The relative importance of solitary bees and syrphid flies as pollinators of two outcrossing plant species in the New Zealand alpine

Pollinators vary in their relative contribution to the conspecific pollen deposited onto receptive stigmas, because of variation in both visitation rate and effectiveness of pollen transfer. Syrphid flies and short-tongued solitary bees are common flower visitors in alpine New Zealand, yet their relative importance as pollinators is unknown. We measured pollinator performance of the New Zealand alpine endemics Hylaeus matamoko (Hymenoptera: Colletidae) and Allograpta spp. (Diptera: Syrphidae) on two New Zealand alpine herbs, Ourisia glandulosa (Plantaginaceae) and Wahlenbergia albomarginata (Campanulaceae). Ourisia glandulosa received visits by solitary bees and syrphid flies at equal frequencies, whereas W.albomarginata was mostly visited by H.matamoko. Based on single-visit pollen deposition to virgin stigmas, H.matamoko was a much more effective pollinator than Allograpta spp., delivering 10 times as much pollen per visit to O.glandulosa stigmas and 3 times as much to W.albomarginata stigmas. By multiplying visitation frequency by single-visit pollen deposition, we estimated that H.matamoko performed 90% and 95% of the pollination of O.glandulosa and W.albomarginata, respectively. Although H.matamoko bees are short-tongued and small in size, they are critically important to plant reproductive success in the New Zealand alpine. These bees contributed most of the pollination, even to a species that received just as many visits by flies, underscoring the need to consider per-visit effectiveness as well as visitation rate in assessing the importance of different pollinators.