Understanding biodiversity patterns and gradients across the globe has long been a central goal of ecology. On macroecological scales, geographic differences in the process of diversification itself may contribute to diversity gradients. For example, many hypotheses for the latitudinal diversity gradient invoke geographic variation in time-for-diversification, rate-of-diversification, or ecological limits to diversity as causal factors. The emergence of comprehensive macroecological datasets and large-scale phylogenies has offered new opportunities to address these hypotheses empirically. However, these advances have mostly been limited to vertebrates and plants, and less is known about large-scale diversification patterns in invertebrate animals. Here we address hypotheses for geographic diversity gradients using an exemplar invertebrate group, ants, an ecologically dominant clade with high but not intractably high diversity. We perform this analysis at two taxonomic scales. First, using a new compilation of geographic distributions of all ant species and comprehensive phylogenies, we look for geographic diversification rate and diversification time differences across 260 ant clades. Second, we investigate diversification patterns of the second largest ant radiation in more detail, the young and hyperdiverse genus Pheidole, as it colonized different continents around the globe.
Results/Conclusions
In the ant-wide analysis we found that the latitudinal gradient in ant richness is associated with a sharp declining trend of richness-corrected phylogenetic diversity with latitude, indicating a role of phylogenetic niche conservatism in climatic affinity. Temperate clades, furthermore, were generally younger and nested within tropical clades. There was no evidence of systematic diversification rate variation with latitude. In the Pheidole radiation, temperate lineages arose recently and are nested within tropical clades, and again there was no evidence of diversification rate variation with latitude despite a dramatic latitudinal diversity gradient. However, there was evidence of widespread diversity regulation in the genus, evidenced by bursts of diversification after colonization of new biogeographic regions followed by slowdown. Taken together, our results are consistent with the hypothesis that phylogenetic niche conservatism and differences in time-for-diversification (i.e. the “Tropical Conservatism Hypothesis”) explains the gradient. There is also evidence for ecological regulation of species richness on large scales in Pheidole, but no evidence that diversification rate varies systematically with latitude in ants.