Ecosystems Curriculum

Section 2: Lesson Plan — Learning About Decay

There are two alternative RECAST activities in this section. The Rotting Log activity has the appeal of getting the concepts across in one lesson and is often very exciting for the students. However, it may be difficult for some teachers to obtain a rotting log or some teachers may not feel comfortable taking apart a rotten log. The Decomposition Chamber activity is slower and offers a clearer sense of process. Teachers may of course choose to do both.

RECAST Activity A: Investigating a Rotting Log

By investigating a rotting log, students will notice different stages of decay and begin questioning why they find soil inside a rotting log. The activity can be done as a class activity in small groups or as a group demonstration. Having students discover soil inside the logs for themselves is a powerful learning experience. However, it involves having enough rotting logs to go around, dealing with the mess involved (certainly worth it!), and dealing with whatever students find in the logs. This can include various types of fungi, insects, and spiders.

The Brown Recluse Spider is a poisonous spider that lives in woodpiles. It is unlikely that one would remain inside a log transported to school because they are reclusive; however, it is not impossible. For these reasons, teachers may choose to do a demonstration instead. If you do the activity, encourage the students to see the log as a habitat and make your best effort to return the creatures with the remaining pieces of the log to an appropriate spot outdoors after the activity. If you can't find a rotten log, a pile of rotting leaves and its residents can be a good substitute.

RECAST Activity B: Creating a Decomposition Chamber

By setting up and observing a decomposition chamber, students will notice changes that indicate that decay is occurring over time. This activity can be done as a small group activity or together as a class. Having students observe the process of decay over time and discover the soil that is generated from the breakdown of matter is a wonderful learning opportunity. It takes approximately 6-8 weeks in order for observable changes to occur, therefore it involves planning ahead by the teacher. This activity should take place in two parts: the first part to talk about predictions and to set up the chamber; and the second part to observe, measure and discuss the changes and the causes of them.

Materials: RECAST Activity A: Investigating a Rotting Log

• A piece of log that is beginning to rot
• Some broken bits of a well rotted log
• Rotting log segments (1 per 4 to 5 students or 1 for a class demonstration)
• Some soil with obvious bits of decaying material
• Hand lenses
• Plastic tablecloths or newspaper to place on desk tops
• Plastic containers for any critters that might need a temporary home
• Metal spoons (or something to examine the log with; to break apart gently or examine nooks with so as not to use hands)
• Gloves for students pulling log apart
• Observation sheets
• Field guides

Materials: RECAST Activity B: Creating a Decomposition Chamber

• Clear plastic two liter soda bottles with the top fourth cut off
• Pieces of cheesecloth or pantyhose
• Rubber bands (2 per bottle)
• Bag of sand
• Bag of garden soil
• Water
• Organic materials in containers: leaves, grass clippings, pine needles, lettuce
• Other items made from organic materials: newspapers, sticks
• Inorganic materials: plastic wrappers, Styrofoam pieces

Prep Steps: RECAST Activity A: Investigating a Rotting Log

• Review understanding goals and background information.
• Read the lesson plan.
• Find rotting logs and get soil from a forest or wooded area. Plan one log or log segment for approximately every four students.
• Gather newspaper or plastic tablecloths, hand lenses, gloves, plastic spoons and containers.
• Protect tabletops or desktops by covering with newspaper or plastic tablecloths, or work outside.
• If students will be working in groups, break rotting logs into segments so that each group of students has one segment to explore.
• Photocopy the two observation sheets.

Prep Steps: RECAST Activity B: Creating a Decomposition Chamber

• Review understanding goals and background information.
• Read the lesson plan.
• Gather materials needed for the decomposition chamber.
• Cut bottle tops.
• Cut cheesecloth or pantyhose.
• Measure out soil and sand.
• Gather organic and inorganic matter to add to chamber.
• Read the Picture of Practice to familiarize yourself with what to expect from the lesson.
• Photocopy the sheet, What is Cyclic Causality? for your students.
• Photocopy the story, The Cyclic Journey of an Atom for your students.

Analyze Thinking

Step 1: Gather Students' Ideas and Questions

Ask, "What happens to a tree when it falls in the forest?" Gather students' ideas and questions. Track these on chart paper. Students will typically say things like, It disappears; It goes into the ground; It disintegrates; It rots; and It turns into little pieces until it is gone.¹ If students use words like decay or decomposers, ask them to define the words.

