Species Identification Using Dichotomous Keys
To help your students unlock the wonderful world of living things, teach them to use a dichotomous key.
By Lynsey Peterson
Without the logic and simplicity of dichotomous keys, many good biology students would be lost. There are so many species out there that memorizing them all is impossible. Unlike many facts that you can simply ‘Google’, identifying an unknown organism can be difficult without the right key. Field guides are great for identifying common species, but they can become a crutch to students. Students can flip through pictures for hours and not correctly identify an organism. Dichotomous keys give students a series of questions about their unknown object or organism. There are only two answers to each question; hence, the name dichotomous. Students simply choose the correct answer for each question and they can identify the organism. The key also simultaneously reinforces vocabulary related to the organisms. Of course, students must know how to correctly use the key.
I introduce dichotomous keys before applying their use in class. First, I give students a very simple key with pictures that illustrate the terms used. We work through a few examples together. I emphasize that no matter what you are identifying you always start at the beginning of the key. Once students can easily use the simple key, I give them a more complex one. At this point I may also give the students a few organisms to identify. There are many simple, easy to use keys available online to help students. I have found and used keys for leaves, flowers, stream invertebrates, and fungi. There are even good keys available for identifying rocks and minerals. For homework, students create a dichotomous key of their own. This can be challenging, so I work with the class on an example. You simply choose any group of similar objects, such as shoes, jewelry, foods, or dishes. Then, you divide the group in two based on a characteristic. This characteristic forms the basis for the first question on the key. You continue the process, writing the key as you go, until all of the objects have been singled out on the key.
Once my students have mastered a dichotomous key, I no longer worry about losing them when they use a field guide. They still always want me to identify things for them, but what fun would that be? You can also help your students master dichotomous keys with the following lessons.
Species Identification Lessons and Activities:
Students use a dichotomous key to classify a variety of trees. They identify leaf types and how trees are classified, create identification notebooks, and record the number of trees they have identified.
Students analyze how a dichotomous key can help explain and identify unknown organisms. They create a dichotomous key, developing a list of three observable characteristics per organism.
Students discuss the benefits of using a dichotomous key to identify and classify organisms and objects. They design a dichotomous key for identifying sea shells using oyster shells or images.
Students use a dichotomous key and levels of taxonomy in this lesson. They examine samples and discuss how to classify organisms, and memorize a mnemonic device for the taxonomic levels.
Students construct a dichotomous key to classify a group of shoes. They divide the shoes into two piles according to the characteristics of their choice. They continue doing this for both piles and construct a key on the board.