Hymenopteran Pollinators Prefer Yellow Flowers to Red Ones in Scotch Broom (Cytisus scoparius (L.) Link), But Not Enough to Negatively Affect Plant Fitness

In well-established flowering plant invasions, floral phenotypes that do not attract pollinators are predicted to be eliminated through natural selection. We explored this hypothesis in the invasive plant species Scotch broom (Cytisus scoparius), which has yellow flowers frequented by a diverse community of bee species, as well as red-tinged flowers that are predicted to be pollinated at a significantly lower rate. We predicted that plants with the red-flower phenotype will have fewer flowers visited, produce fewer seed pods per flower, and produce fewer seeds per pod than the yellow-flowered type. To investigate this, we measured the proportion of flowers pollinated for red- and yellow-flowered phenotypes, observed the number of seed pods produced per flower, and counted seeds per pod. Although we found lower pollination in the red-flowered phenotype, we did not see differences in female fitness. Scotch broom is an invader predicted to be limited in fecundity by the rate of pollinator visits and would be expected to lose phenotypes that attract fewer pollinators. We found the persistence of a less-attractive phenotype, likely because the reduction in male fitness is not paired with a reduction in female fitness.