Step 2: Expand Students' Ideas

Ask, "Does whatever happens only happen with trees, or does it happen with other things? What about a piece of fruit, for example?"

Encourage some dissenting ideas by asking, "How many of you think that it completely disappears? If it doesn't disappear, what else might happen to it?" Some students may have compost bins at home and may know that it becomes a part of the soil.

RECAST Thinking

Step 3a: Explain the Activity—RECAST Activity A: Investigating a Rotting Log

Explain to students that they are going to break apart a piece of rotting log and examine what they find. This will help them think about what happens when something decays. Have students think about the following questions as they work:

• What happens to a log as it decays?
• What causes the log to decay?

Note to Teacher: Do the activity in small groups or as a class demonstration. Read the background information to help you decide which is best for your class. Try to have your class work outside for this activity if at all possible. If not, have them work on newspaper or on plastic tablecloths.

Divide students into groups of four. Hand out the two observation sheets. The student who is actually breaking the log apart should be wearing gloves because there might be insects and fungi in the log that shouldn't be handled. If anyone notices a spider, they should NOT touch it. Have students call you over instead.

Step 4a: Explore the Log

Give the students about 15-20 minutes to explore the log and to record their findings on their observation sheets. Circulate while they are working and help them to think about the kinds of evidence that they are finding in the log to suggest what might be happening to it.

Many students are unaware that the log is turning into a part of the soil. They may try to find explanations for how soil got into the log. Having students notice the soil is the first step towards inviting them to question how it got there and helping them to see that decay is a cyclic process.

Step 5a: Discuss Students' Findings

Regroup for a class discussion. Ask, "How would you describe the log? How is it different from a log that is not as decayed? What are some of the things that you noticed inside the log?" List what students found. Review the list. If the list doesn't include soil, ask, "Did anyone find any soil? Why was soil in the log?" Gather some contrasting ideas.

Step 6a. Compare Logs at Three Points of Decay

Show students: 1) a piece of a log that is beginning to rot; 2) smaller, broken pieces of a log that is further in the process of decay; and 3) some soil.

Ask, "Do you see any ways that these things are related? In what ways are they alike and similar? In what ways are they different?" Explain that the piece of log, the bits of broken down log and the soil were all at one time the same material—wood from a tree. Students are looking at wood at different stages of decay.

Ask: "Have you heard the word 'decay' before?" Explain that another word for decay is rot. Decay is when dead matter is broken down. When a log begins to decay, it breaks down into its basic parts, which end up as part of the soil and as gases that are given off.

Step 7a: Consider How Decay Happens

Engage students in a conversation to consider how the log is decaying. Ask, "What kinds of creatures did you find and what might they be doing in a rotting log? What did you observe about the creatures? Do you think that the creatures play a role in the log decaying and turning into a part of the soil? How might this be?"

See if anyone thinks that the creatures are eating the log. Ask students to think about what they know about the job of earthworms and other decomposers. Many students think that decay just happens, like something falling apart. They don't think that something actually causes it, so they may need help in making the connection between the decomposers and the decay.

Continue with Step 8 below

Step 3b. Explain the Activity—RECAST Activity B: Creating a Decomposition Chamber

Explain to students that they are going to build a chamber filled with organic and inorganic matter and observe the changes that take place within it over a period of a month or two.

Ask, "Does anyone know what organic matter and inorganic matter are?" Gather ideas. Explain that organic matter is stuff that is alive or made from substances that are alive. Inorganic matter is matter that is not alive and has never been alive in its current form. Have students consider different types of organic and inorganic matter that could be added to their decomposition chambers.

Step 4b. Make Predictions

Ask, "What will happen over time to organic matter, like fruits and plants, which are placed in the decomposition chambers?" Then ask, "What do you predict will happen to inorganic matter like Styrofoam cups, plastic toys and packaging and so on?" Record their predictions and save them to be reviewed later.

Ask, "How long do you think it will take to observe changes in the bottle? What are you basing your prediction on?"

If students predict that changes will take place, ask, "What will cause the changes that you are predicting?" Many students think that weather, animals or humans are responsible for the decay and breakdown of organic matter. Explain how the decomposition chamber is a "controlled environment" and discuss what this implies for the causes of decay. Encourage some initial conversation on this issue. Section 3 addresses the causes of decay in greater depth.

