Section 1

Activity: 

Graphing the Population Explosion of Weeds

Overview: This session will illustrate the exponential explosion of invasive plant populations.  Students will explore the general path of the seed cycle and the potential barriers in that cycle that prevent populations from exploding. Students will reinforce these concepts by calculating the growth of a hypothetical but realistic Yellowstar Thistle population.

Grade Level: 4-6
 

Focus: Life Science, Investigation, Experimentation, Resources, Mathematics, Ecology
 

Objectives:

• Students will be able to outline the path of the seed cycle.
• Students will list the natural and human barriers to the population explosion of a species.
• Students will calculate the explosive rate of reproduction of Yellowstar Thistle over a three year period. (Note: Less experienced students could be given the data and asked to graph the corresponding points.)
• Students will graph the growth occurring within the seed bank and plant population of the Yellowstar Thistle plant over a three year period.

Time Format: 1 session

Materials:
The Yellowstar Thistle Seed Cycle Worksheet package. 
Yellowstar Thistle - Plant Invasion &  Yellowstar Thistle Seed Invasion Graphs

Note to MAC users: Accessing these materials via the link above may not load properly. View these files by opening the file called pdf on the CD and open the document toc.pdf.
This will allow you to access the printable version of these materials. You must have Acrobat Reader to access the print version of these files.

Advanced Preparation:

1. Make copies of::

The Yellowstar Thistle Seed Cycle Worksheet package  (Note: Make enough copies to hand this cycle out. It can be used as a guide for extracting the problem solving information from the Yellowstar thistle information sheet.)

Yellowstar Thistle graphs:  Yellowstar Thistle - Plant Invasion & Yellowstar Thistle Seed Invasion ( Graph Activity Sheets)

Activity

1. Begin by listing and discussing with students factors which prevent populations of plants and animals from growing without limit.

• List:  Natural factors (examples:  disease, predators, fire, insects, etc.)
• List:  Human barriers (examples:  herbicides, collecting, habitat destruction, etc.).
• Ask Students: Which of these controls is absent in the case of weeds? (examples: Nothing eats them, spread by humans) Note: This varies greatly depending on species and conditions. It may be necessary to investigate the conditions in your local ecosystem for examples to introduce to students.

2.  Discuss animal populations that have not been kept in control, and reasons for this. (Teacher Help: Many populations of ground squirrels are exploding because natural predators, such as bobcats and hawks, have been eliminated and the squirrels have adapted to human presence.)
 

3.  Explore the seed cycle of weeds with students by drawing in the direction and position of flow arrows through the cycle.   Have students determine the direction and position of the flow arrows through the cycle.
Ask students: Is anything decreasing the growth rate of the weeds in this example? 
Teacher Help:  No. 
What types of factors could decrease the spread of these weeds?

(Teacher Help:  Point out that each step of  the Seed Cycle can have a control.
Example:

• Dispersal = Washing vehicles and equipment, Staying on trails (Disturbance increases Dispersal)
• Germination = Disease, Low Water, Low Nutrients
• Seedlings = Low Sunlight, Herbivores, Pulling, Herbicides
• Plants and New Seeds = Seed Eaters (Insects), Pulling

4. Distribute the Yellowstar Thistle information sheet , Yellowstar thistle blank graphs, and the Yellowstar Thistle Seed Cycle image. Explain that we are going to calculate the growth rate of an invasive weed called Yellowstar Thistle. Have students read the Yellowstar Thistle information sheet or read through it with them. While reading through the Yellowstar Thistle information sheet have students fill in the missing information from the Yellowstar Thistle Seed Cycle image. After students have read the Yellowstar Thistle information sheet discuss the following questions with students.

• How long does a Yellowstar thistle plant live? 1 year
• What makes Yellowstar thistle a weed? Produces large amounts of seed, No predators, invades space, and dominates nutrients/water, harmful to horses, and prevents grazing by all other animals.
• How many seeds does one plant produce? 1,000
• How long are the seeds viable? 10 years
• How many of these seeds germinate? 4 % or 4 out of every 100.
• What happens to the other 96% of the seeds?  They survive in the seed bank for up to 10 years. Note: The seedbank consists of the non-germinated seeds that lay dormant in the soil waiting for suitable conditions for germination.
• How long does a seed take to germinate? 1 year
• How many seedlings survive to become mature plants? 25%

  Note: The answers to some of these questions could be organized into the following table to help students organize the parameters needed for the calculation problem:
 

Yellowstar Thistle Reproduction Information

5.  Using the information from the Yellowstar thistle information sheet, have students calculate the rate of reproduction (plants/seeds produced) of  Yellowstar thistle over 3 years given the problem below.  Note:  This could be assigned as a homework problem.
 

