High abundance but low diversity of floral visitors on invasive Heracleum mantegazzianum (Apiaceae)

Currently, plant invasions affect native ecosystems across the Earth. Although much attention has already been paid to their effect on local communities, we still lack basic information on the associations between alien and local species. Here, we present the results of our survey of pollinators of the invasive plant Heracleum mantegazzianum (Apiaceae) in central Europe. At 20 sites within the westernmost part of the Czech Republic, which is strongly affected by the invasion of H. mantegazzianum, pollinators on the flowers of H. mantegazzianum were examined and compared to the species composition of pollinators on native vegetation in the surrounding area. While the flowers of H. mantegazzianum were frequently visited by high abundance of insects, the communities of H. mantegazzianum pollinators were relatively species poor, and the proportion of abundances of H. mantegazzianum pollinators was very uneven, with few species of generalist Diptera and the honey bee (Apis mellifera) dominating over all other flower visitors. Significantly larger species of the family Syrphidae visited flowers of giant hogweed than of other plants. Thus, giant hogweed is not a necessary part of flower communities for flower visiting insects, and it should be eradicated because of its negative effects on other plants, landscape and humans. Our results highlight the need for more detailed studies on direct interactions between alien plant species and native pollinator communities as well as indirect interactions between alien plants and native plants through competition for pollinators.