PS 88-264
Density effects on competition for pollination between two wetland plants
Sympatric plants that flower in synchrony often share pollinators and may therefore compete for pollination services, a crucial element of their reproductive success. Competition’s influence on reproduction is likely to vary with density, yet this relationship is not well understood especially in the context of natural habitats. Mimulus ringens and Verbena hastata are sympatric, co-flowering wetland plants visited by many of the same pollinating insects, including bumblebees, solitary bees, and butterflies. Thus, they have the potential to compete for the resource of pollination much like they might compete for light and nutrients. We combined an experimental and observational study to investigate whether the density of V. hastata influences its effects on pollination services to M. ringens in its natural habitat. We explored how different V. hastata densities impacted M. ringens stigmatic pollen receipt and seed set, two important factors of pollination and reproductive success.
Results/Conclusions
We found no significant effect of V. hastata’s density on M. ringens stigmatic pollen load or seed set in the experimental study, and no significant relationship between these variables in the observational study. Therefore, V. hastata density may not be a determinant of the quantity and quality of pollinator visits to M. ringens. This neutral effect may be due to a number of reasons: 1. The two species share visitors but not pollinators, 2. The pollinators to both species exhibit different foraging behavior/posture 3. Resource availability is a more important factor influencing M. ringens reproductive success or 4. M. ringens is pollen-limited at our field sites. Indeed, M. ringens pollen receipt and seed set varied significantly between the 6 wetlands in this study, suggesting that environmental factors may be influential to its pollination. In addition, M. ringens stigmas showed slightly lower pollen load deposition (4951 pollen grains±431 [mean±1SE]) than the mean number of ovules (5000) it produces, suggesting that the populations in this study may indeed be pollen-limited.