Rapid disruption of pollination function by the invasive plant Impatiens glandulifera

Biological invasions can disrupt plant-pollinator interactions by altering pollinator behaviour and pollen transfer dynamics, yet the mechanisms and timing of these effects remain poorly understood. Most studies rely on observational comparisons or removal experiments in long-established invasions, but little is known about changes in pollination function at the onset of invasion. We investigated the pollination of Stachys sylvatica by combining field comparisons between pristine and invaded sites with an experimental introduction of Impatiens glandulifera into a previously uninvaded site. We quantified pollinator visitation, pollen loads carried by bumblebees, and conspecific and invasive pollen deposition on S. sylvatica stigmas. Across multiple field sites, stigmas of S. sylvatica in pristine habitats received approximately three times more conspecific pollen than those in invaded sites. Bumblebees dominated the pollinator assemblage across all sites, and in invaded habitats, they carried pollen loads strongly dominated by I. glandulifera. During the experimental introduction, bumblebees rapidly incorporated I. glandulifera into their foraging, while conspecific pollen deposition on S. sylvatica stigmas declined sharply, with 81.5% reduction within 4 days. Our results demonstrate that invasion by I. glandulifera can rapidly impair pollination function of a co-flowering native species through changes in pollen transport and transfer efficiency, even before strong shifts in visitation patterns become apparent. By capturing early invasion dynamics through experimental introduction, this study highlights the importance of direct pollen-based metrics for understanding how plant invasions disrupt pollination processes.