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

In animal-pollinated plants, adaptive floral divergence to distinct
pollination environments can lead to pollination shifts, promoting the
evolution of new pollination syndromes that confer reproductive isolation.
A longstanding but untested hypothesis proposes that reduced bee
visitation in tropical montane cloud forests has repeatedly driven the
evolution of hummingbird-pollinated plant species in diverse lineages.
Here, we test whether recently diverged bee and hummingbird-pollinated
syndromes in two sister species are locally adapted to their native
pollination environments, and whether this adaptation reflects declining
bee activity at higher elevations. Alternatively, we ask whether higher
pollen transfer efficiency can drive adaptation to hummingbirds regardless
of bee availability. We measured visitation and per-visit pollen
deposition to calculate pollinator effectiveness as a quantitative measure
of pollinator performance, 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 as well as at
their elevational boundary. In their respective ranges, the species are
specialized on their respective pollinator functional groups, bees or
hummingbirds. However, C. wilsonii received more pollen grains per visit
despite C. kuntzei experiencing higher visitation rates. Consequently,
pollinator effectiveness was greater for C. wilsonii. In reciprocal
translocations, C. kuntzei showed uniform bee visitation and effectiveness
across habitats and elevations, whereas hummingbird visitation and
effectiveness increased with elevation for C. wilsonii. Our results show
that pollination systems are adapted to their native pollination
environments, and that floral adaptation to hummingbird pollination is
likely driven by higher hummingbird visitation in montane environments
combined with their higher per-visit pollen deposition efficiency, rather
than by a decline in bee visitation with elevation as previously assumed.
This study provides the first thorough experimental test of the drivers of
evolutionary transitions to hummingbird pollination in the American
tropics.