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?