Section 3: Resources

Picture of Practice

Discovering Non-Obvious Causes of Decay: A 5th Grade Lesson on the Role of Microbe Decomposers in the Ecosystem

The following picture of practice describes a lesson in which students discuss the possible causes of decomposition when there are no visible or obvious decomposers present. Through discussion with their teacher, students discover that sometimes the causes of things are difficult to detect because they are invisible and/or take time to have a noticeable effect. Here we look in on a portion of the conversation early in the unit and then a little later in the unit, after students have had a chance to discuss their worm tank experiments.

• Mr. Miller: Let's review a little from yesterday, can anyone remind us of what decomposition is?
• Hannah: It's when something breaks down dead matter and turns it to soil.
• Mr. Miller: Okay, and we've talked about several types of decomposers. Can anyone name some of them?
• Jake: Earthworms.
• Becca: Sow bugs.
• Emily: Mushrooms.
• Carlos: Fungi.
• Mr. Miller: Now I want you to think about what would happen if something died and there were no earthworms, sow bugs, mushrooms, or fungi to break it down. What do you think would happen?
• Becca: Uh, nothing. It would just be there.
• Mr. Miller: It would just stay there? Okay. What are some other ideas?
• Jake: It would turn to soil.
• Mr. Miller: And can you say how that would happen?
• Carlos: Maybe the heat would turn it to soil.
• Sarah: Or maybe the rain would wash it away. Or wind.
• Mr. Miller: Okay, what are some other ideas?
• Ian: Maybe it would just fall apart on its own, you know, because it's so old.
• Katie: I don't think it could just fall apart on its own. I think nothing would happen, that if there aren't any worms and things, it just stays the same.
• Mr. Miller: Okay, today, we are going to set up an experiment to help us think about this question. We'll compare what happens to dead plant matter when there are worms and when there are no worms. We'll set up two tanks to compare what happens. In each of the tanks we'll put the same amount of dirt, some grass and some shredded leaf matter from outside. Then, in one of the tanks we'll put some earthworms and in the other tank we won't. We'll watch these two tanks carefully for several weeks, making observations of what's going on in each of the tanks every couple of days. What do you think will happen in each tank? Take a minute to think about what you think will happen in these two tanks.

Mr. Miller records the students' predictions on a piece of chart paper, which he saves to revisit later.

• Jacob: I think the leaves in the worm tank will get smaller and smaller. The worms will eat them.
• Emily: I think the tank with the worms will have more soil than the tank with no worms.
• Mr. Miller: Okay. And what about the worm free tank?
• Becca: I don't think anything will happen in there.
• Carlos: Yeah, in the worm tank the leaves will go down, but in the worm-free tank they will stay the same.
• Mr. Miller: Okay, and how will we be able to know that? What can we do to test some of these predictions?
• Jacob: We could see how high the stuff is in the tanks.
• Mr. Miller: Okay, we could measure it. Can anyone think of some ways that we could measure the two tanks?
• Ian: We could use a ruler to measure how tall the stuff is.
• Mr. Miller: Okay. Now, in order to have a fair test, we will need to mark the height of the matter in both tanks and to be sure that we put the same amount of stuff in each. That way we will know that if there is a difference later on, it's because of what happened and not because we put more dirt in one than in the other.

Mr. Miller goes on to explain how the class will care for the worms and observe the changes occurring in the tanks over the next 6 to 8 weeks. After this time has passed, Mr. Miller and the class gather to discuss what they have observed in the worm and worm-free tanks.

• Mr. Miller: Boys and girls, today I'd like to talk about what we have been observing in the worm and worm-free tanks. Would anyone like to share some observations with us?
• Ian: The stuff in the worm tank got higher at first and there were lots of worm trails, but then it got lower. There was less stuff after a while.
• Emily: Yeah, the stuff was disappearing.
• Mr. Miller: Do you think that it was actually disappearing, or was something else happening to it? Can it just disappear? What do we know that could help explain what happened?
• Jacob: The worms were eating the leaf matter.
• Mr. Miller: That's right. The worms were eating the leaf matter and breaking it down into smaller particles. When things are smaller, they can fit into less space. He demonstrates by taking a couple of leaves and crunching them up so there is no air between them. Also, some of the stuff turns into gases as the worms digest it. Therefore, there seems to be less matter in the worm tank over time. Let's talk about the worm-free tank. What happened there?
• Becca: The stuff went down over time.
• Mr. Miller: How would you explain this change? Any ideas?
• Becca: I think the stuff just broke apart as it got old.
• Ian: Maybe there were some worms in there that we didn't see.
• Carlos: Or maybe there were some other tiny bugs in there.
• Sarah: There could be some other stuff in there like bacteria that maybe we don't see.
• Mr. Miller: That's an interesting idea, Sarah. Let's talk a little bit about that. Bacteria, and something else called mold, are what we call microbes. Microbes are tiny living things that you can't see without a really good microscope. But, although we can't see microbes, we can see their effects.
• Emily: But I've seen mold before. It grew on some bread we had at home.
• Jacob: There was mold on our pumpkin from last Halloween. We left it out on our steps and after it rained and it was out there a long time, it had mold stuff all over it.
• Mr. Miller: That's right, when there's enough mold all together, you actually can see it. Did you notice what happened to the pumpkin after the mold had been growing on it?
• Jacob: Yeah, it just sort of got all mushy and stuff.
• Mr. Miller: Yeah, as it grows, mold can be seen spreading like a type of fuzz on things like bread or old fruits and vegetables. Bacteria do the same kinds of thing, but because it is so small you can't really see it like you can mold. Do you think this could explain what caused the change in the worm-free tank?

Some students nod in agreement, others shake their heads and still others seem unsure one way or another.

• Mr. Miller: Well, some of you seem unconvinced that this is what's happening in the worm-free tanks. Sometimes when you can't see something, it makes it difficult to know what is going on. For example, because we can't see microbes, it makes it hard to understand the job they do in the ecosystem. Let's start an activity to help us find out more about microbes.

Mr. Miller then sets children up for collecting evidence of non-obvious decomposers using bread samples for growing mold.