Producers, Consumers and Decomposers

Every living thing in an ecosystem plays one of three main roles. Producers make their own food from sunlight or chemicals. Consumers eat other living things to get energy. Decomposers break down dead bodies and waste, recycling the nutrients so plants can use them again. Together these three groups keep every food chain and ecosystem on Earth running. Take any one out and the whole system collapses.

  • ProducersPlants and algaeMake food from sunlight via photosynthesis
  • Primary consumersHerbivoresEat plants
  • Secondary consumersCarnivoresEat herbivores
  • DecomposersBacteria, fungi, wormsBreak down dead matter
  • Without producersNo energy in ecosystemEverything else starves
  • Without decomposersNutrients run outPlants slowly die off

Producers: the food-makers

Producers (also called autotrophs, meaning "self-feeders") are organisms that make their own food from non-living raw materials. Almost all producers on Earth do this through photosynthesis: using sunlight, water and carbon dioxide to make glucose (sugar). They are the foundation of almost every food chain on Earth.

Land producers include all green plants: grass, trees, herbs, vegetables, flowers. In the oceans, the main producers are tiny floating algae called phytoplankton, plus larger seaweeds like kelp. Phytoplankton are so abundant that they produce roughly half of all the oxygen on Earth.

A few unusual producers do not need sunlight at all. Around deep-sea volcanic vents, bacteria called chemoautotrophs make food from chemicals like hydrogen sulphide. They are the producers in the strange dark ecosystems thousands of metres below the ocean surface.

Consumers: who eats whom

Consumers (or heterotrophs) get their energy by eating other living things. There are several types, depending on what they eat.

  • Herbivores (primary consumers): eat plants. Cows, rabbits, caterpillars, manatees.
  • Carnivores: eat other animals. Lions, sharks, foxes, eagles.
  • Omnivores: eat both plants and animals. Bears, pigs, raccoons, humans.
  • Insectivores: specialise in eating insects. Anteaters, bats, hedgehogs.
  • Scavengers: eat dead animals (carrion). Vultures, hyenas, jackals.
  • Parasites: live on or in other organisms and feed off them without killing them quickly. Tapeworms, fleas, mistletoe.

Decomposers: the recyclers

When any plant or animal dies, decomposers move in. They break down dead organic matter into simple chemicals that wash back into the soil and become nutrients for new plants. Without decomposers, dead bodies would pile up endlessly and the nutrients locked inside them would be lost. The world would slowly fill with corpses and run out of plant food.

The main decomposers are:

  • Bacteria: the most important by far. A single gram of soil can contain over a billion bacteria, many of them busy breaking down dead matter.
  • Fungi: especially good at breaking down tough plant materials like wood and leaf litter. The mushrooms you see in a forest are the fruiting bodies of large underground fungal networks (sometimes covering hectares) that quietly recycle nutrients.
  • Detritivores: small animals that eat dead organic matter and chew it up so smaller decomposers can finish the job. Earthworms, woodlice, dung beetles, millipedes.
Fact A single mature oak tree dropping its leaves in autumn can produce around 700,000 leaves in one season. Without decomposers, the forest floor would be buried metres deep in dead leaves in a few years. Earthworms, woodlice, fungi and bacteria break it all down within months, releasing the nutrients back into the soil so the tree itself (and every other plant nearby) can use them again next spring.

How the three groups link up

The roles connect into an endless loop.

  • Producers capture energy from the Sun and lock it into sugar molecules.
  • Consumers eat producers (and each other) to get that energy.
  • When producers and consumers die, decomposers break them down.
  • The chemicals decomposers release become the raw materials producers need to grow.
  • The cycle starts again.

This is why energy flows through an ecosystem (it comes in from the Sun and is eventually lost as heat) but matter cycles within it (the same atoms get used over and over).

What happens if one group is missing

You can see how important each group is by imagining what would happen if it disappeared.

  • If producers went extinct, no new energy would enter the ecosystem. Consumers would starve in days or weeks. Almost all life would end.
  • If all consumers went extinct, plants would still grow, but nothing would eat them, no animals would exist, and the ecosystem would become much simpler (essentially just plants and microbes).
  • If decomposers went extinct, dead bodies and dead plants would pile up. Nutrients would slowly get locked up in the dead matter. After a few years, producers would start failing for lack of nutrients, and the whole ecosystem would collapse from the bottom up.

All three roles are essential. The next time you walk through a forest, remember that the bacteria, fungi and worms quietly working in the soil under your feet are doing some of the most important work on the planet.

Did you know? Some creatures act as more than one of these things in their lives. A young carnivorous beetle larva that eats other insect larvae is a secondary consumer. The adult beetle that emerges later might be a herbivore eating leaves. Some animals (like bears) eat almost anything: a bear can be a herbivore (eating berries), a carnivore (eating fish or deer), an insectivore (eating ants and bee larvae) and a scavenger (eating dead bodies), all in the same week.
Deeper dive: chemoautotrophs and life without sunlight

For most of biology's history, scientists assumed every food chain on Earth had to be powered by the Sun. Photosynthesis, after all, was the only known way for living things to make food from non-living raw materials. Almost every food web traced its energy back to sunlight one way or another.

That picture broke in 1977, when a research submarine called Alvin made the first ever visit to a deep-sea hydrothermal vent, on a mid-ocean ridge near the Galapagos Islands. The vents were spewing super-hot water (350 °C, kept liquid by the pressure) loaded with chemicals like hydrogen sulphide and methane. And around the vents, in pitch darkness 2.5 km below the surface, was an astonishingly rich community of life: huge tube worms 2 m tall, giant clams, blind shrimp, fish, crabs, all of them living far from the Sun.

The base of these strange ecosystems turned out to be chemoautotrophic bacteria: microbes that can make food from chemicals released by the vents, in a process called chemosynthesis. The bacteria either live freely in the water or inside the tissues of larger creatures like tube worms (which have no mouth or gut, getting all their food directly from the bacteria living inside them). The whole ecosystem is independent of the Sun.

This discovery has huge implications. Life can exist anywhere there is energy, water and some basic chemistry. That means living things may also survive deep below the surface of Mars, in the ice-covered oceans of Europa or Enceladus (moons of Jupiter and Saturn), or in countless other places we have not yet looked. Hydrothermal vents have rewritten our ideas of where life can exist.

For how energy moves through these roles, see food chains and food webs.