Effects of an invasive plant species, giant goldenrod (Solidago gigantea) on pollinator communities

Abstract

Invasive plant species can have an effect on native plant and related pollinator insect communities both in anthropogenically disturbed and semi-natural habitats. Due to the dominance of invasive plant species, the impoverished vegetation often results in a decline of native pollinator diversity. Furthermore, invasive species may have a negative effect on flower-visitation of the native wild plant or crop species. We studied the effect of giant goldenrod (Solidago gigantea), a common insect-pollinated invasive plant species in Hungary, on bee (Hymenoptera: Apoidea) and hoverfly (Diptera: Syrphidae) communities. We compared 10 invaded (relative cover of S. gigantea >50%) with 10 uninvaded (<5% S. gigantea) sites. Pollinators were sampled along two 100m long 2m wide transects per study site, twice: before and during the peak flowering of S. gigantea. We recorded floral resources at ten 1×1 m quadrates/transect. We analysed the data by generalized linear mixed models, where the response variables were the abundance and species richness of wild bees, hoverflies and abundance of honeybees; explanatory variables were invaded-uninvaded effect, number of flowering species, flower abundance, sampling occasion; random effect was the sampling site, hence we could compare the site-pairs. The flower abundance was larger at the uninvaded sites before and similar between the invaded and uninvaded sites during the flowering of S. gigantea. Abundance of pollinators increased with flower abundance in all investigated pollinator groups. Wild bees were the most abundant in the uninvaded sites, before flowering of S. gigantea. Honey bees were most abundant in the invaded sites during the flowering of S. gigantea. Hoverflies were more abundant in the uninvaded than in the invaded sites before flowering of S. gigantea, but showed the opposite during flowering of S. gigantea. Our results suggest that sites invaded by S. gigantea can host less pollinators due to the lack of native wildflower resources, while later in the season the flowering patches of S. gigantea are attractive for honeybees and hoverfly species. We suggest that further similar studies are needed on other invasive plant species to highlight trait-specific differences in the response of pollinator insects. Furthermore, studies are needed at finer temporal scale to reveal the whole seasonal effect of invasive species on plant-pollinator networks.