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How are living things linked by what they eat in an ecosystem?

Describe ecosystems and feeding relationships using food chains and food webs

A focused answer to the O-Level Biology outcome on ecosystems. The key ecological terms, the trophic levels of a food chain, how food webs link chains, and the effect of removing an organism.

Generated by Claude Opus 4.88 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to understand the key ecological terms (ecosystem, habitat, population, community, producer, consumer) and to describe feeding relationships using food chains, food webs and trophic levels. You should be able to read a food chain, identify each organism's role, and predict the effect of removing one organism.

The answer

Key ecological terms

  • A habitat is the place where an organism lives.
  • A population is all the organisms of one species living in a habitat.
  • A community is all the populations (all the different species) living together in a habitat.
  • An ecosystem is a community of organisms together with the non-living parts of their environment (such as water, soil and air), interacting as a unit.

Producers and consumers

  • A producer is an organism, usually a green plant, that makes its own food by photosynthesis. Producers capture energy from the Sun.
  • A consumer is an organism that gets its food by eating other organisms.
    • A herbivore (primary consumer) eats producers.
    • A carnivore (secondary or tertiary consumer) eats other consumers.
  • A decomposer (such as bacteria and fungi) breaks down dead organisms and waste.

Food chains and trophic levels

A food chain shows the feeding relationship between organisms, with arrows pointing in the direction the energy flows (from the eaten to the eater). Each stage is a trophic level:

grass→grasshopper→frog→snake\text{grass} \rightarrow \text{grasshopper} \rightarrow \text{frog} \rightarrow \text{snake}

  • Grass is the producer (first trophic level).
  • Grasshopper is the primary consumer (second trophic level).
  • Frog is the secondary consumer (third trophic level).
  • Snake is the tertiary consumer (fourth trophic level).

Food webs

In a real habitat, most organisms eat, and are eaten by, more than one kind of organism. A food web is several food chains linked together, giving a fuller picture of who eats whom.

Effect of removing an organism

Because organisms are linked, removing one affects others. For example, if all the grasshoppers in the chain above were removed, the frogs would lose their food and their population would fall, while the grass might increase as fewer grasshoppers eat it. Knock-on effects can spread through the web.

Examples in context

Example 1. A pond ecosystem. A pond contains producers (algae and pond plants), primary consumers (small water animals), and predators (fish), plus decomposers in the mud, all interacting with the water and light. Together these living and non-living parts form an ecosystem.

Example 2. Overfishing a top predator. Removing a top predator fish from the sea can let its prey multiply, which then over-eats the next level down. This shows how disturbing one part of a food web can unbalance the whole community.

Try this

Q1. Define a producer. [1 mark]

  • Cue. An organism (usually a green plant) that makes its own food by photosynthesis.

Q2. In the chain grass to grasshopper to frog, state the trophic level of the grasshopper. [1 mark]

  • Cue. The second trophic level (it is the primary consumer).

Q3. Explain what would happen to the grass if the grasshoppers were removed from the chain grass to grasshopper to frog. [2 marks]

  • Cue. The grass would increase, because fewer grasshoppers would be eating it.

Exam-style practice questions

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

Original6 marksConsider the food chain: grass to grasshopper to frog to snake. (a) Name the producer and the secondary consumer. (b) State the trophic level of the frog. (c) Predict and explain what would happen to the frog population if all the grasshoppers were removed.
Show worked answer →

(a) The producer is the grass. The secondary consumer is the frog (it eats the grasshopper, which is the primary consumer).

(b) The frog is at the third trophic level.

(c) If all the grasshoppers were removed, the frog population would fall (decrease), because the frogs would lose their food source (the grasshoppers), so many frogs would starve or move away.

Markers reward grass as the producer and frog as the secondary consumer, the frog at the third trophic level, and the frog population falling because it loses its food source.

Original4 marksDefine the terms producer and consumer, and explain why all food chains begin with a producer.
Show worked answer →

A producer is an organism (usually a green plant) that makes its own food by photosynthesis. A consumer is an organism that gets its food by eating other organisms.

All food chains begin with a producer because producers capture energy from the Sun by photosynthesis and store it in the food they make. This is the way energy enters the food chain; consumers then get their energy by eating producers or other consumers. Without producers, there would be no energy entering the chain.

Markers reward correct definitions of producer (makes its own food by photosynthesis) and consumer (eats other organisms), and the explanation that producers bring energy from the Sun into the chain.

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