One of the most ubiquitous patterns in ecology is the systematic increase in species richness from temperate to tropical latitudes. Theory predicts that conspecific negative density dependence (CNDD) resulting from specialized host-enemy interactions maintains higher biodiversity in the tropics. However, this remains untested at the global scale. Moreover, different strengths of CNDD across common and rare species may determine the ways in which CNDD maintains diversity. We measured CNDD as the degree to which conspecific adults suppress recruitment of saplings relative to negative effects from heterospecific densities. We measured CNDD across species as well as the mean strength of CNDD in each of 24 stem-mapped forest plots that are part of the Smithsonian Center for Tropical Forest Science Forest Global Earth Observatory (CTFS-ForestGEO) network.
We show that global patterns in tree-species diversity reflect not only stronger CNDD at tropical latitudes, but also latitudinal shifts in the relationship between CNDD and species-relative abundance. Across 24 forest plots worldwide, species richness and diversity increased with the strength of CNDD. Moreover, CNDD was stronger for rare species at tropical latitudes but stronger for common species at temperate latitudes. These results indicate fundamental differences in the nature of local-scale biotic interactions that maintain diversity in tropical and temperate forests and contribute to the latitudinal-diversity gradient.