Materials

• Parachuting Cats Into Borneo Story
• The Lemmings of Norway: An Example of Balancing Ecosystems by Flux Story
• Lynx-Hare Cycles Sheet

Prep Step

• Review understanding goals and background information.
• Review the lesson plan.
• Photocopy the story, Parachuting Cats Into Borneo, if you plan to have them read it (alternatively, this story may just be read aloud to them).
• Photocopy the Lynx-Hare Cycles Sheet for each student.

Analyze Thinking

Step 1: Gather Students' Ideas and Questions

Explain to students that they have talked a lot about ecosystems and what happens to populations of animals in ecosystems. Ask:

• Do you think a population always has the same number of animals in it, or can it change?
• If you think it changes, how does it change? Yearly? Monthly? Once in a great while? What might cause changes at these intervals?
• Do you think that change in an ecosystem helps or hurts? Why?

Note to the Teacher: If students did the simulation in the last section, draw their attention back to the patterns in the accompanying population size graphs. Go back to the simulation and discuss the up and down patterns that are evident.

Gather reactions to the idea of change in ecosystems. Most students view the changes, especially the boom and bust patterns, as negative.

RECAST Thinking

Step 2: Investigating the Important Role Predators Play in Controlling Population Size

Read the story, Parachuting Cats Into Borneo. Ask: "What questions do you have about what happened?" Make sure that the students understand what happened and why.

Step 3: Map the Interdependencies Amongst Populations

Ask, "What happened and why did it happen? What would a diagram showing what made what happen look like?" On the board, map out what the story revealed about the interdependencies amongst the populations. Your mapping might look something like this:

This mapping shows a two-way relationship between the wasps and the caterpillars because the wasps give energy to the caterpillars and the caterpillars keep the size of the wasp population under control. Insects give energy to lizards that in turn give energy to cats. The cats get energy from the rats but also keep the size of the rat population in balance.

Make sure that students understand that the predators play an important role in keeping certain population sizes under control. For instance, the wasp keeps the thatch-eating caterpillars to a minimum and the cats keep the numbers of rats down.

Ask, "Is there ever a time when keeping the numbers of a population down can help the animal whose population size is kept down? Where it actually helps the animal who is being eaten?" Collect and discuss students' ideas.

Step 4: Discuss Why Flux is Not Always Bad

Explain that the example above shows how balance can be important in the ecosystem. Often people think that ecosystems must be balanced in order to be "healthy." However, change or "flux" is not always harmful for an ecosystem. It is an important part of the complex tapestry of ecosystem dynamics. In fact, some might say that flux is a part of balance—that balance is not a static thing. When there is change, it can offer opportunities for new populations to become established. For instance, if a rabbit population dies down, other animals that eat the same kinds of things and live in the same sorts of places might start to establish themselves, adding stability to the ecosystem. (In general, the more species, the greater the stability.) Balance AND flux are parts of the dynamic of ecosystems.

Step 5: Read the Story, The Lemmings of Norway

Show the image of a lemming. Explain to your students that they are going to hear a story about a tiny rodent who lives in the far north where it is often cold and snowy. You may want to get a map of the world and show the students where the arctic regions are and where the country of Norway is in particular. Reinforce that there are no landmasses off the coast of Norway.

Read the story out loud to your students. Use the follow-up questions to discuss the story with your students and gather their ideas.

Step 6: Read the Lynx-Hare Cycles Sheet

Read and discuss the page about the Lynx-Hare Cycles as another example. Make sure that the students understand how flux is an essential dynamic that alternately results in balance.

Explore Causality

Step 7: Review Balance and Flux Concepts

Make the following points about the nature of balance and flux:

• Balance and flux in one population can have ripple effects to other populations—they become affected by the flux as well. Populations are intimately connected.

• Even if populations are affected by certain changes, this does NOT always mean that they will die off. If a certain food source disappears, the animals that ate that food may shift to other diets. Chance events can cause this to happen. For example, if a storm washes an abundance of mussels on shore, it might result in a feeding frenzy among shorebirds, but this depletes the mussel beds. The birds must again shift their diets (which might have previously included small amounts of mussels) to other foods.

• Flux can open up opportunities for other species within an ecosystem. When the dinosaurs became extinct, they left a huge gap in the ecosystem. The mammals were able to fill that gap and increased their numbers and their diversity. Without flux, mammals (don't forget humans are mammals!) would not be as prevalent as they are today. In this case, flux created both positive and negative results. The event was negative for the dinosaurs because they died out, but it was positive for the mammals, which could then evolve without intense pressure to compete for resources and without the worries of dinosaur predation.

• Balance and flux are valuable dynamics in ecosystems. An ecosystem in balance is a stable ecosystem doing well. Flux can provide opportunities for weaker populations to gain a stronger foothold in their ecosystem. Balance and flux in a system can cause complex effects, and these effects can be positive, negative, or both for the populations involved.

Review, Extend, and Apply

Step 8: Connect Back to the Web of Life

Ask your students to recall the first activity that they did in this module where they made a "Web of Life." Ask:

• How does what you learned in this section change the way that you think about what you learned from that game?
• How would you revise the game in order to make it reflect what you now know?