The Latitudinal Diversity Gradient

The latitudinal diversity gradient describes the fact that there are less species in the temperate regions than in the tropics. The latitudinal diversity gradient is observed in most groups of animals, plants, and microorganisms, and remains one of the oldest and most famous mysteries in ecology. The ubiquity of this pattern suggests the possibility of a single general explanation for the latitudinal diversity gradient, and scientists since von Humboldt and Darwin have formulated dozens of hypotheses to explain the causes of this gradient. This article reviews the literature describing the main evolutionary hypotheses related to the fundamental processes that can explain why some places (like the tropics) have more species than others: speciation, extinction, colonization (dispersal), and the time necessary for diversity to accumulate. The recent advances in global-scale datasets of species distributions, the fossil record, and molecular mega-phylogenies give some hope of determining the primary cause(s) of the latitudinal diversity gradient.