Broom and honeybees in Australia: an alien liaison.
Facilitative interactions between non-indigenous species are gaining recognition as a major driver of invasion success. Cytisus scoparius (L.) Link (Fabaceae), or Scotch broom, is a cosmopolitan invasive shrub that lacks the capacity for vegetative reproduction and is a good model to study facilitative interactions. Its success in pioneer environments is determined by constraints on its reproduction. We determined whether pollinators were required for seed set in C. scoparius at Barrington Tops, NSW, Australia, where the species has infested ca. 14,000 ha across the plateau. Field and laboratory experiments showed that C. scoparius is an obligate outcrossing species at Barrington Tops. Monitoring of plants (10.7 h) showed that the flowers of C. scoparius have to be tripped to effect seed set and the only pollinator to do this was the introduced honeybee, Apis mellifera L. Most floral visits by honeybees result in fruit set (84 %) and because fruits have many ovules (10 - 18 per ovary) a single bee on an average foraging day can effect the production of over 6000 seeds. A review of C. scoparius pollination across four continents revealed major differences in pollen quantity, which may explain differences in the efficiencies of honeybees as pollinators of C. scoparius. The incorporation of pollinator management in an integrated approach for the control of C. scoparius is discussed.