Solidago canadensis impacts on native plant and pollinator communities in different-aged old fields

Secondary succession in former arable fields (i.e. old fields) might be altered by the colonization of invasive alien species, with possible community-wide impacts, hindering the ability of old fields to become species-rich communities. However, the effects of invasive species on local communities have rarely been addressed in the light of secondary succession. Therefore, we studied the impact of the highly invasive Solidago canadensis on plant and pollinator communities along a gradient of invasion severity in old fields with different ages (1-20 years since last ploughing) in Southern Transylvania, Romania. We asked whether the invasion of S. canadensis causes shifts in (1) the composition and diversity of plant communities, and (2) pollinator communities along the successional gradient. Further, we asked (3) to what extent the presence of S. canadensis affected flower visitation of native plant species by pollinators. According to our results, the invasion reduced the native plant species richness throughout succession, although the most profound negative effect on plant diversity and vegetation naturalness was exerted in older successional communities. The invasion of S. canadensis had a negative effect on the abundance of bees irrespective of the old field age; however, there was no similar negative effect on hoverflies. Native flowers experienced reduced visitation by wild bees, honey bees and hoverflies due to the augmented presence of S. canadensis. Therefore, the invasion of this perennial plant species diverts the trajectory of vegetation succession, alters the mutualistic links between the native elements of these old fields, and causes a non-desired alternative stable states to be installed.