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Using a Dichotomous Key∗†
Judy Olacke
Grade level: Grade 11.
Provincial curriculum links: Ontario.
Subject: Science; Biology.
Keywords: Dichotomous key, tree, coniferous, leaves, human interaction, leaf.
Description
Using a dichotomous key and observation skills, students will classify various specimens of
trees.
Curriculum Framework
Specific Lesson Goals
ˆ Classify specimens by applying the principles of taxonomy
ˆ Use a dichotomous key as part of the data collection to describe and evaluate an
ecosystem
Preparation
Preparation time: 1 hour
Preparation time includes:
ˆ Copy, laminate and assemble the tree key “wheels”
ˆ Copy and laminate cards with pictures of leaf characteristics
ˆ Put out numbered stakes beside the trees to be identified. There should be at
least 5 different types of trees.
∗
Source of Lesson Plan: Written by Judy Olacke and based on lessons originally found in: Ministry of
Natural Resources. Focus On Forests. Toronto: Ontario Ministry of Natural Resources. 1989.
†
Submitted by: Judy Olacke <[email protected]>
Using a Dichotomous Key
2
Length of lesson: 2 hours. One hour for the introductory lesson on the role of taxonomy
as well as constructing and using simple dichotomous keys. One hour for learning the
terms used for leaf characteristics.
Resources required:
ˆ Cards with pictures showing the terms used on keys to describe leaf characteristics (pp 27 & 28, Focus on Forests - see Resource Required below)
– Leaf Types (compound, simple, scale-like, needle-like, in bundles or singly
on twigs)
– Arrangement on Twig (opposite, whorled, alternate)
– Leaf Margins (smooth, toothed, wavy)
– Leaf Shapes (linear, oval, reversed oval, chordate, connate, heart-shaped)
ˆ Copies of a few varieties of dichotomous keys (insects, whales, flowers) (see Gr.
11 Biology texts - any publisher)
ˆ One coniferous tree “wheel” key per student (pp 32, 33 Focus on Forests)
ˆ Numbered stakes for indicating the trees to be identified
ˆ Ruler (cm)
ˆ Pencil and record sheet
Procedure
Part I (1 hour)
1. In class, introduce the students to the importance and purpose of classifying things
(telephone book, CD collection, etc). Relate classifying things to the role of a taxonomist and the “tools” they use.
2. Construct a dichotomous key using 8 of the students’ shoes.
3. Give the students an opportunity to use a variety of dichotomous keys for different
species.
Part II (1 hour)
1. As a whole group activity, have the students observe and describe leaf characteristics
using common scientific terms. Distribute leaf cards that show examples of each
characteristic. (See resources)
2. Write the leaf characteristic terms on the chalkboard. Have students stick their leaf
card under the appropriate term and discuss.
3. Students can then write the leaf characteristics in notebooks and sketch an example
of each.
4. Outside, distribute tree wheels. Walk through how to identify a tree with all students.
5. Individually, students will then use the tree key and identify each of the designated
trees. Students will record their results in their science notebooks.
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Using a Dichotomous Key
3
Discussion and Questions
ˆ Discuss identifications. What characteristics were difficult to decide between?
ˆ What were some easy characteristics to decide between? Are there other characteristics that could have been used in the key?
ˆ Would all groups of species in an ecosystem be as easy to identify (e.g. insects)?
ˆ What job or careers would involve the use of dichotomous keys? For what purposes?
ˆ Why is it important to know what species exist in an ecosystem? What human
interactions can have an impact on the variety of species (diversity) and the population
of individual species in an ecosystem?
Student Evaluation
Rubric or rating scale for students to determine how well they completed the goals of the
lesson.
Enrichment and Extension Activities
Students could classify and identify other species in the naturalized area (e.g. Flowers,
shrubs, deciduous trees or insects). Students could also extend their dichotomous keys to
other areas of interest such as music, or video games.
Educator Notes
This lesson plan should be used after teaching the binomial naming system and the Classification Levels and species. This will develop a greater understanding of the need for
taxonomy and its related career opportunities. Other keys may be used if there is not a
wide variety of coniferous trees in the outdoor area (e.g. shrubs, flowers).
References
ˆ Focus on Forests: A Resource Manual for Intermediate and Senior Teachers on Forest
and Forest Management. Minister of Supply and Services, Canada. 1989. Ministry of
Natural Resources. http://www.oforest.on.ca/programs/Focus/indexM.html
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