While community-wide studies of plant-pollinator interactions have recently received great attention, studies comparing pollination webs across populations or habitats remain little studied. For example, the effects of elevation on plant-pollinator interactions have been examined for individual plant species, but little has been done at the community level. The island of Hawai‘i is a particularly interesting place to study the effects of elevation on pollination webs as there is a long elevation gradient, along which substrate age is constant and vegetation composition changes little. We quantified pollinator visitation rates to all of the flowering plant species in communities on a Hawai‘ian lava flow dating from 1855 to understand how pollination webs and the integration of native and alien species changes with elevation. We conducted pollinator observations on eight flowering plants species at six sites ranging from 880-2400m.
Results/Conclusions
Plants at mid-elevation sites (1570-1820m) received the highest visitation rates, with steep declines above and below these elevations. In addition, the proportion of native pollinators changed along the elevation gradient, where at least 40-50% of visits were from alien pollinators at low elevation, as opposed to 4-20% of visits by alien pollinators at mid to high elevations. In particular, four species of native Hylaeus bee made 63-84% of all visits at these mid and high elevation sites, whereas these native bees were in extremely low abundances at low elevations. Our finding of low visitation rates at low elevations, where Hylaeus were rare, suggests that important native pollinators may not be easily replaced by aliens. Unlike other studies, in which hymenoptera decline in importance with elevation, native Hylaeus bees were the most abundant visitors at the highest elevation, and made up to 85% of all visits. This is one of the first community-wide studies of plant-pollinator interactions in Hawai‘i . It is also one of the few studies quantifying pollinator visitation rates to native Hawaiian plants or examining the importance of Hawaii's only native bee genus. Our study highlights the importance of native bees for plant reproduction in early succession habitats in Hawai‘i.
