Differential effectiveness of bee and beetle pollinators creates flower colour polymorphism

Floral diversity is the hallmark of Angiosperms and is often shaped by pollinator-mediated selection, which drives floral divergence as plants evolve traits that attract specific pollinator groups. Mediterranean red, bowl-shaped flowers are specialized on pollination by glaphyrid beetles in areas with beetles and bees but evolved different colours and specialize on bees as pollinators when beetles are absent. What explains this geographic variation in specialization and concomitant flower colour differences? We assessed pollination effectiveness of honeybees and glaphyrids for the colour-polymorphic Anemone pavonina in its natural habitat. We recorded the duration and number of flower visits and quantified single-visit seed set. Per-visit effectiveness (relative seed set) was threefold higher for Pygopleurus (36.3%) than for honeybees (9.5%). Flower visitation lasted substantially longer for beetles (56.3 sec. on average) than for honeybees (2.3 sec.). For beetles, pollinator importance (single-visit seed set x visitation frequency) was highest at low elevations but declined with increasing elevation, whereas for bees pollinator importance remained low yet constant across the elevational gradient. The high pollinator effectiveness of beetles indicates that floral specialization for this pollinator group is advantageous, even to the point that excluding low-quality bee pollinators - by producing inconspicuous flower colours - is beneficial. Non-red flowers may only be beneficial at higher elevations, where bee pollinator importance exceeds that of beetles.