Organisation of an ecosystem

Ecosystems are structured into feeding levels that show how energy and biomass pass through living organisms — see "Trophic levels" for how these levels and their pyramids of biomass work in detail.

• •Producers (green plants and algae) synthesise glucose using light energy from photosynthesis.
• •Primary consumers eat producers, secondary consumers eat primary consumers, and tertiary consumers eat secondary consumers.
• •In a stable community, numbers of predators and prey rise and fall in predictable cycles.

All materials in the living world are recycled to provide the building blocks for future organisms — the carbon cycle and the water cycle are the two named cycles AQA expects you to know.

Carbon moves between the atmosphere, living organisms and the ground:

1. 1Photosynthesis removes carbon dioxide from the atmosphere — plants use the carbon to build glucose and other organic compounds.
2. 2Carbon passes along food chains as animals eat plants, and other animals.
3. 3Respiration (in plants, animals and microorganisms) releases carbon dioxide back into the atmosphere — this happens continuously, not just at night.
4. 4When organisms die, decomposers (bacteria and fungi) break down the remains, releasing carbon dioxide and returning mineral ions to the soil.
5. 5Combustion of fossil fuels and wood releases carbon that has been locked away for millions of years, adding it back to the atmosphere.

Water moves between the sea, the air and the land:

1. 1The sun heats water in oceans, rivers and lakes, causing it to evaporate into water vapour.
2. 2Plants lose water vapour from their leaves by transpiration, adding more vapour to the air.
3. 3Water vapour rises, cools, and condenses into tiny droplets that form clouds.
4. 4Droplets grow heavy and fall as precipitation (rain, snow, hail), collecting in rivers and groundwater that flow back to the sea.

Microorganisms decay dead plant and animal material, returning mineral ions and carbon to the environment.

• •The rate of decay is increased by higher temperature, sufficient moisture, and high oxygen availability.
• •Compost produced by decay is used as a natural fertiliser to return nutrients to soil.
• •Anaerobic decay occurs without oxygen, producing methane gas which can be burned as a biofuel in biogas generators.
• •Extremely high temperatures slow decay by denaturing decomposer enzymes.

Environmental conditions continuously change and directly affect where organisms can live and reproduce.

• •Changes in temperature alter species distribution, driving migrations to warmer or cooler habitats.
• •Variations in water availability affect plant and animal survival during seasonal droughts or floods.
• •Changes in atmospheric gases like carbon dioxide or sulfur dioxide affect photosynthesis and organism health.
• •Environmental factors can change due to seasonal patterns, geographic location, or human interaction.