Can the pollination biology and floral ontogeny of the threatened Acacia carneorum explain its lack of reproductive success?

Many Acacia species in arid areas of eastern Australia have been severely impacted by grazing, habitat degradation and fragmentation. These factors have been at the core of proposed explanations for the reproductive failure and numerical decline of Acacia carneorum and other threatened acacias. Paradoxically, the sympatric Acacia ligulata is thriving and highly fecund. Although these species have superficially similar floral displays, differences in sexual reproductive success may reflect interactions between flower and inflorescence ontogeny and pollinator assemblages. We compared the floral biology and flower visitor assemblages of A. carneorum and A. ligulata at four sites per species. Both species displayed similar floral ontogeny and synchronicity of display, with inflorescences simultaneously hermaphroditic for 4-5 days. However, A. ligulata displayed a higher density of flowers than A. carneorum and, while both species received a range of flower visitors, A. ligulata was visited by relatively few species and was serviced primarily by the non-native honeybee Apis mellifera, which typically made many within-plant movements during foraging bouts. In contrast, A. carneorum was visited by a diverse suite of native insects that carried little pollen and made fewer within plant movements. On average, Apis mellifera carried 98.4 % A. ligulata pollen, whereas the native insect visitors of A. carneorum carried only 45 % A. carneorum pollen. Differing floral ontogeny or lack of native pollinators does not explain the reproductive failure of A. carneorum. The success of A. ligulata may reflect pollination services provided by A. mellifera and interactions with plant mating systems.