Introduction

Imagine an alien visitor arriving on Earth from a distant galaxy for the first time — the first thing to strike them would almost certainly be the sheer diversity of life here. Even for us, sharing this planet with all of it, that variety never stops being remarkable: more than 20,000 species of ants, roughly 300,000 species of beetles, about 28,000 species of fish, and nearly 20,000 species of orchids. Confronted with numbers like these, ecologists and evolutionary biologists keep coming back to the same big questions — why are there so many species in the first place? Has this much diversity always existed through Earth's history, or did it build up over time? How did it actually come about? Why does it matter to the biosphere — would things function differently with far less of it? And, in the end, how do we as humans actually benefit from it?