Why are the tropics so diverse?

AS YOU MOVE FROM THE POLES towards the equator, species richness increases.

- By Ella Davies

It's a pattern that's been termed the 'latitudinal biodiversity gradient' by ecologists, and Victorian naturalist Alfred Russel Wallace was a pioneer in recognising and attempting to explain it.

Wallace attributed this abundance to time: specifically that land closer to the equator was the earliest formed and hadn't been disturbed by glacial activity at higher latitudes. He theorised that the relative stability of the tropical climate through the year, and through the ages, allowed species to accumulate here.