By fifth grade, most children have become increasingly curious about the complexity of the world around them and about the relationships between the living and nonliving elements in their environment. News stories, community and school projects, and books that promote ecology introduce them to environmental issues. They are fascinated by ways they can help save the environment and the organisms in it.

No organism on earth lives isolated and independent from all others. Every organism--including humans--exists within a community of living and nonliving things, called an ecosystem. Ecosystems, a 16-lesson unit designed for fifth-graders, helps students understand the web of relationships that links organisms to one another and to their natural environment. By constructing, observing, discussing, and reading about both land and water ecosystems in this unit, students can develop a growing sensitivity to living things and what they need to survive.

In the unit, students create a model ecosystem by connecting a terrarium and aquarium. This model ecosystem contains both living and nonliving elements. Throughout the unit, students nurture the ecosystems within the ecocolumn, observe changes in each environment, and prepare for pollution experiments to determine the effects of pollutants on organisms in general in an ecosystem.

In the first lesson, students discuss what they already know about "ecosystems" and what they would like to learn. In Lesson 2 each pair of students begins to set up their own terrarium with soil, seeds (grass, mustard, and alfalfa), gravel, sticks, twigs, and rocks. Students make initial observations and begin to practice recordkeeping by noting precisely what they have put into their terraria.

In Lessons 3 and 4 students set up their aquaria, stocking them with algae, duckweed, elodea, snails, and mosquito fish. While continuing to observe and record information on their terrarium plants from Lesson 2, students begin to record their observations about this newly formed model ecosystem: the aquarium. Students then read about their aquatic organisms and begin to grapple with the concept of interdependence.

In Lesson 5, the class continues to observe and discuss the completed aquaria and growing terrarium plants, and to make predictions about what changes might occur. Having observed plant growth firsthand, students now can learn more about germination by reading about how plants grow from seeds. In Lesson 6, the students add live animals to the terrarium to form yet another model ecosystem, one complete with crickets, isopods, and plants. After physically connecting the two bottled ecosystems into an upright "ecocolumn" in Lesson 7, students are ready to speculate about how one ecosystem might influence the other.

By Lesson 8, most ecosystems are showing obvious signs of change; for example, the crickets may have eaten some of the plants or seeds in the terraria, and newborn mosquito fish or snail eggs may have appeared in the aquaria. When such changes disrupt the ecosystem (for example, with more mosquito fish, the food supply dwindles), the stage is set for a class discussion on "stable" and "disturbed" ecosystems. Students discover that disturbing forces are either natural or human-made. They begin to define an environmental pollutant as "anything that can harm living organisms when too much of it is released into the environment." Student groups prepare for upcoming pollution presentations and experiments by reading about three common pollutants or pollutant by-products: acid rain, road salt, and fertilizer.

In Lesson 9, each of the student groups assigned in Lesson 8 reports on one of the three pollutants, and students begin to see how each pollutant can become a disturbing force in some ecosystems. Lesson 10 offers the class experience in systematically planning an experiment and introduces students to the necessity of experimental controls.

Then, in Lessons 11 through 13, student teams use ecocolumns that do not contain animals to conduct several experiments simulating the effects of the three pollutants on the ecosystems. After observing and recording these effects, students analyze their experimental data and draw some initial conclusions about how such pollutants might affect the ecocolumns that contain animals.

In Lesson 14, students report on their team experiments and pool data to arrive at a group consensus. The class begins to perceive that experimental results sometimes vary and are not always clear-cut, and that differing results can provide useful food for thought. As a final activity, students read about a real ecosystem in danger, the Chesapeake Bay.

Switching their focus from the model ecosystems to the vast ecosystem of the Chesapeake Bay, students discover that many of the bay's pollution problems mirror those in the class experiments. Armed with firsthand knowledge gained through the experimental experience, they are ready to break into small groups to analyze the bay situation from several points of view: the waterman, dairy farmer, land developer, recreational boater, and resident of the bay watershed area.

In Lesson 15, students identify their group's contribution to the bay's problems, propose possible solutions, and begin to grapple with the trade-offs involved. These trade-offs become even more evident in Lesson 16, when each group applies role-playing techniques to make a presentation outlining its particular point of view.

Faced with a number of different perspectives, students come to understand that environmental problems are complex. These problems involve many different interest groups and often require the groups to make trade-offs or compromises in order to reach workable solutions. At the same time, students are left with a powerful realization: There are a number of steps every one of us can take to help address the environmental problems in the world around us.

This is an exciting unit for students. They get to work with living creatures, become investigative experimenters, and draw parallels between the dramatic events occurring inside their model ecocolumns and those that occur every day, all over the world. Don't be surprised if some of the questions students ask go beyond what you know or can find out. Because ecological relationships are complex, research findings continue to change, and people have widely differing opinions about environmental issues. No one can provide all the answers. What you as a teacher can do is help students learn how to continue to find out for themselves.