Introduced honeybees (Apis mellifera) in orchid pollination: surrogate pollinators or pollen wasters?

Biological invasion is one of the leading threats to global biodiversity. Invasive species can change the structure and dynamics of landscapes, communities, and ecosystems, and even alter mutualistic relationships across species such as pollination. Orchids are one of the most threatened plant families globally and known to have established specialised pollination mechanism to reproduce, yet the impact of invasive bees on orchid reproduction has not been comprehensively assessed. We conduct a literature survey to document global patterns of the impact of invasive honeybees on orchids’ pollination. We then present a study case from Australian orchids, testing the extent to which introduced honeybees can successfully pollinate orchids across different degrees of habitat alteration, using Diuris brumalis and D. magnifica (Orchidaceae). Globally, Apis mellifera is the principal alien bee potentially involved in orchid pollination. We show that pollinator efficiency and fruit set in D. brumalis is higher in wild habitats in which both native bees and invasive honeybees are present, relative to altered habitat with introduced honeybees only. Pollen removal and fruit set of D. magnifica rise with native bees’ abundance whilst pollinator efficiency decreases with honeybee abundance and increases with habitat size. Complementarily to our findings, our literature survey suggests that the presence of introduced honeybees adversely impacts orchid pollination, likely via inefficient pollen transfer. Given the worldwide occurrence of introduced honeybees, we warn that some orchids may be negatively impacted by these alien pollinators, especially in altered and highly fragmented habitats where natural pollination networks are compromised.