Food chains, Food webs & Pyramids

Every living thing needs energy, and that energy ultimately comes from the Sun. A food chain shows how this energy is passed from one organism to the next as food.

A typical food chain looks like this:

grass → grasshopper → frog → snake

The arrows show the direction the energy flows — from the thing being eaten to the thing eating it. This is the single most important rule of food chains.

Each organism in the chain occupies a trophic level (feeding level):

Trophic level	Name	Example	What it does
1st	Producer	grass	Makes its own food by photosynthesis
2nd	Primary consumer (herbivore)	grasshopper	Eats the producer
3rd	Secondary consumer (carnivore)	frog	Eats the primary consumer
4th	Tertiary consumer (carnivore)	snake	Eats the secondary consumer

Almost all food chains begin with a producer — usually a green plant or alga that traps light energy from the Sun and converts it into chemical energy (food) by photosynthesis. Consumers cannot make their own food, so they must eat other organisms.

Exam tip. If you draw the arrows the wrong way round, you lose the mark. Read the arrow as "is eaten by": grass is eaten by grasshopper. The energy travels in the same direction as the arrow.

In a real habitat, most animals eat more than one kind of food, and most organisms are eaten by more than one kind of animal. A single food chain is too simple to show this. A food web joins several food chains together to show all the feeding relationships in a community.

For example, in a garden:

• A caterpillar and a slug might both eat lettuce.
• A bird might eat both the caterpillar and the slug.
• A frog might also eat the caterpillar.

Drawing these as separate chains would miss the links — a food web shows them all at once, with arrows again pointing in the direction energy flows.

Food webs are useful because they help us predict what happens if one organism is removed or changes in number. If a disease kills all the caterpillars, the birds and frogs that ate them have less food, so their numbers may fall — but they can switch to eating more slugs instead. A food web shows these knock-on effects, which a single chain cannot.

Exam tip. When a food web question asks "what would happen to the number of X if Y decreased?", trace the arrows. Look for every organism connected to the one that changes — there is usually more than one effect to describe.

A pyramid of number is a bar chart, drawn on its side, that shows how many organisms there are at each trophic level. The producer goes at the bottom, with each level stacked above it. The width of each bar is proportional to the number of organisms at that level.

For an ordinary food chain like:

grass → grasshopper → frog → snake

there are very many grass plants, fewer grasshoppers, fewer frogs and only a few snakes — so the bars get narrower as you go up, giving a neat pyramid shape.

But pyramids of number can be an irregular shape. This is because the pyramid counts how many organisms there are, not their size:

• One big tree (the producer) can feed many insects. The bottom bar (1 tree) is then narrower than the bar above it (hundreds of insects), so the "pyramid" is upside down at the bottom.
• A few large animals (e.g. one rabbit) can be host to many tiny parasites (e.g. thousands of fleas). The top bar is then much wider than the one below — a bulge at the top.

So a pyramid of number does not always look like a true pyramid, because a single large organism counts as just "1", the same as a single tiny organism.

Exam tip. When asked why a pyramid of number is an unusual shape, the key idea is that it counts numbers, not size/mass, so one large organism (a tree) or many small organisms (parasites) distorts the shape.

Because a pyramid of number can be misleading, biologists often use two other pyramids that take size into account.

Pyramid of biomass. This shows the total mass of living material (biomass) at each trophic level, not the number of organisms. Now one big tree counts as a large mass at the bottom, and the many insects feeding on it count as a much smaller mass above. So even the "tree" example gives a true pyramid shape: the biomass gets smaller at each level going up.

Pyramid of energy transfer. This shows the amount of energy stored in the organisms at each trophic level. As energy passes up the chain, most of it is lost at each level — used in respiration (and released as heat), lost in movement, and lost in waste (faeces and urine). Only about 10% of the energy is passed on to the next level. So the energy gets less at each level going up, again giving a true pyramid shape.

Here is a pyramid of biomass drawn for a simple food chain:

This is why pyramids of biomass and energy are almost always true pyramid shapes: you simply cannot end up with more mass or more energy at a higher level than the level that feeds it, because mass and energy are lost at every step.

Exam tip. The reason a pyramid of biomass or energy is always a true pyramid is that energy/biomass is lost at each level (in respiration, heat, movement and waste), so less is available higher up.

It is easy to mix up the three pyramids, so it helps to compare them side by side.

Pyramid	What it shows	Usual shape	Can it be irregular?
Number	How many organisms at each level	Often a pyramid	Yes — counts numbers, not size (e.g. one big tree, or many parasites)
Biomass	Total mass of living material at each level	True pyramid	Almost never — mass is lost at each level
Energy transfer	Energy stored at each level	True pyramid	Almost never — energy is lost at each level

The crucial difference is that a pyramid of number ignores the size of each organism, so a single large organism counts the same as a single tiny one. That is why it can be an odd shape.

Pyramids of biomass and energy take size and energy content into account. Because energy and biomass are always lost as you go up the chain (mostly through respiration and heat loss, and through movement and waste), there is always less at each higher level — so they form true pyramid shapes.

Remember: number can lie about size; biomass and energy cannot.

Exam tip. A classic question gives a food chain with a tree and asks which pyramid would be an unusual shape. The answer is the pyramid of number (because the tree counts as one organism), while the pyramid of biomass would be a normal pyramid.