Invasive plant species often enter a community full of naïve species. While this may become an advantage in terms of enemy release and novel weapons, it can be a disadvantage in terms of pollination, especially to self-incompatible, non-clonal species. Scotch broom (Cytisus scoparius L. Link) has successfully invaded the Pacific Northwest of America after multiple introductions. As such, this plant population contains substantial genetic diversity, allowing it to colonize a variety of habitats, and giving it phenotypic diversity to attract a range of pollinators. Recently, a biocontrol species has been introduced to feed on Scotch broom seeds, and this species is still expanding its range. We surveyed sites along an elevation gradient to determine the biodiversity of pollinators and breadth of Scotch broom’s pollination niche. We also looked at seed predation rates to determine how well the biocontrol is matching Scotch broom’s habitat exploitation.
We found that Scotch broom has a broader biodiversity of pollinators in the Pacific Northwest than has been previously reported in any other invaded range. The majority of broom pollinators are native Bombus bees, representing a guild that is common in urban areas and recently logged sites, both of which are disturbed and thus highly susceptible to invasion. Extinction of some pollinators is unlikely to be a viable method of controlling broom, unlike in New Zealand. Although we found biocontrol species at many sites, they were not evenly distributed and damage varied. Biocontrol beetles were less common at higher elevation and in small broom populations, potentially representing a lag time between broom establishment and effective biocontrol.