Pollinator efficiency, rather than bee decline, explains a shift to hummingbird pollination in tropical montane forests

A longstanding but untested hypothesis proposes that reduced bee visitation in tropical montane cloud forests has repeatedly driven the evolution of hummingbird pollination. Here, we test whether recently diverged bee and hummingbird pollination syndromes in two sister species are adapted to their pollination environments, and whether this reflects declining bee activity at higher elevations. Alternatively, we ask whether higher pollen transfer efficiency drives adaptation to hummingbirds regardless of bee availability. We measured visitation and per-visit efficiency to estimate pollinator effectiveness and conducted reciprocal translocations of Costus kuntzei, with ancestral bee pollination, and Costus wilsonii, with derived hummingbird pollination, across an elevational gradient in Costa Rica, including sites within and outside each species' range and at their elevational boundary. In their ranges, the species are specialized on bees or hummingbirds. However, pollinator effectiveness was higher for hummingbird-pollinated C. wilsonii because of greater per-visit efficiency, despite bee-pollinated C. kuntzei experiencing higher visitation. In reciprocal translocations, C. kuntzei showed uniform bee visitation across habitats, whereas hummingbird visitation increased with elevation for C. wilsonii. Our results show that floral adaptation to hummingbird pollination is likely driven by higher hummingbird visitation in montane environments combined with greater per-visit efficiency, rather than declining bee visitation with elevation.