Presence of an invasive plant species alters pollinator visitation to a native

The degree to which pollinator-mediated interactions assist or impede plant invasions is currently poorly understood. Here we describe the findings of an experiment designed to investigate how pollinator-mediated interactions between invasive Lythrum salicaria (Purple loosestrife) and a closely related native North American species, Decodon verticillatus (Swamp loosestrife) are influenced by the stage of invasion (i.e., early or late). By comparing pollinator preference and constancy to plants in experimentally introduced arrays of L. salicaria and D. verticillatus in invaded and uninvaded communities, we were able to simulate the ecological implications of two different stages of L. salicaria invasion. Invasion status had no significant effect on pollinator visitation to L. salicaria and D. verticillatus when all pollinator taxa were considered together. However, when bumblebees, the dominant pollinator at all sites, were considered alone, we found a significant interaction between pollinator preference and the invasion status of the site, with D. verticillatus preferred at uninvaded sites only, and no preference exhibited at invaded sites. In addition, for all pollinator taxa, we found that interspecific pollinator movements were overrepresented at uninvaded sites and underrepresented at invaded sites, suggesting that heterospecific pollen deposition could be a significant impediment to pollinator-mediated reproduction for both species in the early stages of an invasion. We discuss the potential consequences of our findings to the establishment of animal-pollinated invasive plants and the persistence of native species in the face of invasion.