Why are there so many flowering plants? A multi-scale analysis of plant diversification

The causes of the rapid diversification and extraordinary richness of flowering plants (angiosperms) relative to other plant clades is a long-standing mystery. Angiosperms are only one among 10 major land plant clades (phyla), but include ~90% of land plant species. However, most studies that have tried to identify which traits might explain the remarkable diversification of angiosperms have focused only on richness patterns within angiosperms, and tested only one or a few traits at a single hierarchical scale. Here, we assemble a database of 31 diverse traits among 678 families and analyze relationships between traits and diversification rates across all land plants at three hierarchical levels (phylum, order, family) using phylogenetic multiple regression. We find that most variation (~85%) in diversification rates among major clades (phyla) is explained by biotically mediated fertilization (e.g., insect pollination) and clade-level geographic-range size. Different sets of traits explain diversification at different hierarchical levels, with geographic-range size dominating among families. Surprisingly, we find that traits related to local-scale species interactions (i.e. biotic fertilization) are particularly important for explaining diversification patterns at the deepest timescales, whereas large-scale geographic factors (i.e. clade-level range size) are more important at shallower timescales. This dichotomy might apply broadly across organisms.