Associations between floral specialization and species diversity: cause, effect, or correlation?

It has been proposed frequently, from Darwin's time onwards, that specialized pollination increases speciation rates and thus the diversity of plant species (i.e. clade species richness). We suggest here that the correlation between clade species richness and floral specialization is real, but that clade species richness is frequently the cause, not the result of floral specialization. We urge a broader, variance-partitioning perspective for assessing the causes of this correlation by suggesting four models of how the diversity-specialization correlation might come about: (1) floral specialization promotes initial reproductive isolation ("Initial-RI" model), (2) floral specialization promotes reinforcement of reproductive isolation upon secondary contact ("Reinforcement" model), (3) floral specialization reduces the extinction rate by promoting tighter species packing ("Extinction" model), (4) floral specialization is the result of high clade species richness, which increases the number of related species in communities, and thus selects for floral character displacement ("Character-Displacement" model). These hypotheses are evaluated by comparing the relationships between species richness, speciation mechanisms, and pollination precision, accuracy, and specialization in the broader literature and, more specifically, in four study systems: Dalechampia (Euphorbiaceae), Collinsia (Plantaginaceae), Burmeistera (Campanulaceae), and Stylidium (Stylidiaceae). These systems provide stronger support for the character-displacement hypothesis, wherein local species diversity drives the evolution of specialized pollination. Although the two reproductive-isolation hypotheses may hold for plants like orchids, with extremely precise pollination systems, the reproductive character-displacement hypothesis seems likely to be more important for plant groups with less precise pollination systems.