Decreased insect visitation to a native species caused by an invasive tree in the Cape Floristic Region

Abstract

Flowering invasive plants can potentially reduce pollination rates in co-flowering native plant species by attracting shared flower visitors. We investigate the effect of a prolifically flowering invasive species, Acacia saligna (Fabaceae), on native insect–flower interactions in surrounding communities. We surveyed flower visitation to A. saligna and selected native species at invaded and uninvaded areas and assessed the extent of flower visitor overlap of A. saligna and native species across four sites in South Africa’s Cape Floristic Region. Median visitation rates of bees and all insects were significantly lower to one native species (Roepera fulva) in both field seasons at the same site when A. saligna was present. This native species also had the highest flower visitor overlap with A. saligna which was driven primarily by the native honeybee, Apis mellifera subsp. capensis. Observational data showed that A. saligna inflorescences were visited most frequently by minute beetles and the native honeybee. Our findings indicate that native honeybees are important visitors to A. saligna, and that at least one native plant species experiences reduced flower visitation due to competition with A. saligna for honeybee visits. Flowering A. saligna could have wide-reaching effects on native plants by competing for visits from a super generalist flower visitor – the native honeybee – which may jeopardize the reproductive success of natives whose flowering time overlaps with that of A. saligna. Our study further highlights the importance of understanding the effect of a flowering invasive species on native species and the potentially important mutualism between A. saligna and honeybees.