High species turnover and unique plant-pollinator interactions make a hyperdiverse mountain.

We studied α- and β-diversity of pollinators, flowering plants, and plant-pollinator interactions along the altitudinal gradient of Mt. Olympus, a legendary mountain and biodiversity hotspot in Central Greece. We explored ten study sites located on the north-eastern slope of the mountain, from 327 to 2,596 m a.s.l. Insect surveys were conducted once a month using hand netting (years 2013, 2014, and 2016), and they were combined with recordings of flowering plant diversity (species richness and flower cover). We then calculated α- and β-diversity of pollinators, plants in flower, and plant-pollinator interactions, and explored their demographic response along the altitudinal gradient. Alpha-diversity of pollinators, plants, and plant-pollinator interactions were altitude-dependent; α-diversity of all pollinators, bees, non-bumblebee bees, bee flies, and butterflies showed linear declines with altitude, whereas those of hoverflies and bumblebees showed unimodal patterns. Beta-diversity and its turnover component of all pollinators, hoverflies, bees, bumblebees, non-bumblebee bees, butterflies, and plants showed linear increases, whereas those of bee flies and of plant-pollinator interactions varied independently from the pairwise altitudinal difference. The high dissimilarity and uniqueness of pollination networks, which is probably a result of the high biodiversity and endemism of Mt. Olympus, is driven by species turnover and the formation of new interactions between new species. Contrasting to the monotonic decline of the remaining groups, the unimodal patterns of hoverfly and bumblebee α-diversity are probably the effect of a higher tolerance of these groups to high-altitude environmental conditions. Our findings highlight that the high turnover of species and of pollination interactions along the altitudinal gradient are the mainstay of hyperdiverse mountains, a fact that conveys important historical, ecological, and conservational implications.