How are organisms organised into ecosystems, and how do they depend on each other?
The terms population, community, habitat and ecosystem, the difference between producers, consumers and decomposers, food chains and food webs, and the interdependence of organisms including predator and prey relationships.
A focused CCEA GCSE Biology answer on ecological relationships, covering the terms population, community, habitat and ecosystem, producers, consumers and decomposers, food chains and webs, and interdependence including predator and prey relationships.
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What this dot point is asking
CCEA wants you to define population, community, habitat and ecosystem, distinguish producers, consumers and decomposers, build and read food chains and food webs, and explain the interdependence of organisms including predator and prey relationships.
Key ecological terms
Producers, consumers and decomposers
Food chains and food webs
A food chain shows the feeding links in order, with arrows pointing in the direction the energy flows (from the food to the feeder). A food web is many food chains linked together, showing the more realistic picture that most organisms eat, and are eaten by, several others.
Examples in context
- Example 1. A predator and prey cycle
- In a community of foxes and rabbits, when rabbits are plentiful the foxes have lots of food, so the fox population rises. The rising number of foxes eats more rabbits, so the rabbit population falls. With fewer rabbits, some foxes starve, so the fox population falls, which lets the rabbits recover. This produces the classic out-of-step cycle, with the predator peak following the prey peak. CCEA often shows this as a graph and asks you to explain the link.
- Example 2. Why interdependence matters
- In a food web, removing one species rarely affects just that species. If a disease wiped out the rabbits, the foxes would lose a major food source and might switch to other prey or decline, while the grass the rabbits ate would grow more thickly. This ripple effect shows interdependence: organisms rely on each other for food, and a change in one population spreads through the whole web. Conservation works because of this connectedness.
- Example 3. The difference between a food chain and a food web
- A single food chain, such as grass to rabbit to fox, makes it look as though each organism has only one food source and one predator. In reality a fox also eats mice and birds, and rabbits are also eaten by stoats and buzzards. A food web links many chains together to show this fuller picture. The advantage of a web is that if one food source disappears, an animal can often switch to another, making the community more stable. CCEA expects you to read a food web and predict the effects of a change, so practise tracing the links in both directions.
Try this
Q1. What is a producer, and give an example. [2 marks]
- Cue. An organism that makes its own food by photosynthesis, such as a green plant.
Q2. In the chain grass to rabbit to fox, what does the rabbit-to-fox arrow show? [1 mark]
- Cue. That the fox eats the rabbit (energy flows from rabbit to fox).
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20204 marksDefine the terms population, community, habitat and ecosystem.Show worked answer →
Four marks for four clear definitions.
A population is all the organisms of one species living in the same area at the same time.
A community is all the populations of different species living together in the same area.
A habitat is the place where an organism lives.
An ecosystem is a community of organisms together with the non-living (abiotic) parts of their environment, interacting as a unit.
Markers reward precise definitions; a common slip is confusing population (one species) with community (all species).
CCEA 20193 marksExplain what would happen to a food web if the number of one prey species fell sharply.Show worked answer →
Three marks for following the knock-on effects through the web.
If a prey species fell, the predators that feed on it would have less food, so their numbers would fall too.
Predators might eat more of an alternative prey species instead, so that species would decrease as well.
The plants or prey that the original species fed on might increase, because fewer of them are now being eaten.
Markers reward the predator decreasing, an effect on an alternative food source, and the idea that the whole web is interdependent, so a change spreads.
Related dot points
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Sources & how we know this
- CCEA GCSE Biology specification — CCEA (2017)