One hundred Yellowstar thistle seeds are introduced in an area inadvertently ( livestock feed, roadways, animals, recreation, etc.) . How many Yellowstar thistle plants and seeds will there be after 3 years?
 

Plant Facts about Yellowstar Thistle (from the Yellowstar thistle information sheet)

• A Yellowstar thistle plant produces 1,000 seeds per plant.
• Four percent of Yellowstar thistle seeds in the seedbank germinate (sprout) each year, leaving 96 percent for next year's seedbank.
• twenty-five percent of the seedlings that sprout survive to become mature plants.
• Yellowstar thistle seeds remain viable in the soil for up to 10 years.
• It takes 1 year for Yellowstar thistle to germinate and produce seed.
• Yellowstar thistle plants live for 1 year.

Clues for students:  Keep your eye on the Seedbank!!
 

 
 

Answer:

 The first part of this section explains the steps used to calculate the problem for the first year and second year. The second part is a table showing the calculated results.

Year 1
From 100 seeds, 4 (four percent of 100 seeds) will germinate the first year.

Ninety-six will remain in the seed bank ( 100- 4 to germination = 96 seeds).

Twenty-five percent of the sprouts (one of the four germinated seeds) will survive.

That one surviving Yellowstar Thistle plant will produce 1,000 seeds the first year. At the end of the first year the seed bank = 1096 seeds

Year 2

In the second year, 44 (4% of 1096) seeds from the seed bank will sprout and 11(25% of 44) of those plants will survive.

Remember that the previous year plant dies so only the 11 plants remain. These 11 plants produce 11,000 new seeds plus the 96 percent of the second year's seeds (1096 - 44 = 1052 ) that did not germinate for a total of 12,052 seeds in the seedbank.

Continue for the third year, taking 4 percent of the previous year's seedbank (4% of 12,052 = 482) and take 25 percent of those seedlings ( 25% of 482 = 120 ) for the new number of plants (120) in the third year.

Multiply 120 plants times 1,000 for seeds produced in the third year and add to 96 percent of the previous year's seedbank for the new seedbank.

Note: Remember the plants only live for 1 year so don't add them to the next year.

The results can be organized into a table like the one below.

Yellowstar Thistle Growth After 3 Years

Year	Yellowstar Plants	Seeds Produced	Seed Bank
0	0	0	100 Introduced
1	1	1,000	1,096
2	11	11,000	12,052
3	120	120,122	131,653

Point out to students the final (year 3) Ask students: What could be done to stop this from happening? (Teacher Help: Don't allow the 100 seeds to be released. Pull the first Yellowstar seedling. Keep searching for new seedlings until the seed bank is gone.) Explain to students that it may be impossible to stop all the spreading of seeds everywhere. This is why it is important for everyone to do their part to look for these invaders and help protect the native plants and animals by removing the weeds. Ask students: What do you think will happen to the native plants if this weed population explosion continues?
 

6. Have students graph the results of the Yellowstar thistle invasion using the Plant and Seed Yellowstar Thistle - Blank Graphs.

Note: If you feel this problem is too difficult.   Give students the answers and have them fill in the Yellowstar Thistle graphs, showing the rate of growth over time.

Note: Students could design or decorate their own graphs.  These graphs could be placed in their Weed Journals or be turned into visuals for use  in a public awareness campaign.

Wrap-Up/Evaluation

Have students answer the following :

1. What natural and human barriers prevent populations of plants and animals from growing without limit?
Natural barriers ( disease, predators, fire, insects)
Human barriers ( herbicides, collecting, habitat destruction).
 

2. Draw and label the weed seed cycle.

3. What makes Yellowstar thistle a weed?
Produces large amounts of seed, no predators, invades space, dominates nutrients/water, harmful (toxic) to horses, and prevents grazing by farm animals.