name both species and state the benefit each one gets. 'Clownfish gets protection from predators; anemone gets nutrients and protection from anemone-eating fish.'
• Decomposers are BACTERIA and FUNGI — they release ENZYMES outside their cells, return CO₂ to the atmosphere AND mineral ions to the soil.
• Carbon cycle: name all four processes — PHOTOSYNTHESIS (removes), RESPIRATION, DECOMPOSITION, COMBUSTION (return). OCR will dock marks if you forget combustion.
• Food-web cascade questions: write the chain of effects step by step. 'More X → fewer Y → more Z → effect on the producer.'
• Always finish food-web answers with the OCR phrase: 'organisms depend on each other, so a change in one population affects others'.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions