We explored how the presence and abundance of a competitor for pollination influenced pollinator visitation patterns, conspecific and heterospecific pollen deposition, and seed set. We established replicate experimental arrays of the wetland perennial Mimulus ringens and manipulated the presence and abundance of Lythrum salicaria, a co-flowering invasive exotic. We quantified pollinator visitation to Mimulus flowers, pollen deposition on Mimulus stigmas and seed set when a) no Lythrum was present, b) an equal frequency of Lythrum and Mimulus were present, or c) Lythrum plants were 3X as frequent as Mimulus plants.
The rate of pollinator visitation to Mimulus flowers was reduced in the presence of Lythrum, but did not decline further with increased relative abundance of the competitor. However, patterns of pollinator movement were strongly influenced by Lythrum abundance. When Lythrum was more abundant than Mimulus pollinator probes of Mimulus flowers were often immediately preceded or followed by visits to Lythrum. The mean number of conspecific pollen grains deposited onto each Mimulus stigma was significantly influenced by the abundance of the competitor. In addition, the mean number of Lythrum grains on Mimulus stigmas nearly doubled with increasing Lythrum abundance. The reduction in conspecific pollen on Mimulus stigmas with increasing Lythrum abundance is likely to reflect the increased rate of pollinator transitions between Mimulus and Lythrum, leading to heterospecific pollen loss. The mean number of Mimulus seeds per fruit was highly variable among days, but tended to be lower in the presence of the competitor. These results suggest that changes in bumblebee foraging due to an increased abundance of competitor plants has a direct effect on conspecific and heterospecific pollen deposition and seed set in Mimulus ringens.