Why do so many more species live in the tropics than near the poles?

The honest answer is that scientists do not agree. Species richness increases from the poles toward the tropics for a wide variety of land and sea organisms, a pattern called the latitudinal diversity gradient. It is one of the most widely recognized patterns in ecology, yet there is a lack of consensus among ecologists about what causes it. A quarter of a square kilometre of Ecuadorian rainforest has been found to hold more than 1,100 tree species, so the effect is dramatic.

Many explanations have been proposed, and they come in families. One is about energy: more sunlight and water at low latitudes mean more plant growth, which can support more individuals and so more species. Another is about stability: a constant climate lets species specialize on predictable resources and so squeeze into narrower niches. A third is about speed: warmth may mean faster evolution and so more new species. And a fourth is about time: cold regions may be poor in species because glaciation left too little time to recolonize.

Each of these has critics. The stability idea struggles with tropical mountaintops, which are stable but often poor in species. The speed idea faces evidence that speciation rates in marine fish and flowering plants actually fall toward the equator. Another idea, that predators and other interactions are stronger in the tropics, was weakened by a large test with ocean fish, where predation peaked at mid-latitudes.

For the Amazon, one famous proposal came from the scientist Jürgen Haffer in 1969. He suggested that during the last ice age, patches of rainforest were separated by stretches of non-forest, and species formed in these isolated refuges. The theory is still debated, and genetic evidence suggests that some speciation happened before the Pleistocene. Many ecologists now suspect that several mechanisms together make the gradient.