9.1 Ecosystems

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. The same nitrogen atom in a protein in your muscle has cycled through soil bacteria, plant roots, herbivores and carnivores countless times. Stable ecosystems balance the gains and losses at each level. 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. Edexcel makes you reason explicitly about these knock-on effects, particularly in woodland food webs.

How to learn this topic

Build on what you already know

• Edexcel CB1: photosynthesis — plants use CO₂ and water to make glucose using light energy.
• Edexcel CB1: aerobic respiration — glucose + oxygen → CO₂ + water + ATP.
• Edexcel CB1: proteins are built from amino acids; amino acids contain nitrogen.
• KS3: food chains, decomposers, the water cycle, the basics of the carbon cycle.

1. Build up the hierarchy: individual → population → community → ecosystem.
2. Biotic vs abiotic factors and how each affects population size.
3. Producers, consumers and decomposers and what each role does.
4. Interdependence in a woodland food web — knock-on effects.
5. Carbon cycle — photosynthesis, respiration, decomposition, combustion.
6. Nitrogen cycle — nitrogen-fixing, nitrifying, denitrifying bacteria, decomposers.
7. Water cycle — evaporation, transpiration, condensation, precipitation.
8. Predator-prey cycles and stable communities.

Key terms

ecosystem

The interaction of a community of organisms with the abiotic (non-living) environment. (Edexcel 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 factor

A living factor that affects a population — predators, competitors, food availability or pathogens (disease). (Edexcel example: more predators → higher death rate → smaller prey population.)

abiotic factor

A non-living factor that affects a population — temperature, light intensity, water, soil pH, mineral ion availability, oxygen. (Edexcel example: lower temperatures reduce survival → smaller population.)

producer

An organism that makes its own biomass from inorganic materials, usually by photosynthesis. On land: plants. In water: algae.

decomposer

A microorganism (bacteria or fungi) that breaks down dead organic material by releasing enzymes onto it and absorbing the small molecules. Returns CO₂ to the atmosphere and converts proteins and urea into ammonia in the soil. (For the nitrogen cycle: 'decomposers break down proteins and urea into ammonia'.)

interdependence

When organisms in a community depend on each other for survival — food, shelter, pollination, seed dispersal. A change in one population affects others. (Edexcel phrase: 'organisms depend on each other for survival'.)

stable community

A community in which the populations of all species and the abiotic factors remain roughly constant over time.

nitrogen cycle

The continuous recycling of nitrogen between the atmosphere, soil and living organisms via nitrogen-fixing, nitrifying and denitrifying bacteria, and decomposers.

nitrogen-fixing bacteria

Bacteria that convert atmospheric nitrogen gas (N₂) into ammonia. Found free-living in soil and in root nodules of legumes (peas, beans, clover). (Edexcel marking phrase: 'nitrogen-fixing bacteria convert nitrogen gas to ammonia'.)

nitrifying bacteria

Aerobic soil bacteria that convert ammonium ions to nitrites and then to nitrates. (Edexcel marking phrase: 'nitrifying bacteria convert ammonium to nitrite then nitrate'.)

denitrifying bacteria

Bacteria found in waterlogged, low-oxygen soils that convert nitrates back to nitrogen gas, returning N₂ to the atmosphere.

ammonia

NH₃ (dissolves to form ammonium ions, NH₄⁺). The first nitrogen compound produced by nitrogen-fixing bacteria and by decomposers breaking down proteins and urea.

nitrate

NO₃⁻. The form of nitrogen plants absorb from soil through their roots, used to make amino acids and proteins. (Edexcel phrase: 'plants absorb nitrate through roots' and 'nitrate used to make proteins'.)

carbon cycle

The continuous recycling of carbon between the atmosphere, living things and the geosphere through photosynthesis, respiration, decomposition and combustion.

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. Learn the hierarchy in order:

• 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.
• Ecosystem — the community plus the abiotic factors (light, water, temperature, soil pH, mineral ions) that it interacts with.

Biotic and abiotic factors

Population sizes are set by two kinds of factor:

• Biotic factors (living): predators, competitors, food availability, pathogens (disease).
• Abiotic factors (non-living): temperature, light intensity, water, soil pH, mineral ion availability, oxygen.

Worked example — rabbits in a meadow. If temperature falls, lower temperatures reduce survival — fewer rabbits make it through winter, the population shrinks. If predators (foxes) move into the meadow, increased predator numbers increase death rate of rabbits, reducing population. Notice the structure of the Edexcel answer: name the factor, then explain the mechanism (lower → reduce survival; more predators → more deaths).

