How do decomposers break down fallen leaves?

1. Every autumn the trees drop their leaves, and a thick layer covers the ground. Yet the forest never fills up with old leaves. Something must be clearing them away.
2. First come the small animals: woodlice, millipedes, snails and worms. They chew holes in the leaves and tear them into little pieces. Small pieces are much easier to rot.
3. Next come the fungi. Their thin white threads grow over and through the leaf pieces. The threads give off juices that dissolve even the tough parts, and then soak up the food.
4. Too small to see, bacteria cover every scrap. A pinch of soil holds millions of them. They finish off the soft leftovers and the droppings of the animals.
5. After a year or two, the leaves have become dark, crumbly stuff called humus. The goodness that was locked inside them is now loose in the soil.
6. The tree's roots take up that goodness with water. In spring it helps build new leaves. The same material goes round and round, from leaf to soil to leaf.

This is a slice through the forest floor. Tap each part to see who is at work there.

• Fresh leaves. The top layer is this year's leaves. They are still whole, dry and tough, and full of stored goodness.
• Shredders. Woodlice, millipedes and snails chew the damp leaves into small pieces. Their droppings are even smaller pieces, ready for fungi and bacteria.
• Earthworm. At night an earthworm pulls leaves down into its burrow. It eats them with soil and mixes the leftovers deep into the ground.
• Fungus threads. A web of white fungus threads spreads through the leaves. It dissolves the tough parts that animals cannot digest. Mushrooms are how it spreads its spores.
• Bacteria. Bacteria are far too small to see without a microscope. They live on every scrap in enormous numbers and finish off the soft remains.
• Humus. This dark, crumbly layer is what is left when the leaves are almost fully broken down. It holds water like a sponge and feeds the plants.
• Tree roots. Fine tree roots spread through the humus. They take in water with the goodness that the decomposers set free, and the tree uses it to grow.

Run time forward and watch the pile shrink. Then switch off the animals, the fungi or the bacteria and compare how much is left.

Experiment: What happens to a pile of leaves if the small animals, fungi and bacteria are kept away?

1. Press "Fast forward" with all three helpers switched on. Watch the pile.
2. Slide time back to the middle. Switch the helpers off one at a time.
3. With everything off, slide time to the end. How much is left?

What we found: With all the helpers at work, most of the pile turned into dark soil in about two years. With no helpers it barely changed. The animals alone only make small pieces; it takes fungi and bacteria to turn them into soil.

Turn on JavaScript to try it yourself.

Turn this pile into soil within one year. Decide how wet it should be and whether to let the small animals in.

A tree spends all summer packing its leaves with useful materials. In autumn the leaves fall, and all of that lands on the ground. If nothing happened to them, the forest would be buried under old leaves, and the soil would slowly run out of the things that trees need.

Instead, a team of decomposers goes to work. Small animals such as woodlice, millipedes, snails and earthworms chew the leaves into bits. Fungi spread their white threads through the bits and dissolve the tough parts. Bacteria, far too small to see, finish off what is left. Each helper makes the job easier for the next one.

What remains is humus, a dark and crumbly material that makes soil rich. The goodness from the leaves is now loose in the soil water, where roots can take it up. Decomposers are nature's recyclers: the same material is used again and again.

How fast it goes depends on the weather. Decomposers like it warm and damp, with some air. In a warm rainforest a leaf can vanish in a few months. In a cool forest it can take two or three years. Our recycling comparison breaks down in one way: nobody is in charge here. Each helper is just eating its dinner.

Key words

Decomposer

A living thing that breaks down dead plants and animals.

Detritivore

A small animal that eats dead leaves and other dead bits.

Humus

The dark, crumbly part of soil made from rotted plants.

Nutrients

The goodness that plants need to grow, which they take from the soil.

Enzyme

A special juice that living things make to break food into tiny pieces.

Check your thinking

1. What do decomposers turn fallen leaves into?

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   Dark soil with goodness that plants can use. The leaves become humus, and their goodness goes back into the soil.

2. Why does it help when small animals chew the leaves into bits first?

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   Small bits give fungi and bacteria many more places to work. Fungi and bacteria work on surfaces. Many small bits have much more surface than one big leaf.

3. A pile of leaves is left in a very dry shed for two years. What do you expect to find?

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   Leaves that look almost the same. Decomposers need water. Without it they cannot live and work, so the leaves just sit there.

4. Why do leaves rot faster in a warm, wet rainforest than in a cool forest?

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   Decomposers work faster when it is warm and damp. Warmth and moisture keep the decomposers busy all year round.

5. In a bog the ground is soaked all year, and dead plants pile up as peat instead of rotting. What is the best explanation?

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   Waterlogged ground has very little oxygen, so decomposers work very slowly. Most decomposers need oxygen. In waterlogged ground the air spaces are full of water, so decay almost stops and plant material builds up.