Patterns of floral visitation to native Hawaiian plants in presence and absence of invasive Argentine ants.
Invasive ants can influence abundance and diversity of insect pollinators, and this effect may be expected to be especially strong on oceanic islands. We examined abundance of native Hylaeus bees and patterns of flower visitation to three native Hawaiian plant species (Leptecophylla tameiameiae, Santalum haleakalae, and Geranium cuneatum) in presence and absence of invasive Argentine ants (Linepithema humile) in the subalpine shrubland of Haleakalā volcano. Native bees, nonnative honeybees, and microlepidoptera were responsible for the majority of visits to flowers of the three plant species. Ants visited flowers of S. haleakalae and G. cuneatum relatively commonly in ant-invaded zones but were less common or absent on flowers of L. tameiameiae in the same zones. Zones with ants had fewer Hylaeus bees in yellow pan traps than the zone without ants, and overall flower visitation rates to one plant species, G. cuneatum, were six times lower in sites containing ants. In one of two observation years, flower visitation to L. tameiameiae was also reduced in sites containing ants. Results suggest that Argentine ants may cause a reduction of native Hylaeus bee abundances; however, other factors correlated with elevation cannot be ruled out as the cause of differences in Hylaeus abundances across our sites.