Exploitation of the invasive Acacia pycnantha pollen and nectar resources by the native bee Apis mellifera

The importance of broad ecological perspectives has been well recognised with regard to the spread of alien species. However, less attention has been paid to single plant-animal relationships such as the mechanism through which interactions with pollinators take place, which is responsible for the evolution of new mutualistic relationships and the outcome of sexual reproduction and distribution of plants. Because the majority of plant-pollinator relationships are based on food collection by pollinators, we investigated the extent of exploitation of Acacia pycnantha by Apis mellifera. A. pycnantha is native to Australia, but is invasive in many Mediterranean countries, including Italy. We investigated whether visits by the native pollinator A. mellifera to a non-native food source (A. pycnantha) were occasional, or dictated by a precise pattern matching resource availability. Showing that a native pollinator’s foraging strategy has readily adapted to a potential resource offered by an invasive plant allows us to understand better the mechanisms promoting invasiveness of alien angiosperms. We collected data on pollen and nectar characteristics and variation in their availability, and conducted behavioural observations of foraging honey bees. Acacia pycnantha flower heads and extrafloral nectaries are sources of food actively selected by honey bees, which display behaviours strongly associated with resource availability. Our results showed clear evidence of a strong invasive plant-native pollinator relationship, enhancing the success of sexual reproduction of the alien species. Future studies may highlight to which extent this relationship has influenced the spread of this invasive plant.