Key Stage 4 science · Biology – Ecosystems

GCSE ecosystems, decomposers and the carbon cycle

An ecosystem is a community of living things interacting with each other and with their non-living surroundings. At GCSE, students learn the levels of organisation within an ecosystem, how abiotic and biotic factors affect communities, how materials such as carbon and water cycle between living and non-living parts, and why decomposers matter so much. It builds on earlier food chain work and leads into biodiversity and human impact.

Ecosystems and material cycles, explained

Ecologists describe nature in levels. A single living thing is an organism. All the organisms of one species in an area form a population. The populations of every species living together make a community. An ecosystem is the community plus its non-living environment, such as soil, water, air and climate, and the interactions between them. Examples might be a pond, a woodland or a hedgerow. In a stable community, the species and the environmental factors are in balance, so population sizes stay fairly constant over time.

Many factors affect which organisms can live somewhere and in what numbers. Abiotic factors are non-living: light intensity, temperature, moisture, soil pH and mineral content, wind and, for aquatic life, oxygen levels. Biotic factors are living: the availability of food, new predators arriving, pathogens and competition. Organisms compete with other species and with members of their own species: plants for light, space, water and mineral ions, and animals for food, mates and territory. Interdependence means that a change to one species can ripple through the whole community, for example if the prey of a predator declines.

The atoms that make up living things are limited, so they have to be recycled. In the carbon cycle, plants and algae take carbon dioxide from the air and use it in photosynthesis. Carbon passes to animals through feeding, returns to the air by respiration in plants, animals and microorganisms, and is released from dead material by decay. Burning wood and fossil fuels also releases carbon dioxide. In the water cycle, water evaporates from oceans and land and is lost from plants by transpiration, condenses into clouds, and falls as precipitation, which provides fresh water for living things. Many schools concentrate on these two cycles, although others, such as nitrogen, also exist.

Decomposers, mainly bacteria and fungi, are what keep the cycles turning. They release enzymes onto dead plants and animals and waste, breaking the material down into small molecules which they absorb. Their respiration returns carbon dioxide to the air, and the mineral ions that are released go back into the soil for plants to take up. Decay is quickest in warm, moist conditions with plenty of oxygen, which is why a compost heap needs turning and why food lasts longer in a fridge. Without decomposers, dead material would pile up and nutrients would run out.

Words your child will hear

Population: All the organisms of one species living in an area.
Community: All the populations of different species living together in an area.
Ecosystem: A community of living things together with the non-living environment they interact with.
Abiotic factor: A non-living part of the environment, such as temperature or soil pH.
Biotic factor: A living part of the environment, such as a predator, food supply or disease.
Decomposer: A microorganism, mainly bacteria and fungi, that breaks down dead material and waste.
Interdependence: The way species in a community depend on each other, for food, shelter and other needs.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about levels of organisation within an ecosystem
Students should be taught about some abiotic and biotic factors which affect communities; the importance of interactions between organisms in a community
Students should be taught about how materials cycle through abiotic and biotic components of ecosystems
Students should be taught about the role of microorganisms (decomposers) in the cycling of materials through an ecosystem

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A model exam answer

Explain how carbon in a dead oak leaf on the woodland floor can become part of a leaf on a new tree. (4 marks)

The leaf falls and is attacked by decomposers, which are bacteria and fungi. They release enzymes that break down the carbon compounds in the leaf and absorb the products.
The decomposers respire. Their respiration releases carbon dioxide into the air, so carbon from the leaf returns to the atmosphere.
A new tree takes in carbon dioxide through its stomata. In photosynthesis, the carbon is used to make glucose.
The tree uses the glucose to make cellulose and other compounds, which build new leaves. The carbon from the dead leaf is now part of a living one.

Decomposers break down the dead leaf and respire, releasing carbon dioxide into the air. The new tree takes in that carbon dioxide and uses it for photosynthesis to make glucose, then cellulose and other compounds for its leaves.

Common mistakes and how to help

A community and an ecosystem are the same thing

Both words describe living things in an area, and everyday speech uses them loosely.

What helps: A community is only the living things. An ecosystem includes the community and the abiotic environment too. Ask: is the soil part of the community? It is part of the ecosystem.

Plants only take in carbon dioxide and animals only give it out

Photosynthesis and respiration are often shown as a simple swap between plants and animals.

What helps: Plants, animals, bacteria and fungi all respire and release carbon dioxide. Plants also photosynthesise, and in the light they usually take in more carbon dioxide than they give out.

Dead material just rots away and disappears

Decay seems to happen on its own, and we rarely see the microorganisms doing it.

What helps: Decomposers are living things, and the atoms are not destroyed. They are released as carbon dioxide, water and mineral ions that other organisms can use again.

Competition only happens between different species

The picture of a predator and its prey, or two kinds of plants, comes to mind first.

What helps: Individuals of the same species compete too, since they have very similar needs. Two oak seedlings under one tree compete for light.

How to help at home

Go on a micro-habitat hunt. In a garden or park, look under a log, in leaf litter or along a hedge. Ask your teenager to name a population, a community and an ecosystem, and one abiotic and one biotic factor that affect it.
Compare two spots. Take readings in a sunny and a shady spot with a thermometer, or compare how damp the soil feels. Link the differences in the plants growing there to the abiotic factors.
Watch decay happen. Leave a piece of fruit peel on the soil in a garden or in a compost bin and check it each week. Talk about what is feeding on it and why it rots more slowly in cold or dry weather.
Draw a carbon cycle without help. Ask for the cycle from memory, with arrows labelled photosynthesis, respiration, feeding, decay and combustion. Check that the arrow for respiration comes from plants and decomposers as well as animals.