Food Chains, Food Webs and Decomposers

A plant does not need someone to bring it lunch. Its green leaves make sugar, which is its food. This food-making process is called photosynthesis.

What goes into a leaf?

Roots take up water from the soil. Water travels up the stem to the leaves. The leaves also take in carbon dioxide, a gas in air. It enters through tiny holes, often underneath the leaf.

A green substance called chlorophyll catches sunlight. Using that energy, the leaf turns water and carbon dioxide into sugar. It also gives off oxygen, the gas you need to breathe.

Follow the food in an apple tree

1. Roots take up water, and leaves take in carbon dioxide.
2. Chlorophyll catches light, giving the leaves energy to make sugar.
3. The tree uses some sugar to stay alive and grow.
4. It sends some sugar into its apples. That helps make them sweet.

The tree also builds food into roots, stems, seeds and new leaves. It can store extra food. A potato stores starch; a carrot stores sugar.

Does a plant eat soil?

Plants build most of their bodies from air and water. Long ago, Jan Baptist van Helmont tested a willow tree. Over five years, it gained about 164 pounds. The soil lost only about 2 ounces. He thought water explained the growth. Later scientists found that carbon dioxide from air supplies much of the material too.

Roots take small amounts of nutrients, or minerals, from soil. Nutrients help plants stay healthy, but give them no food energy. The “plant food” sold in stores is fertilizer: it supplies nutrients. A plant in the dark cannot replace light with extra fertilizer. It uses stored food, becomes weak and eventually dies.

Other food makers

Algae make food from light too. They include seaweed and many tiny floating living things called phytoplankton. About half the oxygen made on Earth comes from the ocean, mostly from phytoplankton.

A Venus flytrap still makes food from sunlight. Its trapped insects give it extra nutrients. A mushroom cannot make food from light. It is part of a fungus and has no chlorophyll.

When you run, grow or heal a cut, you use energy. Animals get that energy from food. Plants catch sunlight and store some of its energy in the food they make.

Follow a meal backward

Think about a cheese pizza. Start with each ingredient and ask where it came from.

• The crust comes from wheat. Wheat plants made food using sunlight.
• The sauce comes from tomatoes, grown by tomato plants.
• The cheese comes from cow’s milk. The cow ate grass and hay. Those plants made food using sunlight too.

The cheese took an extra step through an animal. Its food energy still began with sunlight.

A hawk can get energy without eating a plant. Suppose it eats a snake, which ate a mouse, which ate seeds. Work backward: hawk, snake, mouse, seeds, plant, sunlight. The plant first caught the energy that passed along this feeding path.

Where does food energy go?

Animals use much of their food energy for living. Movement, breathing and body repair all need energy. Mammals and birds also use energy to keep warm. Some energy stays in growing bodies or is stored as fat. That energy can reach another animal when it eats them.

As living things use food energy, it ends up as heat. The heat spreads into air, water and the ground. It has not disappeared, but living things cannot use that spread-out heat to make new food. Food chains need a fresh supply of sunlight.

A lizard on a sunny rock gets warm. It still needs to eat. Sunlight can warm an animal without giving it the food energy it needs to grow and move.

Energy on a food label

US food labels use Calories to tell you how much energy food holds. A medium banana has about 100 Calories. The banana plant stored that energy in its food.

Almost all food chains begin with sunlight. A few deep-ocean chains begin differently. Near hot springs on the seafloor, tiny living things make food using chemical energy.

A tree, a squirrel and mold on fallen fruit get food differently. You can sort them by how they get it.

Three feeding jobs

All animals are consumers. Even a tiny insect must eat. Producers can be tiny too: algae in a pond make food from light.

Sort a backyard

Start with an oak tree and grass. Both make food from light, so both are producers. A squirrel eats food made by living things, so it is a consumer. A robin is a consumer too.

Now look at mold on a fallen apple. The mold breaks down the apple and takes in food. It is a decomposer. A mushroom on a rotting log is part of a fungus feeding on the wood. That fungus is a decomposer too.

The fallen apple itself is dead material. It is food for decomposers, not a living decomposer.

Tiny workers and their helpers

Fungi and bacteria break dead material down with special juices. They then soak up food from the broken-down material. Most bacteria are too small to see without a microscope. Many do useful work in soil.

