Floral neighborhood and pollinator functional group affect the outcome of pollinator-mediated interactions between native and exotic plants

Background/Question/Methods: Native and exotic forbs share pollinators, potentially resulting in fitness consequences for plants when pollinator-mediated competition or facilitation influences reproductive success. Reproductive success of plants may thus depend on the independent behavior of functional groups of pollinators whose visitation patterns are not static, but are determined by floral neighborhood. Determining the dynamics of plant-plant indirect interactions mediated by pollinators is critical, because conservation of rare plants depends on an understanding of how frequently abundant exotic plants may indirectly affect seed production through their effects on pollination.

To determine how pollinator-mediated indirect interactions between native and exotic plants influence reproductive success, we compared pollinator visitation and seedset of two western Washington native forbs (Microseris laciniata and Eriophyllum lanatum) and an introduced Eurasian forb (Hypochaeris radicata) in three floral neighborhood types: (i) high native floral density (low exotic H. radicata floral density), (ii) high exotic H. radicata floral density, and (iii) low H. radicata floral density and low native floral density (produced by clipping H. radicata inflorescences).

Results/Conclusions: Pollinator visitation varied by floral neighborhood, plant species, and major pollinator functional group (eusocial bees, solitary bees, and flies). Consequently, pollinator functional groups contributed differing proportions of the total visitation to a plant species depending upon neighborhood context. Exotic H. radicata competed with native M. laciniata for pollinators, as shown by reduced visitation in high exotic density neighborhoods. In contrast, H. radicata facilitated visitation to native E. lanatum, which received a higher rate of visitation in exotic than native neighborhoods. Reproductive success qualitatively reflected visitation patterns, resulting in significant reductions in seed set for M. laciniata in high exotic neighborhoods, but increased seed set for E. lanatum and H. radicata in the same neighborhoods. Low visitation rates to E. lanatum in low exotic density treatments further suggests facilitation. However, visitation and seedset of H. radicata did not decrease when its floral density was reduced, suggesting that H. radicata remains attractive to pollinators at low densities as long as dense native competition is absent.

Our results suggest that pollinator mediated interactions between native and exotic plants in western Washington are influenced by pollinator behavior and floral neighborhood in complex ways, complicating restoration efforts.