Opposing patterns of altitude-driven pollinator turnover in the tropical and temperate Americas

Abiotic factors (e.g., temperature, precipitation) vary markedly along
elevational gradients and differentially affect major groups of
pollinators. Ectothermic bees, for example, are impeded in visiting
flowers by cold and rainy conditions common at high elevations, while
endothermic hummingbirds may continue foraging under such conditions.
Despite the possibly far-reaching effects of the abiotic environment on
plant-pollinator interactions, we know little about how these factors play
out at broad ecogeographic scales. We address this knowledge gap by
investigating how pollination systems vary across elevations in 26 plant
clades from the Americas. Specifically, we explore Cruden’s 1972
hypothesis that the harsh montane environment drives a turnover from
insect to vertebrate pollination at higher elevations. We compared the
elevational distribution and bioclimatic attributes for a total of 2232
flowering plants and found that Cruden’s hypothesis only holds in the
tropics. Above 30° N and below 30°S, plants pollinated by vertebrates
(mostly hummingbirds) tend to occur at lower elevations than those
pollinated by insects. We posit that this latitudinal transition is due to
the distribution of moist, forested habitats favored by vertebrate
pollinators, which are common at high elevations in the tropics but not in
the temperate Americas.