Loading StandardsIn this two-part lesson, students are introduced to the ecology of population growth. First, students grow an aquatic plant called duckweed over a period of two to three weeks. They observe what happens to a population of organisms when it is allowed to grow without predation or competition from other organisms. In the second part of the lesson, students view videos about invasive species that spread quickly and threaten other naturally occurring species. Then they apply what they learned from their duckweed experiment to develop a proposal for controlling the growth of an invasive species in their community.
For each group:
For each student:
Carolina Biological Supply Co.
2700 York Rd.
Burlington , NC 27215
Phone: 336-584-0381
Fax: 800-227-7112
Toll Free: 800-334-5551
Website: www.carolina.com
E-mail: carolina@carolina.com
Lemma Minor CE16-1820 100-150 plants $5.25
Salvinia CE-1860 50-100 plants $5.75
1. Ask students:
List student responses on the board.
2. Tell students that they are going to conduct an experiment over the next two to three weeks to explore this question. They will grow duckweed, an aquatic plant, in an environment without any predators or competition from other species. Show several Duckweed in its natural habitat to help students recognize this common aquatic plant.
3. Contrast the growth of duckweed with the growth of plants that are more familiar to students, e.g., flowering plants. Discuss how most plants reproduce. (They grow flowers, pollinate, produce seeds, and then the seeds become new plants, usually over the course of a year.) Then examine a Duckweed. Point out the thallus (body of plant) and the roots.
Duckweed is an aquatic plant of the genus Lemna. It can be seen floating on top of the water in rivers, ponds, and lakes. A thick growth of duckweed looks like bright green "scum" on the surface of the water. Closer observation reveals that the scum is actually made up of tiny, individual plants. Duckweed plants can reproduce by seeds but seldom do. More commonly, they reproduce asexually by producing a new plant that grows off of the old one. When the new plant grows its own roots, it breaks off from the old plant. The growth of a population of duckweed can be determined by simply counting the number of thalli that appear over a period of time.
4. Divide the class into groups of three to five, depending on the amount of materials you have available. Give each group a plastic container filled with about 250 mL of artificial pond water (see Recipe for Pond Water (PDF)), a small paper cup containing two or three duckweed plants, and forceps. Have students look at the duckweed plants and sketch them in their notebooks. Tell them to label the parts of the duckweed. Then have students write their names on their container, transfer the plants to the container using the forceps, and place the container under a light source.
5.
Over the next ten class periods, have students count the number
of plants they observe in their container and record their
observations in their notebooks. Instruct students to make a
data table in which to record the date and the number of plants
in the container. Then have students plot their data on a graph,
with the x-axis representing time and the y-axis representing the
number of plants. Their graph should look similar to the one below.
Duck Graph
6. Ask each group to prepare a presentation that includes an enlarged drawing of the graph on newsprint and answers to the following questions:
7. Have students present their findings to the class.
8. Show students Biological Invaders. As they watch, have them jot down answers to the following questions (either on the board or on a handout):
9. In groups of three to five, have students discuss the questions, using their notes for reference.
10. Next, have each group of students compare the growth of the tree snake population in a new area to the growth of the duckweed population in their experiment. How are these two situations alike? How are they different?
11. Now show Leafy Spurge. Discuss the following questions after watching the video:
12. Have groups of students prepare short presentations based on the scenario below:
Duckweed has invaded your local rivers, ponds, and lakes! As a team of local ecologists, your job is to develop a proposal for controlling the population of duckweed. In your proposal, include the following information:
Have each group present their proposal to the class. Debate the pros and cons of each.
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