The causes of species richness patterns among clades

Two major types of species richness patterns are spatial (e.g. the latitudinal diversity gradient) and clade-based (e.g. the dominance of angiosperms among plants). Studies have debated whether clade-based richness patterns are explained primarily by larger clades having faster rates of species accumulation (speciation minus extinction over time; diversification-rate hypothesis) or by simply being older (clade-age hypothesis). However, these studies typically compared named clades of the same taxonomic rank, such as phyla and families. This study design is potentially biased against the clade-age hypothesis, since clades of the same rank may be more similar in age than randomly selected clades. Here, we analyse the causes of clade-based richness patterns across the tree of life using a large-scale, time-calibrated, species-level phylogeny and random sampling of clades. We find that within major groups of organisms (animals, plants, fungi, bacteria, archaeans), richness patterns are most strongly related to clade age. Nevertheless, weaker relationships with diversification rates are present in animals and plants. These overall results contrast with similar large-scale analyses across life based on named clades, which showed little effect of clade age on richness. More broadly, these results help support the overall importance of time for explaining diverse types of species richness patterns.