Interdependence

DISCOVERING WHAT MAKES A BALANCED ECOSYSTEM

Grades
4-9

Group size
Individual students

Time required
1 class period initially, then daily observations for about 3 weeks

Materials

Instructional Goal

National standards (9-12) addressed

National standards (5-8) addressed

Student Objectives
Student will:

Prerequisite Knowledge
Students should understand the key terms as well as the concepts about ecosystems.

Advance Preparation Time
About 1 hour

Teacher tips

Background Information

This lesson explores interdependence within the aquatic biome. In this activity, students learn that plants and animals in aquarium are interdependent, and need each other for their continued survival.

Plants are dependent on animals to provide nutrients (through waste products and decomposition) and, to some extent, the carbon dioxide they need for photosynthesis. Animals need plants to supply some or all of their respiration and metabolism. In tact, most of the oxygen used by the world's animals is actually produced by plants that live in the oceans. The three aquariums in this lesson demonstrate interdependence on a small scale, but students can easily extend this concept beyond the aquariums to include the earth's biosphere as a whole.

Lesson Procedure

Step 1
Fill three quart-size canning jars with aged water to within an inch of their tops. Place two or three guppies, or one water snail, in the first jar. Place one 4- to 5-inch long sprig of Elodea in the second jar. Place two or three guppies, or one snail, and the other sprig of Elodea in the third jar. Put an aquarium thermometer in each jar, then screw on the lids to make the jars airtight (see illustration below). Put these mini-aquariums in a place that is out of direct sunlight, and where your students will be able to observe them easily. Maintain the water temperature at approximately 70 degrees Fahrenheit. Try to keep the water temperature in each jar the same so students can determine the effect of plants and animals on each other without introducing another variable.

Step 2
Distribute the student handouts. Have students predict what will happen in each aquarium after several weeks, and they should record their hypotheses in their journals. Students should then observe the aquarium every day and record their observations. In addition to recording the measurable data, such as the daily water temperature, animal population, and plant growth, students should look for subtle changes in the aquariums. Are the fish active? Are they gasping for breath? Are the plants green and healthy? Is the water clear or cloudy? Does it have an odor?

The aquarium with the plants and animals should be reasonably balanced resulting in healthy fish even after several weeks, while the fish that are alone in the first aquarium will quickly run out of dissolved oxygen. (you can remove these fish as soon as they show signs of distress) The plant that is alone in the second aquarium may or may not show much evidence of change, depending on the amount of light it receives and how quickly it uses up its carbon dioxide and available nutrients.

Step 3
After several weeks, have students share their observations in a class discussion. Ask some or all of the following questions during the discussion.

Enrichment Activities
Students can:

Illustration of the three aquariums for the interdependence experiment; Illustration is by Leyla Sezen.

Author: Tugrul Sezen