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.