Food Chains and Ecosystems

Everything is connected

In nature, no living thing survives on its own. Plants, animals, fungi and even tiny bacteria are all linked together by what they eat. Scientists describe these links using food chains, food webs and ecosystems. Understanding them shows us how energy moves through the natural world — and why protecting nature matters.

What is a food chain?

A food chain shows how energy and nutrients pass from one living thing to the next. Each link in the chain is eaten by the next link. Here is a simple grassland food chain:

Grass → Grasshopper → Frog → Snake → Hawk

The arrows point in the direction the energy flows — from the thing being eaten to the thing that eats it. Every food chain has different roles.

The roles in a food chain

Producers make their own food. Plants, algae and some bacteria are producers. They use sunlight, water and carbon dioxide to make sugar through photosynthesis. Because they capture the Sun's energy, producers are the foundation of almost every food chain on Earth.

Consumers cannot make their own food, so they eat other living things. They are grouped by what they eat:

• Herbivores eat only plants (for example, a rabbit or a grasshopper).
• Carnivores eat only other animals (for example, a hawk or a snake).
• Omnivores eat both plants and animals (for example, a bear or a human).

Consumers are also ranked by their place in the chain. A primary consumer eats producers. A secondary consumer eats primary consumers, and so on. A predator hunts other animals, and the animal it eats is its prey.

Decomposers are the recyclers of nature. Fungi, bacteria and creatures like earthworms break down dead plants and animals. As they do, they return important nutrients to the soil. Producers then take up those nutrients to grow, so nothing is wasted.

From food chains to food webs

A single food chain is a bit simple, because most animals eat more than one type of food, and most are eaten by more than one predator. A frog, for example, eats not just grasshoppers but also flies and worms. And a hawk eats snakes, mice and small birds.

When we connect many food chains together, we get a food web. A food web shows the real, tangled network of feeding relationships in a habitat. It reveals how connected everything is — and why a change to one species can ripple through many others.

Energy gets smaller up the chain

Energy is lost at every step of a food chain. Animals use most of the energy they eat to move, stay warm and grow, and a lot escapes as heat. Only about 10% of the energy passes on to the next level.

This is why food chains are usually short — there is rarely enough energy to support more than four or five links. It is also why there are many producers but only a few top predators. You will always see far more grass than hawks.

Ecosystems: the bigger picture

A food web exists within an ecosystem. An ecosystem is a community of living things together with the non-living parts of their environment — the soil, water, air, sunlight and temperature — and all the ways they interact. A pond, a forest, a coral reef and a desert are all ecosystems.

Ecosystems can be delicately balanced. Removing one species can affect the whole system. A famous real example happened in Yellowstone National Park in the USA. When wolves were removed, deer numbers exploded and ate too many young trees. When wolves were brought back in 1995, deer numbers fell, trees recovered, riverbanks became stable, and many other species returned. One predator helped reshape an entire ecosystem.

Investigate your local ecosystem

You don't need a national park to study nature. Choose a small patch outdoors — a garden, a park, or a hedge. Watch quietly for ten minutes and record what you see eating what. Which are the producers? Can you spot a herbivore, a carnivore and a decomposer? Try sketching a food web for your patch. You will quickly see how connected your local nature really is.

To understand how producers capture the Sun's energy, read The Parts of a Plant. To explore the conditions that shape different ecosystems, see Climate and Weather: What's the Difference?.