This activity takes place on the school football field (or other field) in order for students to focus on learning and understanding the field techniques. The activity starts with a guided discussion in which students develop the method they will use set up a study plot. The method is modified as purposeful questioning by the teacher reveals potential problems (abundance of plants, difficulty in counting all of them, significant overhang). Ultimately, a procedure is decided upon, students are shown how to collect coverage data, and each group conducts a study plot.

1. Decide upon plot size. Keep in mind that the plants we are studying (grasses) are very close together and plentiful. A plot size of 1 meter² is sufficient for this activity.

2. Discuss how many plots are needed to make the study worthwhile. Is sampling one plot enough for a valid conclusion to be drawn? Help students understand that the more plots sampled, the better substantiated the conclusion will be. Have students work in pairs; assign each pair a plot.

3. Identify the native vs. alien species. Ideally a football field should be covered with grass. Bermuda grass in the dominant native grass in southern Arizona; it will be considered to be the only native plant for our study. Any other plant will be considered to be an alien (invasive, non-native) species.

4. Pick up materials. Each group will need a piece of graph paper and a meter stick.

5. Gather students on the field. Discuss field techniques and procedures. Ask the students how we should determine the area of the plot that is covered by Bermuda grass. Sample questions are provided.

• How do we determine the area that is covered by grass?

How would we determine the area covered by each plant? (Measure the diameter of the area covered by the plant and calculate the area. Assume that the coverage is circular. For a circle, the area = r² . = 3. 14 , radius = 1/2 of the diameter)

• How easy would it be to measure the diameter of each grass plant in the plot? (Not very easy.)
• What would be an easier way to determine the coverage of the grass in your plot? (It is easier to measure the bare spots than the covered area. This should be plotted along with the area covered by all alien plants.)

Continue to ask students questions to help them develop the method they will use. Ask the students to consider the plants that overhang each other. Some questions you might ask include:

• Are there any plants which overhang one another?
• What should we do with them?
• What would happen if we ignore the overhang?
• What would happen if we counted the area covered by both plants?
• What type of information are we interested in?
• Which method would better answer our question? (Each plant must be considered , therefore we can have over 100% coverage in an area. This is important because if a native plant overhung an alien plant and only the native coverage was counted, the alien plant coverage in the plot would not be accurate. The data would show only the native plant coverage while an alien was present.)

6. Show students how take a diameter measurement of bare places and alien plants and how to make them from two sides of the plot.

• Find an alien plant in your plot. Measure the distance from the center of the alien plant to the two closest sides of the plot. Locate the plant on the grid and draw a small circle.
• Measure the diameter of the alien plant species by holding the meter stick (or a small ruler) above the plant, so that it crosses the plant stem. Measure the distance from one side of the plant to the other. On a separate piece of paper, record the name of the alien plant (if known) and its diameter. Draw a larger circle on your grid to represent the area covered by the alien plant.
• Repeat the same procedure for the bare spots.

7. Determine the coverage by the Bermuda grass and the alien plants. Assume that the coverage of each plant circular. Use the equations given below.

Alien coverage = sum of area covered by each alien plant

Native coverage = total area - alien coverage - area bare places

total area of a 1 meter plot = 100 cm x 100 cm = 1000 cm²

area of a circle = r²

area of a rectangle = length x width

area of a square = side x side

pi = 3.14

diameter = d = 1/2 x radius

1. Determine which shape best resembles the bare spot.
2. Measure the appropriate lengths in centimeters (diameter for a circle; length and with for a polygon).
3. Find the area of the bare spot (use the formulas given above).
4. Record your measurements and calculations.
5. Find the next bare spot in your plot and repeat steps 1 - 4.
6. Add all the areas (for the bare places) together.

1. Identify the non-native plant.
2. Measure the diameter of its primary stem or stems (the primary stem-s connect directly to the root system) in centimeters.
3. Assume the plant overhangs the stem in a perfect circle. Measure the distance between the furthest two points of the circle in centimeters.
4. Determine the area covered by the plants using the formula: area of a circle = r² . The radius, r, equals 1/2 the diameter.
5. Repeat steps 1-4 for all non-native plants.

Determine the coverage area of the native (Bermuda) grass.

1. You will need to know how much of your plot is bare and how much is covered with alien plant species. You have already calculated both these values. Find them, and write them down again: area of bare spots = _______ cm² area of non-native coverage = _______ cm²
2. Determine the area covered by grass by subtracting the area of bare spots and the area covered by alien species from 1000 cm².
3. Then, double the area covered by grass because the grass overhangs itself everywhere. Use the doubled value to calculate the percent coverage by native versus non-native plants.

Students should turn in a grid with the bare places and aliens marked on it. Points will be given for the accuracy of the map, a key at the bottom describing any markings on the map, diameter measurements to go with map markings, and calculations for determining the percentages.