Coaching Centre
Using and building keys
Dichotomous keys in branched and tabular (numbered) form, circular keys, choosing good questions, running a key to identify an organism, and building your own key step by step.
A bushwalker photographs a beetle she has never seen. She doesn't flick through thousands of pictures. She answers a handful of yes/no questions: six legs? hard wing covers? stripes? Each answer halves what's left, and after five or six questions she has a name. That's a key.
What a key is
An identification key is a tool for naming an organism that has already been classified. You look at the organism and answer a series of questions about features you can see. Each answer leads you to the next question, until you reach a name.
The most common kind is the dichotomous key (say "dye-KOT-o-mus"). "Di" means two: every step splits into exactly two choices. Two choices are easier to decide between than five, and every organism ends up on its own branch.
Branched keys
A branched key looks like a tree or a flow chart. Start at the top and follow the branch that matches your organism.
Tabular (numbered) keys
A tabular key holds the same information as numbered pairs of statements. Read pair 1, choose the statement that fits, and it either names the organism or sends you to another pair. Field guides use this form because it fits on a page, even for hundreds of species.
| Step | Statement | Go to |
|---|---|---|
| 1a | Has feathers | Birds |
| 1b | Has no feathers | go to 2 |
| 2a | Has hair or fur | Mammals |
| 2b | Has no hair or fur | go to 3 |
| 3a | Has fins and breathes with gills all its life | Fish |
| 3b | Has no fins as an adult | go to 4 |
| 4a | Has dry skin covered in scales | Reptiles |
| 4b | Has moist skin with no scales | Amphibians |
Circular keys
A circular key puts the start in the centre of a circle and the questions in rings. You move outwards, choosing the segment that matches, until you reach a name on the outer edge. They look great and fit a lot on one page, but they're harder to extend when a new organism turns up.
Running a key
- A beetle from the collection appears. Use the key to identify it, choosing one statement at each step.
- If you get the wrong name, the tool shows the exact step where you went off track.
- Get five right in a row. Then look at the key: why does it only use features you can see on the beetle's back?
Choosing good questions
A key only works if anyone, anywhere, looking at the organism would answer each question the same way. Good questions use features that are:
- Observable: you can see them without special equipment or a long wait. "Has a backbone" is fine in a museum; in the field, "has scales" may be easier.
- Permanent: true of every adult, every day. "Is eating seeds", "is sitting on the ground" and "has a broken leg" change from minute to minute.
- Clear-cut: either yes or no. "Is large" or "is pretty" depends on who's looking. "Wingspan more than 30 cm" doesn't.
- Opposite pairs: the two choices at each step must cover everything, with no overlap. "Has stripes / has spots" fails for a beetle with neither.
Building your own key
List the organisms and their features in a table: kookaburra (feathers), wombat (fur, four legs), echidna (fur and spines), goanna (scales, legs), carpet python (scales, no legs).
Find one feature that splits the group into two. Feathers? Only the kookaburra. Fine, but a more even split works better: "Has fur" puts wombat and echidna on one side, the other three on the other.
Split each side again. Fur side: "Has spines" separates the echidna from the wombat. Other side: "Has feathers" separates the kookaburra; then "Has legs" separates the goanna from the python.
Keep going until every organism is alone at the end of a branch. Then test the key on someone who hasn't seen your table.
- Choose questions to split the animals until every one is alone. The tool draws your key as you go and flags questions that don't split the group.
- Any finished key for six animals uses five questions. Can you arrange yours so nobody has to answer more than three to reach a name?
- Try starting with a different question. Do you still end up with a working key?
Different people can build different keys for the same organisms and both can be correct. What matters is that each one leads every organism to its own name.
Test yourself
Why is "is sitting on the ground" a poor statement for a bird key?
Good key features are permanent.
Use the vertebrate key. An animal has no feathers, no fur, no fins as an adult and moist skin with no scales. It is
1b → 2b → 3b → 4b.
Which statement would work best in a key to bird species?
A permanent, visible structural feature.
A finished dichotomous key separates 8 organisms. How many two-way questions does it contain altogether?
Each question splits one group into two, adding one more group. Going from 1 group to 8 takes 7 questions, however the key is arranged.
Complete the key: put each class at the end of the right branch.
Would each statement work in a key for Australian birds?
Construct a dichotomous key (branched or tabular) to identify these four objects: a pencil, a pen, a ruler and an eraser.
1a Is long and thin (longer than 10 cm) … go to 2.
1b Is short (less than 10 cm) … eraser.
2a Has a flat edge with measurement markings … ruler.
2b Has no markings … go to 3.
3a Is made of wood … pencil.
3b Is made of plastic or metal … pen.
Outline ONE advantage and ONE limitation of dichotomous keys.
An advantage is that each step is a simple choice between two options, so someone without expert knowledge can identify an organism. A limitation is that it only works for organisms the key was written for; a newly discovered species, or a juvenile that lacks the adult features, can't be identified.