Combined effects of Impatiens glandulifera invasion and landscape structure on native plant pollination

Abstract

1. Habitat loss, land use intensification and biological invasions are all threatening pollinator com- munities, but the combined effects of these factors on pollinator diversity and pollination services have not been studied yet. 2. Here, we tested the hypotheses that (i) the invasive plant Impatiens glandulifera outcompetes native plant communities for pollinators, and (ii) pollinator abundances depend on landscape struc- ture, but are modulated by this mass-flowering invader. 3. We selected 14 study sites in riparian habitats along a landscape gradient with decreasing pro- portion of natural land cover. Within each site paired invaded or non-invaded plots were studied. We performed standardized surveys of pollinators and established experimental plots by adding the native plant Raphanus sativus to assess the impact of I. glandulifera on visitation rates and seed set. 4. Impatiens glandulifera was well integrated in the plant–pollinator network, being visited by several native pollinators, mainly bumblebees. The invader received higher visitation rates than simultaneously flowering native riparian plants and the experimentally added native R. sativus. However, visitation rates to the native plant community showed no significant differences between invaded and non-invaded plots, with the exception of honeybees, which slightly increased their visits in invaded plots. With regard to the experimental setting, the presence of I. glandulifera reduced bumblebee visitation to R. sativus pots, but had no significant effects on seed set. 5. We found enhanced visitation rates of bumblebees in intensively used agricultural landscapes. However, in the presence of I. glandulifera this landscape effect was masked by bumblebees being highly attracted to I. glandulifera stands independent of the structure of the surrounding landscape. Surprisingly, wild bees and hoverflies were not affected by landscape structure, but, as also the case with bumblebees, they were principally affected by the immediate community flower abundance. 6. Synthesis. Our data provide no evidence that I. glandulifera outcompetes native plants for pollinators. However, social bees were very attracted to this late-seasonal floral resource. We conclude that both, plant invasions and landscape structure have important effects on the plant–pollinator community studied, but that they operate at different stages of the flowering season.