Food Chains and Food Webs

Every organism needs energy to stay alive, and that energy has to come from somewhere. Almost all of it begins as light from the Sun, captured by plants, and then passes from one organism to the next as they feed on each other. A food chain shows this flow of energy and biomass through a single line of organisms, while a food web joins many chains together to show the whole network of feeding relationships in a community. This lesson, part of Topic B4 of OCR Gateway Science A, teaches you to read food chains and webs, to use the correct terms for each feeding level, to predict how a change to one organism affects others, and (at Higher tier especially) to interpret the predator–prey cycles that emerge from these relationships.

By the end of this lesson you should be able to name and describe producers, the different consumers and decomposers, identify trophic levels, read food chains and food webs, explain how a change to one population affects the rest, and interpret a predator–prey graph.

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Producers: The Start of Every Food Chain

A food chain always begins with a producer. Producers are organisms — almost always green plants or algae — that make their own food by photosynthesis, using light energy from the Sun to convert carbon dioxide and water into glucose. In doing so they fix light energy into chemical energy stored in their tissues (their biomass). Because they capture the energy that everything else in the community will eventually use, producers are the foundation of the whole ecosystem.

Every arrow in a food chain points in the direction the energy and biomass flow — that is, from the organism being eaten to the organism eating it. The arrow means "is eaten by" or, more precisely, "energy/biomass flows to".

flowchart LR
  A["Grass<br/>(producer)"] -->|"eaten by"| B["Rabbit<br/>(primary consumer)"]
  B -->|"eaten by"| C["Fox<br/>(secondary consumer)"]
  C -->|"eaten by"| D["Eagle<br/>(tertiary consumer)"]

Exam Tip: The arrows in a food chain show the flow of energy and biomass, so they always point towards the predator (from prey to predator). A very common mistake is to draw or read the arrow the wrong way — "is eaten by" runs from the food to the feeder, never the reverse.

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Consumers and Trophic Levels

Organisms that cannot make their own food and instead get energy by eating other organisms are called consumers. Consumers are named by their position in the chain:

• A primary consumer eats the producer. Primary consumers are herbivores (plant-eaters) — for example, a rabbit eating grass.
• A secondary consumer eats the primary consumer. These are often carnivores (meat-eaters) — for example, a fox eating a rabbit.
• A tertiary consumer eats the secondary consumer — for example, an eagle eating a fox.

Each feeding stage in a food chain is called a trophic level (from the Greek for "feeding"). The producer is trophic level 1, the primary consumer is level 2, the secondary consumer is level 3, and so on.

Trophic level	Name	What it does	Example
1	Producer	Makes food by photosynthesis	Grass, algae
2	Primary consumer	Herbivore; eats producers	Rabbit, caterpillar
3	Secondary consumer	Often carnivore; eats primary consumers	Fox, small bird
4	Tertiary consumer	Carnivore; eats secondary consumers	Eagle, owl

Predators and prey

Within these feeding relationships, a predator is an animal that hunts, kills and eats other animals, and its prey is the animal that is hunted. A fox is a predator of the rabbit; the rabbit is the fox's prey. The same animal can be both: a frog is a predator of insects but prey for a heron. The numbers of predators and prey are tightly linked, as you will see later in this lesson.

Exam Tip: "Carnivore", "herbivore" and "omnivore" describe what an organism eats; "predator" and "prey" describe a hunting relationship; "primary/secondary consumer" describes a position in the food chain. A question may ask for any of these — read carefully which one it wants.

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Decomposers: Recycling the Dead

Not all feeding involves living prey. Decomposers are organisms — mainly bacteria and fungi — that feed on dead organisms and waste (such as faeces and fallen leaves), breaking them down. As they do so they release the mineral ions locked up in the dead material back into the soil, where plants can absorb them again. Decomposers are essential to every community because without them dead bodies and waste would pile up and the minerals plants need would never be recycled. You will study decomposers and the rate of decay in detail later in B4; for now, recognise that they sit alongside food chains, returning materials to the start of the cycle.

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Food Webs: Joining the Chains Together

In a real community, very few organisms eat only one thing and very few are eaten by only one predator. A food web shows how many food chains in a community are interconnected, giving a fuller and more realistic picture of who eats whom.

Reading the web above, you can see that the rabbit is eaten by the fox, the insects are eaten by both the bird and the mouse, and the fox eats more than one kind of prey. The producers (grass and clover) sit at the bottom, and the top predators (fox and bird) sit at the top with no predator of their own. To read any food web, simply follow the arrows from prey to predator.

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How a Change to One Organism Affects Others

Because the organisms in a food web are interdependent, removing or changing one population sends effects through the whole web. To work out the consequences, trace the arrows.

Suppose, in the web above, a disease wiped out most of the rabbits. Then:

• The fox has lost one of its food sources, so foxes would have less food. They might eat more insects and mice instead, or their numbers might fall.
• The grass, no longer grazed by rabbits, might grow more (at least until something else eats it).
• If foxes switch to eating more mice, the mouse population might fall, which could in turn affect the bird that also eats mice.

The key exam skill is to reason one step at a time: identify the organisms directly connected to the one that changed, decide whether each gains or loses food, then follow the knock-on effects further along the chain. Because a food web has many links, the loss of one species is often buffered — predators with several food sources can switch prey, which is one reason a community with high biodiversity (many species and many links) tends to be more stable.

A worked food-web question

In the woodland web above, the number of insects falls sharply because of a pesticide. Suggest and explain two effects on other organisms.

Step 1 — find what eats insects. The bird and the mouse both eat insects, so both have less food. Their populations may fall.

Step 2 — follow the knock-on effect. If fewer insects eat the clover, the clover may grow more. And if the bird and mouse decline, the fox (which eats the mouse) has less food too.