Earthworms, pill bugs, millipedes and some beetles help. They eat dead material and break it into smaller pieces. Fungi and bacteria can then work on those pieces. These animals are consumers, though many books group them with decomposers because they help things rot.

Not every mushroom belongs to a decomposer fungus. Some fungi get food through a partnership with living tree roots. How a living thing feeds tells you its role; where it grows is not enough.

What about an animal eating a dead animal?

A vulture tears and swallows meat from a dead deer. It is a scavenger, a consumer that eats animals it did not kill. Decomposer fungi and bacteria break dead remains down further. Their work releases nutrients, water and gases that can be used again.

What does an animal eat most of the time? Its usual diet helps you place it in one of three groups.

Three kinds of diet

Seeds, fruit and nectar count as plant foods. Insects count as animals. A ladybug that eats aphids is a carnivore, even though it is small. An elephant is a herbivore, even though it is huge.

Use the whole diet

A rabbit eats grass and clover. Both foods are plants, so it is a herbivore. An owl eats mice and insects. Both foods are animals, so it is a carnivore.

A red fox eats mice, rabbits and insects, but also berries. Its usual foods include animals and plants. That makes it an omnivore. Sharp teeth and hunting do not make its whole diet animal-only.

Teeth give helpful clues. A horse’s broad, flat teeth grind plants. A wolf’s pointed teeth grip and tear meat. People have different tooth shapes for different foods. What an animal actually eats is the best evidence.

Animals can surprise you

Most bears are omnivores, but giant pandas eat almost only bamboo. They feed like herbivores. Polar bears eat mainly seals, so they feed mainly as carnivores. The word “bear” alone does not tell you the diet.

Diet can change during life. Most tadpoles eat algae and plants. Adult frogs eat insects and other small animals.

Place an eater in a chain

A herbivore eats a producer, making it a first consumer. A carnivore eats another consumer, so it comes later. An omnivore’s place depends on that feeding path. A bear eating berries is a first consumer. When it eats salmon that ate other animals, it comes later in the chain.

A spider catches a fly. The spider is the predator and the fly is its prey. These words describe the two sides of a hunt.

One animal can have both roles

Follow this feeding path: “grass → grasshopper → frog → snake → hawk”.

• The frog catches the grasshopper. The frog is a predator.
• The snake catches the frog. The frog is now prey.
• The hawk catches the snake. The snake is prey as well as a predator.

The grasshopper is prey, but it eats grass. Eating a plant does not make it a predator. The grass is a producer, not prey.

A top predator, also called an apex predator, has no animal in its habitat that usually hunts it once it is fully grown. An adult orca is one example. Top predators still become food for decomposers after death.

Diet and hunting tell different things

A carnivore eats animals. A predator catches animals. A vulture mainly eats animals already dead, so it is a carnivore and a scavenger. A bear that eats plants and animals is an omnivore. When it catches a salmon, it is also acting as a predator.

Catching food and escaping

A hawk’s sharp eyesight helps it spot prey. A cheetah uses speed. A spider’s web traps insects. A hidden predator can wait until prey comes close.

Prey have useful features too. Rabbits watch for danger with eyes on the sides of their heads. Turtles have hard shells. Speed, hiding colors and staying in groups can help animals escape. Many hunts fail, leaving prey alive to feed and have young.

Their numbers affect each other

When prey is plentiful, predators find more food and can raise more young. More predators can then eat more prey. If prey becomes scarce, predators have less food.

In northern Canada, long records show snowshoe hare numbers rising and falling in roughly ten-year cycles. Lynx numbers follow the hares they hunt. Food, weather and other changes also matter, so a food web helps explain possible changes without giving an exact count.

A food chain puts feeding steps in order. Each arrow points from the food to the eater. You can read it as “is eaten by” or “gives energy to”.

Build a pond chain

Suppose tadpoles eat algae, small fish eat the tadpoles, and herons eat the fish. The names arrive mixed up: heron, algae, tadpole, small fish.

1. Find the producer. Algae make food from sunlight, so place them first.
2. Find what eats the algae. Put the tadpole next.
3. Add the small fish, then the heron.
4. Join each food to its eater: “algae → tadpole → small fish → heron”.

Check the first arrow aloud: “Algae are eaten by the tadpole.” The direction matches the movement of food energy. An arrow pointing from tadpole to algae would say the algae eat the tadpole.

Count the consumers

The producer is the first living thing, but it is not a consumer. Count the animals after it.

