Food Chains and Ecosystems: KS3 Biology Guide

Food chains and ecosystems are essential KS3 biology topics typically taught in Year 7 or 8. They explain how energy moves through living communities, how organisms depend on one another, and what happens when that balance is disturbed. Understanding these ideas underpins a great deal of GCSE biology and geography content.

What is an ecosystem?

An ecosystem is a community of living organisms interacting with one another and with their non-living (abiotic) environment — including factors such as temperature, light, water and soil. Some useful definitions you need to know:

Habitat — the physical place where an organism lives (e.g., a pond, a woodland floor).

Community — all the living organisms in one habitat, including all the different species.

Population — all the members of a single species in a habitat (e.g., all the rabbits in a field).

Producer — an organism that makes its own food by photosynthesis. All producers are plants or algae.

Consumer — an organism that gets energy by eating other organisms.

Decomposer — an organism (mostly bacteria and fungi) that breaks down dead organic matter and returns nutrients to the soil.

What is a food chain?

A food chain shows the feeding relationships between organisms in a habitat. Each arrow in a food chain represents the transfer of energy from one organism to the next. It is important to understand that the arrow does not simply mean "is eaten by" — it means "energy passes to."

Example food chain

Grass → Rabbit → Fox → Decomposers

In this chain:

Grass is the producer (it photosynthesises sunlight into chemical energy).

Rabbit is the primary consumer (herbivore) — it eats the grass.

Fox is the secondary consumer (carnivore) — it eats the rabbit.

Decomposers (bacteria and fungi) break down the dead fox and return mineral nutrients to the soil, where they can be taken up by plants again.

Trophic levels

Trophic level | Name | Example
1st | Producer | Grass, oak tree, algae
2nd | Primary consumer | Rabbit, caterpillar, snail
3rd | Secondary consumer | Fox, thrush, frog
4th | Tertiary consumer | Sparrowhawk, pike
— | Decomposer | Earthworm, fungi, bacteria

Energy is lost at each trophic level — roughly 90% is lost as heat during respiration, movement and waste. Only about 10% is passed on to the next level. This is why food chains rarely have more than five links; there is simply not enough energy left to support a sixth level.

What is a food web?

Most organisms eat more than one thing, and are eaten by more than one predator. A food web shows multiple, interconnected food chains operating in the same ecosystem. Food webs are more realistic than simple chains because they show how the removal of one species can affect many others.

Example food web (described in text)

In a simple meadow ecosystem, a food web might look like this:

Grass → Rabbit → Fox

Grass → Rabbit → Sparrowhawk

Grass → Caterpillar → Thrush → Sparrowhawk

Grass → Caterpillar → Fox

Grass → Slug → Hedgehog

All dead matter → Earthworms and fungi (decomposers) → back to soil nutrients → Grass

In this web, the sparrowhawk sits at the top of two chains and is a top predator (or apex predator). Removing the rabbit from this web would reduce food for both the fox and the sparrowhawk, while also allowing the grass to grow more freely — which might increase caterpillar and slug populations.

How do predator-prey populations cycle?

Predator and prey populations are closely linked. When prey numbers rise, predators have more food, so their numbers rise too. But as predator numbers increase, they eat more prey, causing prey numbers to fall. With less food available, predator numbers then fall — which allows prey to recover. This creates a cyclic pattern that lags slightly in time.

A classic example is the Canadian lynx and snowshoe hare, data for which has been collected since the early 1800s:

Phase | Hare population | Lynx population
Hares increase | High and rising | Low (lags behind)
Lynxes increase | Peaks then falls (overhunted) | Rises (more food)
Hares fall | Low | Peaks then falls (less food)
Lynxes fall | Begins to recover | Low

The lynx population peak always comes slightly after the hare population peak because predators respond to changes in prey numbers rather than driving them directly.

What is the role of decomposers?

Decomposers are organisms — mainly bacteria and fungi — that break down dead plants and animals, and animal waste, into simpler substances. This process is called decomposition. It is essential for two reasons:

Nutrient recycling — decomposers release mineral ions (e.g., nitrates, phosphates) back into the soil. Plants absorb these minerals through their roots to make proteins and other molecules. Without decomposers, nutrients would be locked up in dead matter and unavailable to producers.

Energy release — decomposers respire the organic matter they digest, releasing energy for their own life processes.

Decomposers are sometimes described as a separate category from consumers, because they do not physically eat their food — they release enzymes onto it and absorb the digested products. However, they occupy a crucial position in every food web and ecosystem.

According to the Department for Education's Science Programmes of Study for Key Stage 3, pupils should learn about the interdependence of organisms in an ecosystem, including food webs and insect pollination, and the importance of plant reproduction through insects and wind pollination for the diversity of living organisms.

BBC Bitesize KS3 Biology covers food chains, food webs, producers, consumers and decomposers as core content for Year 7 and Year 8 science.

Frequently asked questions

What is the difference between a food chain and a food web?

A food chain is a single, linear sequence showing one set of feeding relationships — for example, grass → rabbit → fox. A food web is a more complex diagram that links many food chains together within the same ecosystem, showing how most organisms eat and are eaten by several different species. Food webs are more realistic because most animals have varied diets. A food chain is useful for showing the concept of energy transfer; a food web is better for understanding how ecosystems actually work.

What do arrows in a food chain mean?

The arrows in a food chain show the direction of energy transfer, not simply "is eaten by." An arrow from grass to rabbit means that energy stored in the grass passes into the rabbit when it is eaten. This distinction matters: it is the energy that flows along the chain, not the organism itself. Always draw arrows pointing from the organism being eaten (the source of energy) towards the organism eating it (the receiver of energy).

What happens if a species is removed from a food web?

Removing a species can cause a cascade of changes throughout the food web. If a prey species is removed, predators that depend on it may decline or switch to eating other prey (potentially causing those populations to fall). If a predator is removed, its prey may increase rapidly, which could deplete the producers at the base of the chain. This is known as a trophic cascade. For example, removing wolves from Yellowstone National Park in the 20th century caused elk populations to rise, which overgrazing of vegetation significantly altered the landscape.

What is the role of decomposers in an ecosystem?

Decomposers (bacteria and fungi) break down dead organic matter — dead plants, animals and faeces — and release the nutrients locked inside back into the soil as mineral ions. Plants then absorb these minerals to grow. Without decomposers, nutrients would not be recycled, the soil would become depleted, and producers could not sustain themselves. Decomposers are therefore essential to keeping an ecosystem running indefinitely. They also obtain energy for their own respiration through the process of decomposition.

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