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 rates (BAMM and DR), and regional energy (past and current temperature, productivity).

Results: While regional richness across each of the six taxa peaked in the temperate region, it varied markedly across hemispheres and continents. The effects of time, diversification rates, and regional energy varied greatly from one taxon to another, but high diversification rates generally emerged as the best predictor of high regional richness. The effects of time and regional energy were limited, with the exception of salamanders and cetaceans. 

Main conclusions: Together, our results indicate that the causes of i-LDG are highly taxon-specific. Consequently, large-scale richness gradients might not have a universal explanation and different causal pathways might converge on similar gradients. Moreover, regional diversification rates might vary dramatically between similar environments and, depending on the taxon, regional richness might or might not depend on the time for species accumulation. Together, these results underscore the complexity behind the formation of richness gradients, which might involve a symphony of variations on the interplay of time, diversification rates, and regional energy.

Methods
This release includes the code and data that support the findings as described in the paper "Cerezer et al. (2022) Exceptions to the rule: Relative roles of time, diversification rates, and regional energy in shaping the inverse latitudinal diversity gradient" published in Global Ecology and Biogeography. The detailed methods are in the main text of the associated article.