Who makes food, who takes food?

Every ecosystem starts with producers — usually green plants. They use light energy in photosynthesis to make their own food, so they trap the energy that everything else depends on.

• Producer — makes its own food from light (e.g. grass, algae, an oak tree).
• Consumer — cannot make food, so it eats other organisms.
• Herbivore — a consumer that eats plants.
• Carnivore — a consumer that eats other animals.
• Omnivore — eats both plants and animals.

Remember: a producer makes, a consumer takes. Nearly all food chains begin with a producer because it captures the Sun's energy first.

Producer or consumer?

A food chain shows who eats whom

A food chain is a simple diagram of feeding relationships. The arrows show the direction energy flows — always from the organism being eaten to the organism doing the eating.

Common mistake: the arrow does not mean "eats". A points to B means A is eaten by B, so the energy travels along the arrow. Never draw the arrow pointing at the food!

Which way do arrows point?

Hunters and the hunted

A predator is an animal that hunts and eats other animals. Its prey is the animal that gets eaten.

• The same animal can be both — a frog is prey to a heron but a predator of flies.
• Predators and prey are linked: if one changes in number, so does the other.

Careful: "predator" is a role, not a fixed label. Whether an animal is predator or prey depends on which relationship you are looking at.

Match the organism to its role

Tap an organism on the left, then tap its correct role on the right.

Real ecosystems are food webs

Most animals eat more than one thing, and most are eaten by more than one predator. When we join many food chains together we get a food web. It shows how everything in a habitat is connected.

Key idea: because everything is connected, removing one species affects many others — not just the ones it eats or is eaten by.

Reading the food web

Change one thing, change everything

Organisms are interdependent — they rely on each other to survive. Because a food web is so connected, a change to one population sends ripples through many others.

Suppose disease wipes out most of the rabbits in the woodland web:

• Foxes have less food, so fox numbers may fall.
• Foxes turn to eating more mice, so mouse numbers may fall too.
• With fewer mice, the oak seedlings and insects the mice ate may increase.

Big idea: you can't remove just one species. Take one away and the effects spread far beyond its direct neighbours.

Ripples through the web

The predator–prey graph

Predator and prey numbers rise and fall in a repeating cycle. When prey are plentiful, predators have lots to eat and increase. More predators eat more prey, so prey numbers drop — then predators run short of food and drop too, letting prey recover.

Read it carefully: the prey peak comes first, then the predator peak follows. Predators can only build up after there is plenty of food.

Which curve peaks first?

What do rabbits have to do with oak trees?

Why do top predators get the most poison?

Fighting over resources

Resources in a habitat are limited, so organisms compete for them. Whatever there isn't enough of, living things will compete for it.

• Animals compete for food, water, territory and mates.
• Plants compete for light, water, space and minerals from the soil.
• The best-adapted competitors survive; poor competitors may die out or move away.

Watch out: competition is strongest between organisms that need the same resources — for example two plants growing side by side both reaching for the same patch of light.

What do plants compete for?

A two-way link

Living things are affected by their environment. Light, water, temperature and soil decide what can live and grow where. Living things also change their environment:

• Trees cast shade, so plants below get less light. Their fallen leaves rot and add nutrients to the soil.
• Earthworms burrow. This mixes the soil and lets in air and water.
• Beavers build dams. Water backs up and floods the land, making a pond.
• Humans clear land for farms and towns. This changes the habitat for other species.

Changing the environment

Poisons build up: bioaccumulation

Some toxic materials (like certain pesticides or mercury) can't easily be broken down or removed by the body. When one organism eats another, the poison passes on and builds up. This is called bioaccumulation.

The concentration gets higher at each step up the food chain, so top predators end up with the most. This can poison them or stop them breeding.

Misconception: "a tiny amount in the water can't do any harm." In fact it still harms top predators most, because it is concentrated as it passes up the chain.

Who is worst affected?

Insect pollination

Pollination is the transfer of pollen from the male part of a flower to the female part, so the plant can make seeds and fruit. Many plants rely on insects — especially bees — to carry that pollen for them.

As a bee visits flowers to collect nectar, pollen sticks to its body and rubs off onto the next flower — pollinating it. This lets crops set fruit and seeds.

Why it matters: a huge share of the crops we eat — apples, strawberries, beans and more — depend on insect pollinators. Fewer pollinators means less food for people, so protecting bees protects our food supply.

Why do pollinators matter?

Why do top predators get the most poison?

Build the food chain

Tap the organisms in the correct order — start with the producer and follow the energy up to the top predator.

Short answer challenge

The big ideas to know

You've covered the key ideas in the KS3 "Interactions and interdependencies" topic. Press Finish to see your score.