• Tadpole: first consumer, also called primary consumer.
• Small fish: second consumer, also called secondary consumer.
• Heron: third consumer, also called tertiary consumer.

Every arrow must show a real feeding step. If the heron eats fish, you cannot remove the fish and draw a tadpole-to-heron arrow unless the heron also eats tadpoles.

Does the Sun belong at the start?

A picture may show the Sun sending light to the producer. The Sun supplies energy, but is not alive or eaten. The producer remains the first living thing in the chain.

Try other habitats

In the ocean, “phytoplankton → krill → blue whale” shows tiny food makers feeding small animals that a whale eats. In a desert, seeds can feed a kangaroo rat, then a rattlesnake, then a hawk.

A longer Arctic path is “phytoplankton → zooplankton → Arctic cod → seal → polar bear”. Zooplankton are tiny drifting animals. Size does not decide the order: who eats whom does.

You belong in food chains too. Bread gives a short path: “wheat → human”. Milk adds an animal step: “grass → cow → human”.

A mouse can eat seeds and insects. An owl and a snake can both eat the mouse. One line cannot show all these meals. A food web joins chains that share living things.

Read a meadow web

Here are the web’s nine feeding links:

• grass → rabbit; rabbit → fox
• grass → mouse; grass → grasshopper
• grasshopper → mouse; grasshopper → frog
• frog → snake; mouse → snake; mouse → owl

In a drawing, put each living thing in just once. Add all its arrows. Arrows pointing into the mouse come from its foods: grass and grasshopper. Arrows leaving the mouse lead to its eaters: snake and owl. The mouse eats plants and animals, so it is an omnivore.

Find every complete path

Start at grass and follow arrows forward. At a branch, try each route. Finish where no feeding arrow leads to another eater. Then return to the last branch you have not tried.

The rabbit branch gives one chain: “grass → rabbit → fox”.

The direct mouse branch gives two:

• “grass → mouse → snake”
• “grass → mouse → owl”

The grasshopper branch gives three:

• “grass → grasshopper → mouse → snake”
• “grass → grasshopper → mouse → owl”
• “grass → grasshopper → frog → snake”

That makes six chains. Stopping at the mouse misses its eaters. Going backward along an arrow would invent a meal.

One mouse, two positions

In “grass → mouse → owl”, the mouse is a first consumer. In “grass → grasshopper → mouse → owl”, it is a second consumer. Its position changes with the path you follow.

Fox, snake and owl are the top consumers shown here. A bigger web might include a hawk eating snakes. Then the snake would have another eater above it. A web shows the feeding links included in that model.

Other webs may start with several producers, such as algae and pondweed. Decomposers are often left out to keep the picture clear. When included, they receive dead material and waste from across the web, not just from top consumers.

A fallen log does not vanish by itself. Living things feed on it and break it down. This breakdown of dead material and waste is decomposition, also called rotting.

How the workers feed

A fungus sends thin threads through a log. The threads release juices that break down the wood. The fungus soaks up food from that material. A mushroom is only the part you see above the surface; much of the fungus stays hidden.

Mold is a fungus too. Bacteria also break dead material down. They are too small to see one by one without a microscope. A teaspoon of healthy soil can hold hundreds of millions of them.

Beetles, termites, pill bugs and millipedes help break a log into pieces. Fungi and bacteria can reach more of the wood as it crumbles.

Follow a log over time

First, insects bore into the fallen wood and fungi spread inside. Over years, the log becomes soft and crumbly. Small animals feed there, and parts become dark material mixed with soil.

As decomposition happens, nutrients are released into the soil. Carbon dioxide returns to the air, and water is released too. Young trees can take up the nutrients and water. Their leaves use carbon dioxide to make new food.

Decomposers are nature’s recyclers: they return materials for living things to use again. They also get food energy while doing this work.

What would happen without them?

Dead leaves, bodies and droppings would pile up. Nutrients would stay locked in the remains. Plants would run short of nutrients, grow poorly and provide less food for animals.

Decomposition happens in forests, ponds, oceans and compost piles. Every producer and consumer eventually dies, so all parts of a food web can supply decomposer food.

Spot the changes

A pumpkin that softens, grows fuzzy mold and crumbles is decomposing. An unpleasant smell can be another clue. A freshly cut apple turning brown is different. Air causes that quick color change; browning alone does not show that decomposers are feeding.

