Interdependence

DISCOVERING WHAT MAKES A BALANCED ECOSYSTEM

Group size
Individual students

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

Materials

• 3 one-quart mason or canning jars with lids
• aged water to fill the jars
• 2 sprigs of Elodea or Anacharis about 4 to 5 inches long
• several aquarium animals guppies and/or water snails
• 3 aquarium thermometers
• copy of Handout 1, "Interdependence in the Aquarium," for each student
• each student should have a journal for recording observations

• To demonstrate that plants and animals are interdependent in a balanced ecosystem

National standards (5-8) addressed

Student Objectives
Student will:

• Predict what will happen in three experimental aquariums, only one of which is balanced.
• Record evidence of interdependence in the aquariums and record their observations in their journals.
• Define interdependence in their own words.
• Hypothesize about the effects of heat and light on the aquariums.

Advance Preparation Time
About 1 hour

• Review the "Background Information" and "Lesson Procedure"
• Gather materials. Obtain the aquatic plants and animals.
• Duplicate the student handouts.
• One or two days before the activity, prepare about a gallon of aged water. Allow it to stand for at least 24 hours before placing animals in it.

• The animal(s) isolated in the airtight "animals-only" aquarium (see Step 1 in the Lesson Procedure) will probably not survive very long. I don't think it is necessary to wait until the animal dies for students to learn the concept of interdependence. The animal could be taken out as soon as students see signs of distress and can predict its demise if left in the jar.
• The experiment will take several weeks to complete, and you should provide time each day for students to record their observations in their journals.
• Students need to be reminded about what constitutes good observation. Otherwise, some students may give only a cursory glance to each jar and record that nothing has happened.

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.

• Which aquarium was a model of a balanced eco system? What characteristics did it have that were lacking in the other two aquariums?
• Why were the other two aquariums not balanced?
• What do you think would happen if we placed all three aquariums in direct sunlight for one day? For one week?
• What would happen if we placed all three aquariums in a dark cabinet for one day? For one week?
• Where does the snail (or fish) get its food?
• Where does the oxygen come from that the animal needs to survive?
• What does the plant need to make its food through photosynthesis?
• What was the average temperature in each aquarium?
• How does a balanced aquarium demonstrate interdependence?

• Perform the same experiment with different types of aquatic animals, such as goldfish or tadpoles.
• Introduce other variables into the interdependence experiment, such as water temperature that are significantly higher or lower than the ideal of 70 degrees Fahrenheit, or placing the aquariums in sunlight or in a dark environment.
• Research other types of ecosystems and discover what interdependent components they need to stay in balance. (See Ecosystems Lesson Plan by Al Globus)
• Establish a plant terrarium and see if they can keep it in balance.

Curator: Al Globus If you find any errors on this page contact Al Globus.

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