The effect of interspecific competition for pollinator service on seed production and outcrossing rates in Mimulus ringens (Scrophulariaceae)

Sympatric plant species with similar flowering phenologies and floral morphologies may compete for pollinator service, and as a consequence may potentially influence each other's reproductive success and mating system. Mimulus ringens and Lobelia siphilitica co-occur in wetland habitats of central and eastern North America. Both species produce blue zygomorphic flowers and share a similar suite of bumble bee pollinators. To examine the effect of competition for pollinator service on seeds per fruit and outcrossing rates, I planted four experimental arrays of M. ringens , each consisting of genets with unique combinations of homozygous marker genotypes. In two arrays mixtures of M. ringens and L. siphilitica were planted in a checkerboard design. In the other two arrays only M. ringens individuals were present.; Bumble bee pollinators frequently moved between Mimulus and Lobelia flowers, and 12% of the pollinator movements were interspecific transitions. Seeds per fruit and outcrossing rates for Mimulus ringens fruits were significantly lower in the mixed-species arrays than fruits in the Mimulus-only arrays. This is the first study to demonstrate that competition for pollinator service may directly influence a plant's mating system.