Flipped elevational pattern of pollination mode in tropical vs. temperate Americas

Aim Abiotic factors, such as temperature and precipitation, vary markedly along elevational gradients, and can in turn, shape key biotic interactions, such as herbivory and pollination. Despite the well-known effects of climatic conditions on pollinator activity and efficiency, we know little about the role of climate in pollinator shifts in animal-pollinated plants at broad geographic scales. Here we investigate patterns of altitudinal turnover in pollination mode across the Americas, with a focus on the most common pollinators (bees and hummingbirds). Specifically, we test Cruden’s classic hypothesis that plants are likely to shift to bird pollination at high elevations because endothermic pollinators are more reliable in cold and rainy conditions. Location Americas Time period Current Major taxa studied 2232 plant taxa from 26 clades Methods We collated information on pollination mode (1262 insect-pollinated, 970 vertebrate-pollinated) for the study taxa from the literature, and used GBIF occurrence data to estimate median distributions and bioclimatic attributes of each species. We used (phylogenetic) GLMMs to test for associations between pollination mode and ecogeographic variables. Results To our surprise, we found flipped elevational patterns of insect- and vertebrate-pollination strategies across latitudes, with vertebrate pollination dominating at high elevations in the tropics, but not in temperate zones. We term this pattern the ‘Tropical flip’. We recovered a strong association of vertebrate-pollinated plants with moist, forested habitats across latitudes, while insect-pollinated plants were often found in cool and dry or warm and moist conditions. Main conclusions Altitudinal gradients in temperature may not serve as a universal explanation for shifts among endothermic insect and ectothermic vertebrate pollination. Instead, strong abiotic niche differentiation among insect- and vertebrate-pollinated plants, along with competition for pollination niche space, has likely shaped the ‘tropical flip’.