Every organism in an ecosystem gets energy and matter somehow. In Photosynthesis you saw plants make sugar from carbon dioxide and water using light. That one trick sorts all of life into three roles.
Key idea
Producers make their own food, mostly by photosynthesis (plants, algae, some bacteria). Consumers eat other organisms: herbivores eat producers, carnivores eat animals, omnivores eat both. Decomposers (fungi, many bacteria) break down dead matter and wastes and return nutrients to the soil and water.
Why it works
Matter has to be reused. A meadow has only so much nitrogen and phosphorus. When a fox dies or a leaf falls, those nutrients are locked inside. Decomposers digest the dead matter and release the nutrients in simple forms that plant roots can take up again. Without them, producers would run out of nutrients and everything that eats producers would follow.
Worked example: Name the role
A mushroom grows on a rotting stump. A deer eats clover. A hawk eats a snake. What role does each play?
- The mushroom feeds on dead wood and breaks it down: a decomposer.
- The deer eats a producer: a consumer (a herbivore).
- The hawk eats an animal: a consumer (a carnivore).
- The clover makes its own food from sunlight: a producer.
Answer: Mushroom: decomposer. Deer and hawk: consumers. Clover: producer.
Worked example: A tricky one
Is an earthworm eating dead leaves in the soil a consumer or a decomposer?
- It feeds on dead plant matter and wastes, not on living things.
- Ecologists group animals like earthworms, which break down dead matter, with the decomposers (they are sometimes called detritivores).
Answer: A decomposer (a detritivore).
Common mistake
Students leave decomposers out of an ecosystem, or think they are just consumers that eat living things. Decomposers feed on dead matter and wastes from every level. Check any food web you draw: is there a decomposer that receives matter from the plants and the animals?