Altered plant-pollinator interactions among Scaevola species in Hawai‘i

Background/Question/Methods The genus Scaevola (Goodeniaceae) occurs as shrubs to small trees in diverse habitats in Hawai‘i, from coastal strand to montane rainforests.  Extensive variation in floral characteristics suggests different pollinators among species.  Bees (Hylaeus spp.) and honeycreepers (Drepanidinae) are the putative native pollinators, but have declined over the last century.  Although the floral structure of some species is indicative of macrolepidotera pollinators, no native species in this guild are documented as visitors to Scaevola flowers.  Due to high rates of extinction among the Hawaiian fauna and the introduction of alien generalists, former roles of some native pollinators may remain a mystery, and the timeliness of understanding the roles of those still present is underscored.  We quantified flower visitation rates and visitor behavior for all nine species of Scaevola during the day and night (178.5 observation hours). 

Results/Conclusions Visitation was primarily diurnal, ranging from 0.2 to 3.0 visits/flower/hour during the day, with between three and fifteen visitor taxa per species.  Non-native visitors, mainly honeybees (Apis mellifera) and ants, were the most frequent visitors for most species.  Hylaeus were infrequent visitors to three species and common only at S. chamissoniana.  No native macroleptidoptera visitors were observed.  Birds were the main visitors to S. glabra and S. procera, with the alien Zosterops japonicus a primary visitor to both, and the native Hemignathus kauaiensis also a main visitor to S. glabra.  Visitors approached flowers in a manner conducive to pollination and/or to rob nectar.  The interactions of the most frequent visitors with the plants differed for each species of Scaevola, and in several cases may impose limitations on pollination.  The prevalence of alien visitors has several implications for both the plants and native pollinators.  Non-native visitors may be depriving native pollinators of floral resources and may limit plant reproduction if alien visitors are less effective pollinators than native species.  Alternatively, non-native visitors may provide important pollinator services for Scaevola species whose native pollinators have declined or shifted to new resources for unrelated reasons.