Overview of Dichotomous Keys

Dichotomous keys are used to identify a variety of things including insects, plants, animals and rocks. A key gives pairs of “either-or” choices that direct the user to the next pair of choices (also known as a couplet) or to the point of identification. Dichotomous keys are also used for identifying and classifying new species. Sometimes dichotomous keys use technical terminology which can make it difficult for users who lack experience in the field.

A dichotomous key begins with broad-ranged questions or statements which get more specific. For example, the first question in a dichotomous key about birds might ask if the bird can fly or if it is flightless. Questions further down in the key get more specific and may ask about the size of the bird, the shape of the beak, the color of the feathers, etc.

Dichotomous Key Usage Example – Shade Trees in Tampa Bay

Below are the first 6 of 9 couplets in a dichotomous key to identify 1 of 10 different shade trees found in Tampa Bay, Florida. With stems and leaves from the tree, the user starts at the couplet 1a and 1b and determines if the leaves and buds are opposite or alternate. If they are opposite, the user goes to couplet 2, if they are alternate, they go to couplet 3. The user then works their way through the couplets until they identify the tree species.

Notice that based on couplet 1a and 1b, if the leaf and bud orientation is opposite (Go to 2), the tree is either a Florida maple or a red maple depending on whether the leaf margins are entire or serrated. If the leaves and buds are alternate (Go to 3), the user must go into more detail using the key to identify the tree. In this case, the subsequent couplets go into more detail about the morphology of the leaves.

1a. Leaves and buds are opposite.Go to 2
1b. Leaves and buds are alternate.Go to 3
2a. Leaves are simple, lobed, and have entire margins.Florida Maple, Acer floridanum
2b. Leaves are simple, lobed, and have serrated margins. Leaf petiole and new twig growth are light red in color.Red Maple, Acer rubrum
3a. Leaves are simple.Go to 4
3b. Leaves are compound.Go to 9
4a. Leaves are entire.Go to 5
4b. Leaves are serrated or lobed.Go to 7
5a. Leaf undersides appear silvery or white and are highly aromatic when crushed.Camphor Tree, Cinnamomum camphora
5b. Leaf undersides are not silvery or white.Go to 6
6a. Leaves are leathery and revolute.Live Oak, Quercus virginiana
6b. Leaves are not leathery and are relatively flat.Laurel Oak, Quercus laurifolia

References

  • Martinez, C. (Updated April 25, 2017). How do you read a dichotomous key? Retrieved from https://sciencing.com/do-read-dichotomous-key-8546380.html
  • Koeser, A., Hasing, G., Andreu, M and M.H. Friedman. (n.d.). How to use a dichotomous key: A tutorial featuring 10 common shade trees of the Tampa Bay area. Retrieved October 10, 2017, from http://edis.ifas.ufl.edu/ep510