Are there magnet plants in Australian ecosystems: Pollinator visits to neighbouring plants are not affected by proximity to mass flowering plants
Aggregations of resource-rich plants can act as "magnets" drawing pollinators from other plants. Magnets can have positive and negative impacts on co-flowering neighbours: enhanced pollination via a 'spillover-effect' or a reduction in pollination via competition. Support for the importance of magnets largely comes from studies conducted in the northern hemisphere. We used a comparative approach to test two hypotheses for three pairs of Australian native plants: (1) putative magnets attract a greater number and more diverse suites of pollinators than co-flowering species; and (2) the quantity, diversity and specificity of pollinators varies with distance from putative magnets. We surveyed pollinator activity on co-flowering plants before and after bagging to experimentally exclude pollinators from putative magnets, dominant flowering species with populations ranging in size from 700 to 4000 m2. Selected focal species were found to be pollinator magnets, but did not appear to influence the pollination of neighbours. Prior to bagging, putative magnets received more visits but visitors were predominantly (90-100%) exotic honeybees (Apis mellifera). The number and diversity of pollinators on co-flowering species did not consistently increase when magnets were bagged. Moreover, pollinator visitation, diversity and constancy did not vary with distance from putative magnets before or after bagging. All sampled (n = 388) honeybees had pollen of only one plant species on their bodies and no honeybee sampled on a co-flowering species carried pollen of magnet species (n = 212). We found that interactions between pollinators and co-flowering Australian plants differ substantially from those reported for the northern hemisphere; this is most likely due to the impact of abundant, introduced honeybees.