Producers, consumers, decomposers

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

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

Producers are the base of every food chain. Consumers get their biomass by eating other organisms: primary consumers (herbivores) eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers; apex predators are top of the chain. Decomposers — bacteria and fungi — break down dead organic material by releasing enzymes outside their cells (extracellular digestion). They release CO₂ by respiration and return mineral ions (including ammonia from proteins and urea) to the soil.

Interdependence in a woodland

Organisms depend on each other for survival. A classic Edexcel scenario uses the food web: oak → caterpillar → blue tit → sparrowhawk. Suppose a disease kills most of the caterpillars. Tracing the knock-on effects: blue tits decrease as they have no food; sparrowhawks decline because blue tits decreased; oak trees benefit as no caterpillars eat their leaves. Edexcel rewards this chain of cause-and-effect — name each species and say what happens and why.

The carbon cycle

Carbon moves between the atmosphere, living things and stores 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 glucose down and release CO₂ back to the atmosphere.
• Decomposition — when organisms die, decomposers break down the dead plant material, release enzymes to break down organic molecules, respire the organic compounds and CO₂ released into atmosphere during respiration.
• 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). Burning fossil fuels at the current rate adds CO₂ faster than photosynthesis removes it.

The nitrogen cycle (Edexcel emphasis)

Plants and animals need nitrogen to build proteins (and amino acids, DNA, chlorophyll). About 78% of the atmosphere is nitrogen gas (N₂) — but the triple bond is so strong that plants cannot use N₂ directly. Four groups of bacteria, plus decomposers, do all the chemical conversions in the nitrogen cycle.

1. Nitrogen-fixing bacteria. These bacteria convert nitrogen gas to ammonia. Some live free in the soil; others live inside root nodules of legumes (peas, beans, clover) in a mutualistic relationship — the plant gives them sugars, they give the plant a usable nitrogen source. Lightning also fixes a small amount of nitrogen non-biologically.

2. Decomposers (saprobionts). When plants and animals die, and when animals excrete urea, decomposers break down proteins and urea into ammonia. This puts ammonia (which dissolves as ammonium ions, NH₄⁺) into the soil.

3. Nitrifying bacteria. These are aerobic soil bacteria that convert ammonium to nitrite then nitrate in two steps (NH₄⁺ → NO₂⁻ → NO₃⁻). Nitrate is the form plants can use.

4. Plants take up nitrate. Plants absorb nitrate through roots and use nitrate to make proteins (and amino acids). The nitrogen then passes along food chains as animals eat plants.

5. Denitrifying bacteria. Found in waterlogged soils with little oxygen, these bacteria convert nitrate back to N₂ gas, returning it to the atmosphere and completing the cycle.

A full Edexcel six-mark nitrogen-cycle answer strings the bacteria together: nitrogen-fixing bacteria convert nitrogen gas to ammonia; nitrifying bacteria convert ammonium to nitrite then nitrate; plants take up nitrate from soil; plants use nitrate to make proteins/amino acids.

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 in the atmosphere and condenses into clouds.
• Precipitation — rain, snow, hail fall back to the ground; water returns to rivers, soil and oceans.

Predator-prey cycles and stable communities

In a stable community the populations of producers, consumers and decomposers stay roughly constant. Predator-prey cycles are the classic illustration: when rabbit numbers rise, fox numbers rise (more food), which then drives rabbit numbers down (more predation), which then drives fox numbers down. The predator peak lags the prey peak by about a quarter of a cycle.

Exam tips

• For nitrogen cycle questions, name the bacteria EXPLICITLY: 'nitrogen-fixing bacteria convert nitrogen gas to ammonia' and 'nitrifying bacteria convert ammonium to nitrite then nitrate'. Never write just 'bacteria' — Edexcel wants the specific type.
• Always finish a nitrogen-cycle answer with the plant step: 'plants absorb nitrate through roots' and 'nitrate is used to make proteins / amino acids'.
• Biotic vs abiotic: BIOTIC = living (predators, competitors, food, pathogens); ABIOTIC = non-living (temperature, light, water, pH, mineral ions). Edexcel often asks you to classify factors into the two groups.
• When explaining how a factor affects a population, give the MECHANISM, not just the direction: 'increased predator numbers increase the death rate of rabbits, reducing the population'.
• Interdependence questions in a food web: trace every link. Name the species, say what happens to its population, and say WHY (no food → decrease; no predator → benefit/increase).
• Decomposers and the carbon cycle: 'decomposers break down dead plant material', 'release enzymes', 'respire organic compounds', 'CO₂ released into atmosphere during respiration'. These four phrases score four marks.
• Plants both PHOTOSYNTHESISE AND RESPIRE. Don't write 'plants only do photosynthesis' — that's wrong.

Mark-scheme phrasing

Common misconceptions

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Worked example

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Frequently asked questions