Abstract
The invasion of alien plants has been rapidly proceeding worldwide due to urbanisation. This might be beneficial to wild pollinating insects, since some alien plant species have large flowers and/or long flowering periods, which can increase nectar sugar and pollen availability. To determine the relative contribution of alien plants to floral resource supply and whether resource-rich alien plants, if any, serve as an important food source of pollinating insects, we performed year-round field observations in suburban riverbanks. We quantified the per-unit-area availability of nectar sugar and pollen delivered by alien and native flowering species and counted wild flower visitors (bees and wasps, hoverflies and butterflies) per plant species. The available nectar sugar and pollen per area were predominantly delivered by a few specific alien species, and the relative contribution of other species to floral resource provision was low throughout the period that wild flower visitors were observed. Nonetheless, the resource-rich alien plants were not visited by as many insects as expected based on their contribution to resource provision. Rather, on a yearly basis, these plants received equal or even fewer visits than other flowering species, including resource-poor natives. We show that despite their great contribution to the gross floral resource supply, resource-rich alien plants do not serve as a principal food source for wild pollinating insects, and other plants, especially natives, are still needed to satisfy insect demand. For the conservation of pollinating insects in suburban ecosystems, maintaining floral resource diversity would be more beneficial than having an increase in gross floral resources by allowing the dominance of specific alien plants.