Impact of invasive bees on plant-pollinator interactions and reproductive success of plant species in mixed Nothofagus antarctica forests

Abstract

Invasive social bees can alter plant-pollinator interactions with detrimental effects on both partners. However, most studies have focused on one invasive bee species, while the interactions among two or more species remain poorly understood. Also, many study sites had a history of invasive bees, being hard to find sites with historical low abundances. In Patagonia, Bombus ruderatus (F.) invasion begun in 1993 and B. terrestris (L.) in 2006. Though honey bees (Apis mellifera L.) introduction started in 1859, their density is still low in some parts. By experimentally increasing honey bee densities, we evaluated the effect of honey bees and bumblebees floral visitation on native pollinator floral visitation, pollen deposition, and reproductive success of three plant species in mixed Nothofagus antarctica forests of northern Patagonia: Oxalis valdiviensis, Mutisia spinosa and Cirsium vulgare. Our results show that exotic bees became the main floral visitors. No negative association was found between invasive bee and native pollinator visitation rates, but there was evidence of potential competition between honey bees and bumblebees. Floral neighborhood diversity played an important role in pollinator behavior. Conspecific pollen deposition was high for all species, while deposition of heterospecific pollen was very high in M. spinosa and C. vulgare. Not as expected, honey bees visitation rate had a negative effect on heterospecific pollen deposition in C. vulgare. For O. valdiviensis, exotic visitation rates increased conspecific pollen deposition, which was positively related to reproductive success. Although exotic bees became main floral visitors, their contribution to reproductive success was only clear for one species.