The Trouble with Counting Species

[A] Asking how many species live on Earth sounds like a question about counting. It is really a question about definitions. The working definition most people learn — a group of organisms that can breed together and produce fertile offspring — does useful work for large animals and almost none for the majority of life.

[B] Bacteria do not breed in that sense at all; they divide, and they also swap genetic material across what would otherwise be species boundaries. [C] Many plants hybridise freely and the hybrids are fertile, which would collapse whole genera into single species if the rule were applied strictly. Fossils cannot be interbred with anything, so palaeontologists must rely on shape alone, which means that a single variable population can be recorded as several species by one researcher and one species by another. [D] None of this makes the estimates worthless, but it does explain why they range from about three million to over a hundred million. The spread is not mainly a measure of how much remains undiscovered. It is a measure of how much the answer depends on where the lines are drawn.