Toxic nectar from invasive plant species: Ecological effects on native flower-visiting mutualists and role in invasion process

Background/Question/Methods

Floral nectar commonly contains plant secondary compounds (SC) that may be deterrent or toxic to insects and are usually associated with defense against herbivory elsewhere in the plant.  Studies of nectar SCs have included investigations of function, impacts on plant fitness and on flower-visitor health.  No work to date however, has examined the role these toxins play in plant invasion, or their impacts on native pollinator fauna after successful invasion.  The mutualistic facilitation hypothesis proposes that invasion by alien species often depends on, or is at least enhanced by, establishment of mutualistic relationships with native communities.  We asked whether nectar SCs facilitate invasion, by presenting a “novel weapon” that discourages herbivory but enhances pollination, or limit it by providing a barrier to the establishment of animal-mediated pollination.  We measured nectar SC concentration, fitness, and the incidence of mutualistic (pollinator) and antagonistic (nectar-robber) interactions in an invasive plant species, Rhododendron ponticum, in its native (Iberian peninsula) and invasive (British Isles) ranges.  We also determined how nectar SCs from this plant in invaded environments affect native pollinator health and forage preferences by carrying out controlled laboratory bioassays on three potential pollinators in the invaded environment.  

Results/Conclusions

Nectar SC concentrations in the invasive range were significantly lower than in the native range.  There was also a strong negative correlation in the invasive range between nectar SC concentration and female plant fitness (measured as seed set), but a positive correlation between these two factors in the native range.  Furthermore, pollen limitation has been shown in the invasive range but not in the native range.  In response to R. ponticum nectar SC consumption, the European honeybee Apis mellifera mellifera exhibited rapid mortality, with less than 20% of individuals surviving six hours after consumption.  Nectar consumption did not impact survival of a native solitary bee (Genus Andrena), but individuals exhibited malaise symptoms after consuming nectar SCs.  These results were in stark contrast to the bumblebee: consumption of R. ponticum nectar SCs had no impact on survival, behavior, or foraging preferences of Bombus terrestris.  Additionally, we found no evidence of sublethal impacts of the SC on B. terrestris.  Overall, our results suggest that pollinators may be selecting against nectar SC in invaded environments, but not the native environment.  Nectar SCs may have acted as a barrier to invasion that was overcome because of the tolerance of one effective pollinating taxa.