Organisation of an ecosystem — AQA GCSE Biology
Food chains, sampling with quadrats and transects, predator-prey cycles, the carbon and water cycles, decay and the effect of environmental change.
Revision notes
6 short notes, in the order of the specification. Each one in short:
A food chain represents the feeding relationships within a community. All food chains begin with a producer, usually a green plant or alga, which makes glucose by photosynthesis. Primary consumers eat the producer, secondary consumers eat the primary consumers, and tertiary consumers eat the secondary consumers. Consumers that kill and eat other animals are predators, and those eaten are prey.
Sampling: distribution and abundance
PracticalQuadrats and transects are sampling methods used to find the distribution and abundance of species. A quadrat is a square frame placed at random positions to count organisms and estimate population size. A transect is a line with quadrats placed at regular intervals along it, showing how a species changes as a factor such as light intensity changes.
Predator-prey cycles are the regular rise and fall in the numbers of predators and prey in a stable community. When prey numbers are high, the predators have plenty of food, so their numbers rise. More prey are then eaten, so prey numbers fall, and with less food the predator numbers fall too. On a graph, the prey peak comes first.
The carbon cycle returns carbon from organisms to the atmosphere as carbon dioxide, to be used by plants in photosynthesis. Carbon passes along food chains, and plants, animals and decomposing microorganisms release carbon dioxide by respiration. The water cycle provides fresh water for plants and animals on land through evaporation and precipitation. Decomposers also return mineral ions to the soil.
Decay is the breakdown of dead biological material by microorganisms. It is faster in warm conditions, up to an optimum, in moist conditions and when oxygen is available, because the microorganisms' enzymes and respiration work faster. If it is too hot, the enzymes denature. Gardeners use these conditions to make compost, and anaerobic decay produces methane gas for fuel.
Impact of environmental change
Triple onlyHigher tierEnvironmental changes affect the distribution of species in an ecosystem. These changes include temperature, the availability of water and the composition of atmospheric gases, and they may be seasonal, geographic or caused by human interaction. For example, burning fossil fuels changes the composition of atmospheric gases and contributes to rising temperatures. A species is only found where conditions suit it.
Exam questions
8 exam-style questions (28 marks), each with its mark scheme.
Answer the questionsFlashcards
26 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of ecology on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheets
Method, variables, risks and a results table, with questions to answer.
Print it
Free PDFs to print or save.
Quick check questions
What does a food chain always begin with?
Show answer
A producer, usually a green plant or alga that makes glucose by photosynthesis.
Why do ecologists use a transect?
Show answer
To see how the distribution of a species changes along a line as a factor changes.
In a predator-prey graph, which peak comes first?
Show answer
The prey peak, followed by the predator peak.
Which organisms return carbon dioxide and mineral ions to the environment when material decays?
Show answer
Microorganisms (decomposers).
What does the water cycle provide for plants and animals on land?
Show answer
Fresh water, before it drains into the seas.
Frequently asked questions
How does carbon move through the carbon cycle?
Carbon moves through the carbon cycle when plants and algae take in carbon dioxide for photosynthesis, then carbon passes along food chains as animals feed. Plants, animals and microorganisms release carbon dioxide back into the atmosphere by respiration. When organisms die, microorganisms decompose them and release carbon dioxide by respiration, so carbon is available for new organisms.
How do you use a quadrat to estimate population size?
To estimate population size, place a quadrat at random positions, using random coordinates to avoid bias, and count the organisms of the species inside it each time. Repeat many times, for example 10 or more quadrats, and calculate the mean number per m². Then multiply the mean per m² by the total area of the habitat in m².
Why do predator and prey populations rise and fall in cycles?
Predator and prey numbers rise and fall in cycles because predator numbers depend on their food supply. When prey are plentiful, predators eat more and breed, so their numbers rise. More prey are then eaten, so prey numbers fall. With less food, predator numbers fall too, fewer prey are eaten, and prey numbers rise again.
How does the water cycle work?
In the water cycle, water evaporates from the sea, from the land and from plants. The water vapour rises and cools, and condenses to form clouds. It then falls as precipitation, such as rain, hail and snow. This provides fresh water for plants and animals on land before it drains into rivers and then the seas.
What factors affect the rate of decay?
Triple only The rate of decay depends on temperature, water and oxygen. Warmer conditions speed up decay, because the enzymes and respiration of the microorganisms work faster, until an optimum; if it is too hot the enzymes denature. Moist conditions speed up decay, and most decomposers need oxygen for aerobic respiration, so more oxygen means faster decay.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026