Rotting can spoil food you wanted to eat. On a forest floor, the same kind of breakdown helps new plants grow.

Why does food keep longer in a refrigerator? The conditions around decomposers change how quickly they work. The material they feed on matters too.

What changes the speed?

• Warmth: suitable warm conditions speed rotting; cold slows it. Freezing slows it much more.
• Moisture: damp scraps rot more quickly than dry material. Drying food helps it keep.
• Air: most decomposers need air. Turning a compost pile adds air and helps breakdown. Some decomposition happens without air, but a squashed, waterlogged pile often breaks down slowly and smells bad.
• Material: soft fruit and leaves break down faster than tough wood. Small pieces expose more surface for decomposers to work on.

A material is biodegradable if living things can break it down. Leaves and plain paper are examples. Ordinary plastic, glass and metal break down very little through decomposers’ work and can remain for a long time.

Plan a fair test of warmth

A fair test changes one thing while keeping other important things alike. Ask, “Do apple slices rot faster in warmth or cold?”

1. Cut equal-sized slices from the same apple.
2. Put each slice into its own matching clear bag and seal it.
3. Put half in a warm place and half in a refrigerator. Keep their light conditions similar.
4. Check all bags at the same time each day for two weeks.
5. Record visible mold and breakdown. Compare the warm and cold groups.

Temperature is the change you are testing. The fruit, slice size, bags and checking times stay alike. Several slices in each place help you see whether the same result happens again. Warm samples usually show mold and breakdown first.

Keep rotting samples sealed. Never open or sniff them; have an adult help throw the sealed bags away. Wash your hands after handling soil or compost.

Read your results carefully

A damp-soil test might compare a leaf, paper, an apple core, plastic and foil over four weeks. Record what changed during those weeks. A piece that looks unchanged may need longer. Wood is biodegradable even when four weeks brings little visible change.

You can help decomposers turn plant scraps into compost. Finished compost is dark and crumbly. Mix it into garden soil or spread it on top. It improves soil; it is not soil itself.

Feed the pile

A home pile needs a mix of greens and browns. These names describe the materials, not just their colors.

For each bucket of greens, add two to three matching buckets of browns. Cover food scraps with browns. Keep the pile as damp as a wrung-out sponge. Turn it now and then to mix in air.

Leave out meat, fish, bones, dairy and greasy foods. They can attract animals and cause bad smells. Keep out dog and cat poop, diseased plants, plastic, glass, metal and shiny magazine paper too. They can bring germs, disease or unsuitable materials into the compost.

Solve a pile problem

Imagine a class adds buckets of fruit peels but few dry leaves. The pile becomes wet, squashed and smelly.

1. Add dry browns to soak up some moisture and make spaces.
2. Mix the pile to let air reach the decomposers.
3. Keep using more browns than greens as new scraps arrive.

A dry pile needs some water. A soggy pile needs help with moisture and air, not more water.

When is it ready?

A busy pile can warm to 130°F (about 54°C) or more. Decomposers give off heat as they work. Hot composting can kill many weed seeds and germs, but small piles may stay cooler.

A well-kept pile may finish in three to five months. An untended pile can take a year or two, or longer. Look for dark, crumbly material that smells earthy. You should no longer recognize the original scraps.

A worm bin uses red wiggler worms. Their droppings, called castings, make useful compost. A worm bin usually takes three to six months.

Why the garden benefits

Composting sends less waste to landfills and returns nutrients to soil. Compost helps soil hold water and reduces soil washing away. It supports small soil animals too. Plants still need light to make their own food.

An ecosystem includes living things and the nonliving things around them. A meadow has plants and animals, plus soil, water, air and sunlight. Food links connect its living things.

Follow a change through a web

Use the meadow web with grass, rabbits, mice, grasshoppers, frogs, snakes, owls and foxes. Frogs eat grasshoppers. Snakes eat frogs and mice.

Suppose the frogs disappear.

1. Grasshoppers lose a predator. More may survive.
2. More grasshoppers may eat more grass.
3. Snakes lose one food. They may eat more mice instead.
4. If that happens, owls may find fewer mice to eat.

The change can reach animals with no direct link to frogs. The arrows show who eats whom. They do not show how much each snake eats, so use “may” or “probably” when predicting numbers.

