Differential pollinator importance contributes to flower colour variation along an elevational gradient

Floral diversity is the hallmark of angiosperms, often shaped by pollinator-mediated selection when plants evolve traits that attract specific pollinator groups. Mediterranean red, bowl-shaped flowers are specialized for beetle pollination in areas with beetles (Glaphyridae) and bees but evolved different colours and specialize on bee pollinators where beetles are absent (e.g. at high elevations). What explains this apparent specialization? We assessed pollination effectiveness of honeybees and beetles for the colour-polymorphic Anemone pavonina in natural populations along the slope of Mount Olympus, Greece. We recorded duration and number of flower visits and quantified single-visit seed set. Per-visit effectiveness was almost fourfold higher for Pygopleurus beetles (36.3% of maximum seed set) than for honeybees (9.5%). Flower visits lasted markedly longer for beetles (56 seconds on average) than for honeybees (2 seconds). Beetle pollinator importance (single-visit seed set × visitation frequency) was highest at low elevations but declined with increasing elevation, whereas bee pollinator importance remained low yet constant across the elevational gradient. High beetle pollinator importance indicates that floral specialization for beetles is advantageous. Specializing for bee pollination is only beneficial at higher elevations, where bee pollinator importance exceeds that of beetles. This contributes to an elevational flower colour gradient.