Climbing the ladder: how big is each rank?

Groups inside groups grow fast

You know that each rank holds the one below it. So as we climb from the species to the kingdom, the groups get bigger. But how much bigger? Let us count, for the human and for the mango.

The counts below are of living species, and they are rough. New species are described every year, and experts do not always agree where one group ends. Read them as "about".

The genus Homo has only one living species, ourselves. Other species of Homo lived long ago, and they are extinct. The family Hominidae adds the chimpanzees, the gorillas and the orangutans.

Reading the counts as powers of ten

The numbers run from $1$ to $1\,500\,000$, so a plain bar chart would hide the small ones. Instead we ask: ten to the power what? $500$ is about $10^{2.7}$, and $70\,000$ is about $10^{4.8}$. On this scale, each step of $1$ means ten times as many species.

You should find that the human's ladder climbs slowly at first and then fast. From Primata to Mammalia the group grows about $13$ times, and from Chordata to Animalia about $21$ times. The mango's ladder grows more evenly.

Why the top is so big

The kingdom Animalia is huge because of one phylum: Arthropoda, the insects, spiders and their relatives. Insects alone make up more than half of all described species. So two animals picked at random are most likely to share only the kingdom and the phylum Arthropoda.

More members, fewer shared features

The size of a group and the features its members share go in opposite directions.

• Homo sapiens: one species. Its members share almost everything.
• Mammalia: about $6500$ species. All have hair and feed their young on milk, and little else is shared by all.
• Animalia: about $1.5$ million species. All are many-celled and get food from others, and not much more.

This is why a taxonomist describes a family or a genus by its diagnostic features: the few that every member has.

Try it in Python

This program prints the count of each rank above the human, with the count as a power of ten.

Practise with new numbers

Common mistakes

• Thinking each rank is a fixed number of times bigger. The steps differ a lot from group to group.
• Reading $10^{4.8}$ as $48$. It means about $63\,000$.
• Thinking a genus always holds many species. Homo has only one living species.
• Thinking the members of a kingdom share many features. The bigger the group, the fewer the shared features.

Remember these

• Each rank holds the one below it, so the groups grow as we climb.
• Human: Homo 1, Hominidae about 8, Primata about 500, Mammalia about 6,500, Chordata about 70,000, Animalia about 1.5 million living species.
• Big counts are easier to compare as powers of ten.
• Insects make up more than half of all described species.
• More members means fewer features shared by all of them.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which rank of the human holds the most species?

2. Why does the genus Homo have only one living species?

3. About how many times bigger is Chordata than Mammalia?

4. Going up the ladder, do the members of a group share more features or fewer?