Section 1

Activity: Population Explosion Relay

Author: Jon Detka 

Overview: Students are exposed to the concepts of seed dispersal, germination rate, exponential population growth, and predation by engaging in a bean 'weeds vs. natives' relay race.

Grade Level: 1-4

Focus: Life Science, Ecology, Investigation

Objectives

• Students will be able to illustrate how weeds out-compete other plants for space by crowding them out.
• Students will be able to define the 4 'weedy' characteristics responsible for the population explosion of weeds.
• Students will be able to illustrate the role of herbivores in native plant population.

Time Format: 1 class session

Materials
 

• Dried Beans two very different types (one pound bags)
• 3 Large bowls or containers
• Masking tape
• Marker
• Large sketchpad or copies of the blank " Weeds vs. Natives Score sheet"
• Hula hoop

• Make copies of the blank " Weeds vs. Natives Score sheet"
• Label and arrange the bowls filled with beans (Click on image for bigger version.)

Activity:

1. Organize all the students in a circle and explain that a 'weedy relay race' will take place. The object of the race is to see how weeds crowd out neighboring native plants and why this is a problem.
Ask Students: What are some of the ways that weeds can crowd out native plants?
Teacher Help:  Remember from the "Native or Non-Native? - Scavenger Hunt" lesson that invasive plant experts have found that invasive plants (weeds) frequently:
 
•      Produce many seeds
•      Monopolize nutrients or water
•      Germinate quicker and grow faster
•      Avoid predation from herbivores


Ask the students to imagine that they are weeds of the type that lets the wind carry their seed across a long distance. Ask students: Can anyone think of a weed that uses the wind to transport its seeds?
Teacher Help:

• If students can't think of a weed remind them about: Pampas grass or dandelions.
• Ask students to imagine that they are native plants now. 
• Ask students: Besides the wind what are other means of seed transport that weeds or native plants might use?


Teacher Help: Water or Animals (fur or through digestive tract)
 

2. Split the class into two groups. One will be "The Natives" and one will be "The Weeds". We have a "Plant Area" (This will be base) that both weeds and natives are able to grow. (This is the hula hoop). Now explain that all of the plants (natives & weeds) will produce 2 seeds each year. Explain to students that they will run from "The Plant Area" (Base) to the containers (seed bank) at the one end of the relay. This is where the native and weed seeds are located. Tell students that the seeds must travel back to  "The Plant Area" and be dropped there to germinate and grow into plants. Tell students: The native plants are a main part of many of the animals diet and they eat some of these seeds each year.  After each year we will write down the number of Native & Weed seeds in the "Plant Area" on our "Natives vs. Weeds Score sheet".

3. Tell the students that because native plants have been here longer there are more of them in the beginning. These native plants have remained in balance with the other native animals and plants for thousands of years.  Tell students there will be 10 native plants and 1 weed in the "Plant Area". Place 10 of the native seeds (beans) and one weed seed (different type of bean) in the "Plant Area".  Tell students that  both the weeds and native plants only live for one year so we must count each years total to keep track of the population growth.   Remember weeds and natives produce the same number of seeds per year.
(2 seeds per plant per run )

That means the first years seed production will look like:

• Weeds = 2 weed seeds     (2 seeds X 1 plant)
• Natives = 20 native seeds  (2 seeds X 10 plants)
• Note: After determining the number for next year have students write down the totals on their "Natives vs. Weeds Score sheet" and remove the 1 weed and 10 native plants.


Have one student from the weed group run down to the "Weed Seeds" (bowl) at the end of the yard to get two weed seeds, run back, and place them in the "Plant Area".  Tell two students from the native group  to run down to the "Native Seed" (bowl) at the end of the yard and get 10 seeds each (20 total). Tell the native group that on the way back they must deposit 10 seeds (1/2 of the total each year) into the "Animal Food" bowl because the animals need to eat native plants.  Explain that many animals in an area cannot eat weeds.  The rest  of the native seeds can go into the plant area.

Now have students add up the total amount of each type of seed in the plant area and write it down on their "Natives vs. Weeds Score sheet".
Ask Students: to determine the next years number of weed and native plant seeds based on the population in the hula hoop.

Teacher Help (answer to year 2):

• 2 Weeds X 2 Seeds each = 4 seeds
• 10 Native plants X 2 Seeds each - 1/2 (Eaten by Animals) = 10 seeds


Have the students write down the second year plants and seeds that will be produced.  Remove the previous years plants (beans in hula hoop) and have the students gather the necessary amount of seeds.  Remember the natives must deposit half of their seeds into the "Animal Food" bowl on their way back.  Now the "Plant Area" should have 4 weeds and 10 native plants in it.  Have students compute the next years number of plants.

Teacher Help (answer to year 3)

• 4 Weeds X 2 Seeds each = 8 seeds
• 10 Native plants X 2 Seeds each - 1/2 (Eaten by Animals)= 10 seeds


Have the students log the amount of seeds and plants produced and continue the cycle.  Remember to remove the last years weeds (beans) from the hula hoop each time and have the natives deposit 1/2 of their seeds into the "Animal Food" bowl.
Click here to see the completed "Natives vs. Weeds Score sheet"
 

Wrap-Up

 Have students describe what happened in the "Plant Area" by answering the following questions:

1.   What 'weedy' characteristic was responsible for this population explosion of weeds?
     What are the other characteristics of a 'weedy' plant?

2.   What keeps the native plants from exploding?
3.   How do you think these weeds will affect the native plants and animals in the years that follow?

 Going Further

• Sometimes the weeds will grow so fast and so dense that they use up all of the water and nutrients and they will also die leaving nothing but bare ground.
• Discuss the balance that is kept between native plants and the animals that eat them.