Do mass flowering agricultural species affect the pollination of Australian native plants through localised depletion of pollinators or pollinator spillover effects?
Australian agricultural landscapes are dominated by northern-hemisphere mass flowering species, with the potential to disrupt pollination of adjacent co-flowering plants. In northern-hemisphere systems, mass flowering plants can act as 'pollinator magnets'. Magnets can reduce the pollination success of adjacent co-flowering neighbours by drawing pollinators from these plants. However, magnets can also produce pollination 'spillover-effects' through increased pollinator movements to adjacent co-flowering taxa, potentially either increasing pollination or impacting it through transfer of mixed-species pollen. The impact of agricultural species on co-flowering plants in Australia is unknown. Native Australian plants evolved with unique pollinators and yet agricultural pollination is dominated by the exotic honeybee Apis mellifera. We tested how distance (to 250 m) from each of two populations of Lavender (Lavandula spp.) (2 years), Nectarines (Prunus persica var. nucipersica or var. nectarina) (1 year) and the pasture weed Paterson's Curse (Echium plantagineum) (2 years) affected pollinator diversity and abundance within experimental arrays of native and northern-hemisphere plants. For Paterson's Curse, we tested whether visitation varied when honeybee abundance was experimentally increased. As expected, we found magnets attracted significantly more honeybee visits in all possible comparisons across combinations of species, sites and years. However, contrary to expectation, in 10 of 18 comparisons, co-flowering species supported a slightly higher diversity of pollinators than magnets, with honeybees comprising a significantly lower proportion of flower visitors in 14 comparisons. The exceptions, which included populations of all three magnet species, were cases where honeybees were the only visitor (6 comparisons) or the magnet and co-flowering species were each visited by a single but different pollinator (2 compariosns). Most strikingly, our data provided no evidence of pollinator spillover. More than 95% of all pollinators on co-flowering species carried only their pollen and overall pollinator abundance didn't vary significantly with distance from magnets at any site or time. Honeybee abundance did not vary significantly with distance from magnets. For Paterson's Curse, the addition of managed hives increased honeybee abundance on co-flowering species in two of eight cases, but pollinator abundance didn't vary with distance from the magnet. Overall, we found few direct effects of mass flowering agricultural species on the pollination of either Australian native or exotic neighbours. The lack of spillover-effects may largely reflect the unusually high species fidelity of foraging A. mellifera. However, the numerical dominance of honeybees within Australian agricultural systems may impact pollination at the regional level.