Sympatric flowering plants with similar floral morphologies may compete for pollinator service. Although this competition can influence mating patterns and the extent of gene pollen dispersal, the consequences of competition for pollinator service for plant mating systems remain largely unexplored. Mimulus ringens and Lobelia siphilitica have similar flower shape and color and occasionally co-occur and share pollinators. During the summer of 2001, a replacement-plot design was used to examine the effects of interspecific competition for pollinator service. Sixteen plants of M. ringens with unique homozygous allozyme markers at four loci were grown in four uniform experimental arrays. Two arrays had monocultures of M. ringens and two arrays had mixtures of M. ringens and L. siphilitica. Population spacing of M. ringens remained constant between the pure and mixed arrays, facilitating a detailed analysis of the effects of a competing species on patterns of pollinator movement, seed set, selfing rates, and pollen-mediated gene dispersal. Detailed observations of pollinator foraging patterns indicated that 19.5% of all between flower movements were interspecific. The presence of a competing species resulted in a significantly lower mean seed set for M. ringens in the mixed arrays (mean = 1272) than in the pure arrays (mean = 2076). Self-fertilization rates for M. ringens were quantified for individual fruits and plants with complete paternity exclusion. Selfing rates were significantly higher in the mixed arrays (mean = 0.51) than in the pure arrays (mean = 0.33).

Key words: competition for pollinator service, mating system, Mimulus ringens, outcrossing rates, seed set