Quantifying Undiscovered Species

This may be a stupid question, but here goes.

I’m currently working on a middle school science textbook and teacher’s manual. In one of the lecture notes sections, it discusses everything you need to know about skin mites (which is bad in and of itself–I’ve been itching ever since I worked on it). One of the small factoids I stumbled over is that there are 45,000 (or so) known species of mites, and that’s estimated to be about 5% of the total species of mites on Earth.

How the hell can they know that with any reasonable degree of accuracy? How is it possible to quantify a group of objects that you don’t know exist?

They can’t. But that doesn’t stop them from trying.

These sorts of things are basically SWAGs.

Here’s of a description of how the entomologist Terry Erwin developed his estimate of the number of tropical beetle species:

Beetle diversity

These kinds of extrapolations are still controversial in the scientific community and the subject of ongoing research, including here in Panama. I am not certain on the state of the play concerning Erwin’s estimates at the moment. We have a visitor coming who is working on the issue and if I have a chance to ask him before this thread disappears I’ll let you know.

As for mites, they are very poorly studied and AFAIK it is very easy to find undiscovered species. This estimate is probably from some mite specialist guessing on the basis of the current rate of discovery of species vs. the places we haven’t even looked yet (like 90% of the tropics).