A matter of time for tropical diversity

There is a species-diversity gradient on Earth, with the greatest diversity found near the Equator. Analysis of forest data now reveals a mechanism aiding species coexistence in the tropics that might underlie this phenomenon. See Letter p.105 Forest diversity with latitude The number of tree species that coexist on a hectare scale rises by more than two orders of magnitude from boreal to tropical forests. Jacob Usinowicz and colleagues assess the extent to which latitudinal differences in competition between species shape this gradient, using seed production and seedling recruitment data from ten forests spanning the tropics to the boreal zone. They show that the longer and more stable growing seasons in the tropics permit greater differentiation between species in terms of the seasonal timing of reproduction. This asynchrony in the timing of tree recruitment reduces the degree of competition between species in the tropics, relative to forests at higher latitudes, thereby increasing the potential for species coexistence. The findings highlight the contribution that local-scale ecological processes can make to global-scale gradients in biodiversity.