Importance of Non-Native Honeybees (Apis mellifera) as Flower Visitors to the Hawaiian Tree 'Ohi'a Lehua (Metrosideros polymorpha) across an Elevation Gradient

Pollinator populations are in decline worldwide. These declines are likely to impact native Hawaiian species and their interactions. To explore spatial heterogeneity in interactions between a foundational native Hawaiian tree and flower visitors, we examined how flower visitation varied for Metrosideros polymorpha over an elevational gradient, on the Island of Hawai'i. We conducted a short-term, observational study at sites of high human activity, spanning a 1,500-m elevational gradient. We predicted that native flower visitors would be most important, where importance is defined as the product of the number of flower visitors observed and the number of flowers visited per visitor, at the highest elevations where human impacts have historically been less consistent and human populations are lower. We predicted that non-native bee visitation would be most important at the lowest elevations where human impact is highest. Contrary to our expectations, we found that the non-native honeybee, Apis mellifera, was the most important visitor at both the lowest and highest-elevation sites and second only to native species of Hylaeus bees in visitor importance at mid-elevation sites. We recorded A. mellifera interacting with flowers during 16.3% of all observation blocks and Hylaeus spp. during 9.4% of observation blocks, with all other visitors appearing more rarely. Although this study was short in duration and occurred at only six study sites in disturbed areas, our results suggest that the high importance of A. mellifera as a flower visitor of M. polymorpha is consistent across a range of environmental conditions. Hawai'is shifting suite of pollinators may impact gene flow and reproduction for M. polymorpha, a species with immense ecological and cultural importance.