Step 5b: Creating the Chamber

Follow the instructions for building the chamber. This project can be done in groups or together as a class. Circulate while students are working and gather their individual thoughts on what will happen.

Step 6b: Discuss Students' Findings

In six to eight weeks, regroup for a class discussion about what students observed about the decomposition chambers. Review the list of students' predictions for what might happen.

Ask, "What have you noticed about the chambers?" List the changes that students observed.

Ask, "What do the measurements on the sides of the containers suggest?" Gather ideas and discuss.

Ask, "What do you think happened to the organic matter?" Some of it broke down into smaller particles and some of it was given off in the form of gases. Explain that when the plant matter broke down, some of it became a new part of the soil. This explains why the level of soil seems greater now.

Ask, "Are there any differences between what happened to the organic matter and the inorganic matter?" Students should have noticed more change in the organic matter. Discuss the differences.

Ask, "Do you think that inorganic matter breaks down or not?" Discuss how time is a factor in decomposition.

Step 7b: Consider How Decay Happens

Ask, "Do you have any ideas about how the organic matter in the decomposition chambers decayed?" Gather some ideas. Many students think that decay just happens, like something falling apart. They don't think that something actually causes it so they may need help in making the connection between the decomposers and the decay.

Explore Causality

Step 8: Contrasting Linear and Cyclic Causality

Ask, "Why do people usually show decomposers at the end of the food chain?" Gather ideas. People often show them at the end because they eat or digest the things that die. This makes sense in one respect. They ARE the end of the energy domino discussed in section one. However, decomposers do more than just get energy from the things that they eat, they break the matter in these things down enough to recycle it into its basic substances. They are an important part of the nutrient cycle and so here they are NOT the end. There really is no end to a circle.

Ask, "What is the importance of decomposers in the food web and the ecosystem?" Explain that when organisms die, decomposers recycle the dead matter. Recycling it releases the matter back into the surrounding environment and provides the nutrients that green plants need to live.

Draw a cycle on the board that shows a green plant that dies and is recycled by an earthworm, and the nutrients from this decay contributing to the growth of other green plants. Many students are unaware of non-obvious decomposers, so at this point it is important to stick with an obvious one such as an earthworm.

Explain that the process is circular. This circle is called a cycle. Show how the cycle could also include consumers (mice, for example).

Explain that it takes a long time for the different parts of the cycle to happen, so it may be difficult to see how they are related and think about how they go together. The time delay makes it difficult for students to see the cyclic pattern. This is the focus of Section 4.

Step 9: Show How Cyclic Causality Explains Matter Recycling

Explain that sometimes decomposers, like worms, are called Nature's Recyclers. Ask, "Can anyone explain why that is?"

Write the word recycling on the board. Explain that recycling is the way people take waste and turn it into something that can be used again. Ask, "Can anyone give an example?" We recycle paper and aluminum and turn them into things that can be used again.

Explain that in some ways, decay and decomposition are like the recycling that people do. Dead matter is broken down, becomes part of the soil, and contains basic substances that are then available to plants. As a class, read and discuss the story The Cyclic Journey of an Atom.

As a class, read the sheet, What is Cyclic Causality? Either individually or as a group, have students draw a cyclic diagram showing matter recycling on the bottom of the page.

Review, Extend, and Apply

Step 10: Considering What Would Happen Without Decay

Ask, "What would happen if the cycle were to be broken? What would happen if logs didn't decay? Why is it so important that there is decay? What might the world look like if things didn't decay?" Gather some ideas.

Explain that the forest floor would be covered with dead logs and dead leaves and over years it would pile up so high that it would keep new things from growing. In addition, the dead matter and waste would pile up so high that it would eventually cover plants living on the ground and even the tops of trees. Earth would be covered with waste. Decomposers keep this from happening by breaking down dead matter and putting important nutrients into the soil, which helps keep things growing.²

Ask, "What would happen if dead logs all disappeared instead of being recycled? What might the consequences be?" Gather some ideas.

Step back and compare the two forms of causality introduced so far—domino causality and cyclic causality. Consider why they are important in ecosystems. Domino causality helps us to see that all the organisms in the ecosystem get their energy from the sun, through the crucial link of green plants. It also helps us to see that the energy is "lost" to the ecosystem throughout the process (and by the end of the domino effect). We can refer to this process as the "energy domino." Cyclic causality helps us to see that matter is not lost—it is continually recycled. We can refer to this process as the "matter circle."