Understandings of Consequence > Curriculum Modules > Ecosystems > Section 3

Living things called "decomposers" break down organic matter by digesting it. Some decomposers that we can see are worms, mites, and sow bugs. Other decomposers are microbes, tiny organisms such as fungi and bacteria. They are not easily visible. Whereas larger decomposer organisms that children are familiar with are responsible for some of the physical breakdown of organic matter, microbes are the primary agents that recycle dead matter into its basic elements.

Typically, children identify physical factors as the cause of things like branches or fallen fruit, for instance, breaking down. Their explanations may include people or animals stepping on the thing to make it break apart, or rain and wind as the cause for materials breaking down and disappearing. They may also say that earthworms or bugs eating branches or fallen fruit cause decay. Sometimes children will simply say that over time things just disappear into the soil

Microbes are all around in the physical environment, feeding on and breaking down dead matter. Fungi and bacteria are two common microbes. Individual bacteria are too small to see with the naked eye, as are some fungi. Some fungi can be seen if there are large enough numbers of them growing near each other. Mold seen on and in rotten food is a type of fungus. The white fuzz is called hyphae. Hyphae are threadlike strands of the fungus that take in food. On the hyphae, there are "spores," which look like colored powder, and which the fungi use to reproduce.

Decomposition recycles dead matter by reducing it to its basic substances, which are then released into the physical environment and may become part of living things once again. A nutrient cycle is the flow of nutrients from living things to the physical environment and back again. In a nutrient cycle, nutrients are passed along from plants to animals, from animals to animals, from dead plants and animals to decomposers, and finally from decomposers back to plants via the physical environment (i.e. soil, air and water).

It is common to focus only on obvious causes of events unless there is some evidence to suggest that the obvious causes one can detect do not completely explain what is going on. The activities in this section help students notice the role of non-obvious, or in this case microscopic, decomposers. It does this by comparing two tanks of decomposing matter and revealing that even the one without the obvious decomposers is decaying. This encourages many students to believe that there could be microscopic decomposers at work.

There is still a remaining challenge to understanding decomposition. Children (from infancy) typically believe that you can't have a causeless effect. If something is changing, then something must be causing it, right? However, in the case of decomposition, many students believe that things just break down on their own, similar to how a toy can break into parts. They draw an analogy to objects falling apart rather than being acted upon by tiny decomposers at work. This section attempts to address this by having students observe the growth of mold in a reinforcement activity and to discuss what happens when food is "preserved."

This section often refers to microbes eating or digesting food. Students may think of eating in a very active sense, the way that they pick up a piece of food and eat it. It may not seem to them that the microbes are eating. Also, it can be hard for them to think of something as small and undifferentiated as a microbe being alive and doing things like digesting food, growing, etc.¹ It is a good idea to address this with students. Explain that microbes digest food. They secrete digestive chemicals outside their bodies that break down the food, and then they absorb it. This is really the same as the way animals eat food, even though it might seem more passive.

Note to the Teacher

Timing

The activities in this section require at least two sessions, separated by six to eight weeks (and longer if possible.) They involve setting up two worm tanks and observing what happens in them over time. The initial lesson should occur near the beginning of your unit to give enough time for observable changes to take place.

Detritivores vs. Decomposers

Here is a technical point to consider introducing to your students. Most scientists refer to the larger decomposers that you can see as "detritivores," and call bacteria and fungi "decomposers." To simplify matters, both are referred to as "decomposers" in this module. However, older students could certainly learn to make this distinction.

The Difficulty of Demonstrating Conservation of Matter

This lesson focuses on noticing that a tank of compost without worms undergoes changes (the particles become smaller and the level in the tank goes down) and that tiny microbes are responsible for those changes. Students are asked to make observations. You may be tempted to also try to teach a secondary concept—that the amount of actual matter is conserved. This is difficult to illustrate for a number of reasons. There are a lot of other variables that influence the outcome, such as the exactness of the scales, differences in the weight of the actual worm tanks, differences in how much the tanks are watered or differences in rates of evaporation depending upon where they are put. However, even if you could control all of those variables perfectly, you would still not be able to show that matter is conserved. This is because gases are a product of decomposition. If you include the gases and the matter in the tank, then the matter is conserved, but there is no straightforward way to show this (because the tanks allow gas exchange with the atmosphere.)

If you want to discuss the idea of gas exchange, engage the students in a discussion about how we need air, more specifically oxygen (O₂), to live. We exhale carbon dioxide (CO₂). Worms as animals need oxygen and exhale carbon dioxide. Microbes also need oxygen and exhale carbon dioxide. This helps to explain the loss of matter from the tanks. It also underscores the importance of plants because they need carbon dioxide and give off oxygen. (They also need oxygen and release carbon dioxide as they "burn" their own food energy after making it.) One issue that will come up is that it is cognitively difficult for students to view gases as matter. Therefore, they may have a hard time relating the explanation to the loss of matter from both tanks.