Section 1: Lesson Plan — Exploring the Web of Life

Materials

  • Index cards (large size)
  • Pictures of organisms in an ecosystem and their diet information: Sun; Green Plants; Insects; Mice; Toads; Owls; Snakes; Skunks; Fungi; Earthworms
  • Yarn or String

Prep Step

Analyze Thinking

Step 1: Define an Ecosystem

Ask, "What is an ecosystem?" Gather ideas. Arrive at a class definition. It should include the following ideas:

  • An ecosystem can be thought of as a community consisting of different populations of living things. (A population refers to all the members of a species that live in a particular location.)
  • An ecosystem includes the physical environment.
  • Organisms fill different roles or "niches" in the system.
  • Living things in the ecosystem depend on other living things in the ecosystem.

Step 2: Consider the Physical Environment

Discuss the necessities of the physical environment for an ecosystem. List students' ideas on a chart, which may include:

  • Sunlight
  • Water
  • Soil (specifically the nutrients in the soil)
  • Living things in the ecosystem depend on other living things in the ecosystem.

Step 3: Consider Roles in the Ecosystem

Explain: Each living thing in an ecosystem can be put into one of three categories: Producers, Consumers and Decomposers.

Ask, "Do you know what produce means? What about to consume? What about to decompose?" Explain the importance of each group in the ecosystem, highlighting their interdependence.

RECAST Thinking

Step 4: Introduce the Web of Life Game and Construct the Web

Explain: We are going to play a game that will show how each living thing in an ecosystem relies on other living things to survive. Find an open area. Gather the students around it. Use the diagram for step-by-step instructions on how to build the food web.

Begin with the card with a picture of the sun on it. Give it to a student. Ask, "Which organisms rely directly on the sun for their food?" As students identify organisms (in this case, the green plants), give the green plants card to another student. Have the student wearing the sun card hold the end of the yarn and the student wearing the green plants card hold the other end.

Then move on to the green plants. Ask, "Which organisms rely directly on the green plants for their food?" Refer to the information on each card. Some students may not realize that only plants can make their food directly from the sun. You may need to introduce this fact.

As organisms are identified, add them to the food web and then add those organisms that depend upon them for food. Go through the cards in the following order: sun, green plants, insects, mice, earthworms, toads, snakes, owls, skunks, and fungi. Repeat the questioning and constructing until all of the connections have been made.

Step 5: Analyze the Connectedness of the Web

Draw students' attention to the complex web that has been created with the strings. Ask, "What might happen if the owls disappeared?" Not much would happen. Fungi would have less dead matter.

Ask, "What might happen if the mice disappeared?" Owls, snakes, skunks, earthworms and fungi would be directly affected. Other organisms would be indirectly affected.

Ask, "What might happen if the green plants disappeared?" Everything except the Sun would be affected.

Have the student who has the card for green plants sit down and gently tug on the strings that he or she is holding except for the one connected to the sun. Anyone who feels a tug on their string should sit down to show that they have been affected. Then those students should gently tug on their strings and anyone who now feels a tug on their string should sit down and so on.

Ask, "What does this demonstration show?" If the organisms' food source is affected, they will be affected. Sometimes animals can turn to a different food source, but if the green plants disappear, everything will die. They are our link to the energy from the sun. Have the students return to their seats.

Step 6: Introduce the Forest Food Web Diagram

Hand out A Forest Food Web sheet. Explain that it shows the relationships in an ecosystem such as those illustrated in the game. Emphasize that the arrows on the food web show what gives energy to something else. They do NOT go from the eater to the eaten. Say, "You can remember the right direction by having the arrow go from an organism to the mouth of the thing that eats it."

Explore Causality

Step 7: Contrast Simple Linear and Domino Causality

Say, "Usually when we describe how things happen, we use simple linear or simple direct cause and effect. We say that one thing directly causes another thing to happen. For instance, green plants cause there to be energy for the mice."

Introduce the concept of domino causality. Explain that in domino causality, effects cause new effects just like in dominoes falling over. Set up some dominoes and show how one falls down and knocks down the next one and that one causes another one to fall over, and so on. Causes create effects that in turn become causes. This is important because it shows that ecosystem events can have indirect as well as direct effects.

Pass out the sheet entitled, What is Domino Causality? for your students to read. Discuss the sheet as a class and address any questions that they have. Consider the Questions to Think About together.

Step 8: Show How Domino Causality Explains Food Chain and Food Web Relationships

Explain that the relationships in a food web are like dominoes. One thing causes another thing, which causes another thing. For instance, the green plants cause energy to be available for mice, which cause energy to be available for owls, and so forth. If something happens to the green plants, it causes there to be no energy from green plants for mice, and that in turn causes there to be no energy from mice for owls. The term food chain is often used to talk about a row of dominoes that are set up in a straight line to describe a linear set of connections such as this.

Note to Teacher: You can create a set of dominoes using the pictures included in this section for the Web of Life GameAcrobat icon. It would help to have duplicates of some organisms (mice, insects, toads, earthworms).

But just like with dominoes, sometimes the rows branch out in different ways. Set up some domino examples that have branches in them. For instance:

This is the difference between a food chain and a food web. In a food web, the dominoes branch or radiate out so that there can be many effects of one cause when certain dominoes fall. This web describes what happens in an ecosystem better than a food chain does.

Discuss with students how important the link to the sun is. The energy from the sun is converted into food energy by green plants. As it is passed along the food web, energy is lost. (It is converted to heat energy, which can't be used for food.) When things decay, the remaining energy becomes heat energy that is dissipated into the environment. (The processes of life, such as growing, eating, and even sleeping, require energy and when this energy is used, heat is always a by-product.)

Step 9: Differentiate Between Matter and Energy

Explain that energy is different from matter. Matter is what things are made of—actual particles called atoms. Matter gets recycled during decay. (Section 4 focuses on this). You can't see energy. It doesn't get recycled. Instead, as it is used, much of it gets transformed into heat energy. Eventually, so much of what was available is transformed to heat that there is no more to be used by living things. Therefore, our link to the sun is crucial because it is the first domino in the set and it represents a virtually unlimited source of energy. When we talk about energy, we explain the patterns with domino causality. When we talk about matter, we explain the patterns with cyclic causality (Section 2 focuses on this).

Even though they are different things, once matter and energy enter the living world and become "food," they "travel together" in a sense. This should become clearer to students as they play the Nutrient Cycles Game in Section 2.

Review, Extend, and Apply

Step 10: Extend the Concepts to Other Ecosystems Discussions

Summarize the ideas in the lesson by revisiting the Understanding Goals. As students do other food web activities, have them consider how domino causality is involved.