Fremont Magnet Elementary

Back to 4th Grade Science

Decomposers and Microorganisms

Standard 2c Students know decomposers, including many fungi, insects, and microorganisms, recycle matter from dead plants and animals.

Standard 3d Students know that most microorganisms do not cause disease and that many are beneficial.


1. Review

Last week you learned that a food chain shows who eats who.

  • It starts with a primary source called a producer
  • Everything that does not not make its own food is called a consumer
  • There are 3 types of consumers
    • herbivore
    • carnivores
    • omnivores


In this food chain, which of these is an omnivore?

berries ---> rabbit ---> black bear

black bear

Which of the following organisms could be the primary source of energy for a food chain?

  green algae

Which of these describes something that eats only plants?




2. What is a decomposer?

A decomposer is part of the food chain.  When something dies, like a plant or an animal, a decomposer breaks down dead plants and animals

Decomposers turn dead material into good fertilized soil.  Click here to learn more.


Worms are decomposers.  Their job is to break down things like dead leaves and grass.  Their waste makes the soil healthy.  Having worms in your garden is a good thing!


Suppose all the decomposers disappeared from a forest ecosystem. Which of the following would most likely result?

  the forest floor would be covered with dead plants and animals

  the trees and bushes would stop growing

the trees would grow straighter and taller





What occurs when decomposers feed on dead organisms?

  the decomposers become poisonous
the organisms break down into soil

  the decomposers give off oxygen


Here is a quick quiz to see what you know about decomposers

3. Microorganisms

Many decomposers are microorganismsWhat is a microorganism?  You already know that a living organism is anything that is alive.  So you know it has to do with living things.  Think about the prefix micro.  What other words begin with micro?

What do you think microorganism means? Click and drag your mouse across the green bar.

a tiny living thing that can only be seen with a microscope


4. Are bacteria bad for you?

We usually think that bacteria are dangerous.  We wash our hands and use antibacterial soap to kill bacteria that can make you sick.

Although there are bad bacteria that make you sick, most bacteria are good for us and we need them. You have bacteria in your intestines to break down food.  Yogurt, which is a healthy snack food,  is made with bacteria. We could not exist without bacteria!

Most bacteria are decomposers, so they break down all the dead material.  They are the reason the Earth isn't covered with dead dinosaur bodies. 

Click here for a BrainPop movie about bacteria

Click here to see a movie of bacteria

5. Now try this


A main difference between helpful and harmful bacteria is that harmful bacteria

  feed on oil and break up sewage
  cause some plants to make nitrogen
damage or kill living body cells

Which of the following is a characteristic of most bacteria?

  most bacteria live on animals
  most bacteria are decomposers
most bacteria harm plants

Microbe Zoo

Click here to see a movie of a microorganism