Exceptions to the rule: Relative roles of time, diversification rates and regional energy in shaping the inverse latitudinal diversity gradient

Aim: Inverse latitudinal diversity gradients (i-LDG), whereby regional richness peaks outside the tropics, have rarely been investigated and their causes remain unclear. Here, we investigate three prominent explanations, postulating that species-rich regions have had (1) longer time to accumulate species, (2) faster diversification, and (3) more energy to support more diverse communities. These mechanisms have been shown to explain the tropical megadiversity, and we examine whether they can also explain i-LDG.
Location: Global
Time period: Contemporary
Major taxa studied: Amphibians, birds, mammals 
Methods: We estimated the time for species accumulation, regional diversification rates, and regional energy for six tetrapod taxa (≈ 800 species). Then, we quantified the relative effects and interactions among these three classes of variables, using variance partitioning, and confirmed the results across alternative metrics for time (community phylometrics and BioGeoBEARS), diversification...