Low endemic bee diversity and very wide host range in lowland Fiji: support for the pollinator super-generalist hypothesis in island biogeography

The success of invading plants in island ecosystems has often been inferred to result from 'invader complexes', where cointroduced plants and their specialist pollinators can reciprocally enhance each other's spread. However, it has also been suggested that in islands with low pollinator diversity, those pollinators should evolve into supergeneralists that may be able to pollinate a wide range of exotic plants, enabling the spread of exotic weeds. Fiji has a very depauperate endemic bee fauna and previous studies have suggested that its only lowland bee species, Homalictus fijiensis (Apoidea: Halictidae), has a very wide range of host plants. However, those studies only included a small number of endemic flowering plants. Our study expands observations of bee-flower interactions to explore host plant ranges of H. fijiensis and introduced bee species to include a wider variety of native and introduced plant species. We show that H. fijiensis does have a wider host range than introduced bees, including Apis mellifera (Apoidea: Apidae), and an ability to exploit extrafloral nectaries and poricidal anthers that are not utilised by the introduced bee species. Our results support the hypothesis that super-generalism can evolve in islands where pollinator diversity is low, and that this may make those islands susceptible to weed invasions.