Section 1

Activity: Designing and Using a Dichotomous Key

Author: Jon Detka 

Overview: Students first construct a simple dichotomous key and then use a basic key to identify some of the native plants and the most unwanted invasive weeds of California.

Grade Level: 3-5
 

Focus: Life Science, Investigation, Experimentation 

Objectives:
 

• Students will learn the basic skills used in constructing a dichotomous key.
• Students will be able to describe and draw the characteristics of some common native plants of Monterey and the most unwanted invasive weeds.
• Students will be able to define basic plant terminology.

 

Time Format: 2 class meetings
 

• Session 1: Activity part 1 "Designing a Dichotomous Key"
• Session 2: Activity part 2 "Using Dichotomous Keys"

 

• Session 1
• Thumbtacks, paper clips, wooden ruler, pencil, plastic pen
•  journals

 

• Session 2
• "Pacific Coast Tree Finder" (from toolbox)
• "RON Dichotomous Key"
• "RON Plant Facts"

 

1. Find a suitable "weedy" location close to your classroom or meeting place containing some of the native plants and noxious weeds mentioned in the "RON Dichotomous Key". If a nearby location with these plants is not found, you may need to adapt the key to plants that are in your immediate area. An example would be plants and weeds of your neighborhood/schoolyard. If the activity is limited to indoors use the images from the Native or Non-Native Plant Scavenger Hunt cards or collect plants ahead of time. Remember if you are collecting invasive species: Be Careful! Don't Spread the Seeds!

2. Leaders and educators should become familiar with some local native plants and invasive weeds. Use the "RON Dichotomous Key" and the "Pacific Coast Tree Finder" to become familiar with their content.

Activity (Part 1 "Designing a Dichotomous Key"):

1. Distribute to each group of 3-4 students: Thumbtacks, paper clips, wooden ruler, pencil, and plastic pen 

2. Ask students to look at the objects and, as a group, have them brainstorm how the objects are similar to and different from each other. Then, bring the class together to list on the board the different ways the objects are alike and different. Give the students about 2-3 minutes to do this. 

3. Grouping possibilities could be: metal, plastic, and wood 

4. Once this is done tell the students that science uses these similarities and differences to classify the natural world around them. The tool that they use for this is the dichotomous key. From a dichotomous key, one can learn the name of any object/plant/animal, which has been included in the key. In using a key, the student is led through a series of alternatives until the name of the object/plant/animal is reached. A dichotomous key, in its simplest form, has a few
            basic rules that make writing one easy. 
              

· Rule #1: Each step involves making choices between two characteristics. These characteristics are grouped 1a and 1b, 2a and 2b, and so forth. 

· Rule #2: Each step in a group distinguishes one or more objects (plants, animals, etc.) into two smaller units. 

· Rule #3: Each unit either identifies and names an object (plant, animal, etc.) or gives directions as to where to go next in the key. 
· Rule #4: At each step, students must choose one of the groups, steps, and units at a time and not more than one of each. 
              
              
5. Next you will want to illustrate the process by making a basic key to illustrate the rules. It will not be necessary to have real objects in front of the students because the objects will be well known

. 
Write on the board the following four objects: 
· tennis shoes
· belt
· celery 
· french fries

Ask the students to list the characteristics of these items and place them into two groups. 

Group one should include: "tennis shoes" and "belt" as "Clothing Items" 

Group two should include: "celery" and "french fries" as "Food" 
              

            Example Key: 
            Objects: tennis shoe, belt, celery, french fries 

            1a. Clothing: Go to 2 
            1b. Not clothing: Go to 3 

            2a. Fits on your feet: Tennis Shoes 
            2b. Fits around your waist: Belt 

            3a. Crunchy green vegetable: Celery 
            3b. Crunchy fried snack: French fries 
              
              

6. Now go back to the original objects laid out for the students (Thumbtacks, paper clips, wooden ruler, pencil, and plastic pen) and create a dichotomous key for them as well. Make sure that students understand this format before moving on. 

            1a. Objects made of metal: go to step 2 
            1b. Objects not made of metal: go step 3 

            2a. Flat, without pointed ends, curved on two sides: Paper clip 
            2b. Round base, pointed, sharp end: Thumbtack 

            3a. Objects made of wood: Go to step 4 
            3b. Objects not made of wood: Objects is made of plastic, round, 
writing utensil: Pen 

            4a. Flat measuring device: Ruler 
            4b. Round, writing utensil: Pencil 

              
Wrap-up 
              

Tell students that scientists use dichotomous keys to help them identify a variety of species. 
Scientists who design dichotomous keys are called Taxonomists.  When a taxonomist designs a dichotomous key it will only identify objects that have been previously included in the key. 

Ask students: How might this be a problem?
Maybe there is a new species that is not in the key.
Ask students: Can a dichotomous key be too large? Why? 
If the key is too big it may take a long time to find what you need.