If drought kills most of the grass, all six chains in this meadow web lose their producer. Plant eaters lose food first. Animals that eat them can then lose food too.

Having another food can help

A snake that loses frogs can still eat mice in this web. Having several foods can help a consumer cope when one becomes scarce. A koala depends heavily on eucalyptus leaves. Losing those trees leaves it few food choices.

A real coastal example

Along parts of the North Pacific coast, sea otters eat sea urchins. Urchins eat kelp, a large seaweed. Hunters once greatly reduced otter numbers for their fur.

With fewer otters, more urchins survived and ate kelp. Kelp forests shrank, and fish lost shelter. Where otters returned, they ate more urchins and helped kelp recover. A predator helped protect a producer through its prey.

An added animal can cause changes too

Brown tree snakes reached Guam by accident in cargo after World War II. They ate native forest birds and eggs. Many native forest bird species disappeared from the island.

When you predict a change, name the link behind it: “Grasshoppers may increase because fewer frogs are eating them.” Then look for the next food or eater affected.

People eat food, use land and make waste. Our choices can change the food webs around us. You can follow a choice through feeding links to see who else might be affected.

Removing food and shelter

Clearing land for roads, farms or buildings can remove producers. Animals lose plant food and places to hide or nest. Animals that eat those animals can lose food too. This is habitat loss.

Pesticides kill pests, such as insects eating crops. Some also kill helpful insects. Birds and frogs that eat insects may then find less food.

Oil, litter and chemicals can harm water plants and animals. Some sea turtles mistake plastic bags for jellyfish. Eating plastic can kill them.

Trace a pond choice

A town plans to spray a pond to kill mosquito larvae. Dragonfly nymphs and small fish eat the larvae. Herons eat the fish.

Start with the larvae: fewer larvae mean less food for their eaters. Then follow the next link: if fish become fewer, herons may find fewer meals. A spray that harms other pond animals can reach more parts of the web too.

Taking too many animals

Overfishing and overhunting remove animals faster than their numbers can recover. Predators can lose food. Off Newfoundland, heavy fishing helped northern cod numbers fall sharply. The northern cod fishery closed in 1992. This is a past example of a food-web change caused by people.

Helping a food web

You can protect plants, reduce litter and compost suitable scraps. Native plants give local insects food. Using fewer pesticides helps keep insect food available. Fishing and hunting rules help leave animals to breed.

Imagine a school adds native flowers, bushes and a log pile. Flowers feed insects. Birds can eat the insects and shelter in bushes. Fungi and pill bugs feed in the logs. One planted corner can support several feeding roles.

Protecting a species before it becomes very rare matters. A species is extinct when no living members remain anywhere. Losing an animal from one island is serious, but it is different from losing that species everywhere.

A fallen leaf becomes hard to see as it rots. Its materials are still there, in new forms and places. Matter is the material that makes up everything, including your body.

Follow matter around

Plants take in water, carbon dioxide and soil nutrients. They use these materials to make food and build their bodies. Animals eat plants or other animals, so some of that matter becomes part of animal bodies.

Living plants and animals give carbon dioxide back to the air as they use food for energy. Animals also leave waste. When living things die, decomposers break down the remains. Nutrients return to soil, and water and carbon dioxide are released. Plants can use those materials again.

For example, a maple leaf falls and decomposes. Some of its nutrients return to the soil. Roots take them up, and they become part of new growth. A caterpillar eats a new leaf, and a robin eats the caterpillar. Materials move between the environment and living bodies.

Draw the matter model

Draw four boxes: Plants; Animals; Decomposers; Air, water and soil. Connect them with labeled arrows:

• Air, water and soil → Plants: water, carbon dioxide and nutrients.
• Plants → Animals: food. Add Animals → Animals for animal meals.
• Plants and Animals → Decomposers: dead material and waste.
• Decomposers → Air, water and soil: released materials.

Add carbon dioxide arrows from the living groups to the air. Plants also release oxygen during photosynthesis. The labels tell you what moves along each arrow.

Give energy its own arrows

Add sunlight entering the plants. Then add food-energy arrows between living things and heat arrows leaving them.

Matter can be used again. Energy in food eventually spreads away as heat. Living things cannot turn that spread-out heat back into food. Decomposers use energy too; they recycle materials, not used energy.

Earth gets almost no new matter from space, so its materials must keep being reused. It receives new energy from the Sun each day.