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How is an ecosystem organised, and how do materials cycle through it?

The levels of organisation in an ecosystem, feeding relationships and food chains, predator-prey cycles, the carbon and water cycles, and the role of decomposers, with the required practical on sampling.

A focused answer to AQA GCSE Biology 4.7.2 to 4.7.3, covering the levels of organisation in an ecosystem, food chains and predator-prey cycles, the carbon and water cycles, decomposers, and the sampling required practical.

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  1. What this dot point is asking
  2. Levels of organisation in an ecosystem
  3. Food chains and predator-prey cycles
  4. The carbon and water cycles
  5. Required practical: sampling
  6. Try this

What this dot point is asking

AQA wants you to describe the levels of organisation in an ecosystem, interpret food chains and predator-prey cycles, explain the carbon and water cycles and the role of decomposers, and understand how organisms are sampled in the required practical.

Levels of organisation in an ecosystem

Organisms in a community depend on each other for food, shelter, pollination and seed dispersal, so they are interdependent. If one species is removed, others that rely on it are affected. A community where the species and environmental factors are in balance, so that population sizes stay roughly constant, is called a stable community.

Food chains and predator-prey cycles

A food chain always begins with a producer (usually a green plant or alga) that makes glucose by photosynthesis, transferring light energy into the ecosystem. Producers are eaten by primary consumers, which are eaten by secondary consumers, then tertiary consumers. Consumers that kill and eat other animals are predators, and those eaten are prey.

The carbon and water cycles

  • Carbon cycle: carbon dioxide is removed from the air by photosynthesis in plants and algae, passing carbon into food chains. It is returned to the air by respiration (of plants, animals and decomposers), combustion (burning fuels) and decomposition. This recycles carbon so it can be used to build new organisms.
  • Water cycle: water evaporates from seas, lakes and land, and is transpired by plants, forming water vapour. This condenses to form clouds and falls as precipitation (rain, snow), providing fresh water for organisms on land before it runs back to the sea.

The rate of decay is faster when it is warm (faster enzyme action), moist (decomposers need water) and well oxygenated (for aerobic respiration). This is why gardeners turn compost heaps to add air and keep them warm and damp.

Required practical: sampling

You can use a quadrat placed randomly to estimate the population size or the percentage cover of a species, then scale up to the whole area. A transect (a line across a habitat) is used to study how the distribution of organisms changes, for example from a path into a field, by placing quadrats at regular intervals along the line.

Try this

Q1. Define a community. [1 mark]

  • Cue. All the populations of different species living together in a habitat.

Q2. Explain why a fall in prey numbers leads to a fall in predator numbers. [2 marks]

  • Cue. With less food, fewer predators survive and reproduce, so their numbers fall.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksA field contains grass, rabbits and foxes. Describe how the numbers of rabbits and foxes change over time, and explain why these changes happen.
Show worked answer →

A 4-mark question rewards describing the predator-prey cycle and explaining the cause.

The numbers of rabbits (prey) and foxes (predators) rise and fall in cycles, with the predator peak slightly behind the prey peak. When rabbit numbers are high there is plenty of food, so more foxes survive and reproduce and fox numbers rise. The increasing foxes eat more rabbits, so rabbit numbers fall. With less food, fewer foxes survive, so fox numbers then fall, which lets the rabbits recover, and the cycle repeats.

Markers reward the cyclic pattern, the predator peak lagging the prey peak, and food availability as the cause of the changes.

AQA 20214 marksA student used quadrats to estimate the number of daisies in a field of area 800 square metres. The mean number of daisies in a 0.25 square metre quadrat was 6. Estimate the total number of daisies in the field, and describe how the student should have placed the quadrats to make the estimate valid. Show your working.
Show worked answer →

A 4-mark required-practical calculation rewards correct scaling plus valid technique.

Number per square metre equals 60.25=24\dfrac{6}{0.25} = 24 daisies per square metre. For the whole field, 24×800=19 20024 \times 800 = 19\ 200 daisies.

To make the estimate valid the quadrats must be placed randomly (for example using random number coordinates) so the sample is representative and not biased, and enough quadrats must be used to get a reliable mean.

Markers reward the correct density (24 per square metre), the total (19 200), and random placement with enough repeats.

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