Why do the tropics have the highest biodiversity?

A long-standing problem in biogeography is to explain the gradient of biodiversity that decreases as latitude increases. Why do the tropics have the world’s greatest biodiversity while polar, Arctic, and Antarctic biomes have lower species diversity? A recent study of mammalian and avian biodiversity compared with weather and climate data, summarized here, suggests that “the latitudinal diversity gradient may be due higher species turnover — a higher potential for speciation counterbalanced by a higher potential for extinction — towards the poles than near the equator.”