Can pollination facilitation mitigate the Allee effect

Pollination facilitation is a process by which one plant species increases the reproductive success of another species with which it co-occurs and shares pollinators, by increasing the quantity or quality of pollinator visits. For plants occurring at low densities, facilitation might be critical to population persistence. Through facilitation, plant species may rescue one another from negative effects of growing at low density, by increasing the quantity or quality of pollinator visits, thus enhancing seed production. Evidence from a mathematical model developed (in collaboration with Dr. W. F. Morris and Dr. W. G. Wilson; Chapter 1) suggests that one plant species can facilitate another's pollination when the number of pollinator visits to patches of plants per unit time (the aggregative response) accelerates at low densities. Results from a field experiment (Chapter 2) with the common weed Brassica rapa suggest that although pollinator visits to patches and plant reproductive success increased with plant density, the pollinator aggregative response did not accelerate at low densities. I obtained similar results from an experiment with the common sandhill plant Piriqueta caroliniana (Chapter 3), which showed strong positive effects of increasing plant density on pollinator visitation (visits to individual plants per hour) and reproductive success. P. caroliniana plants also receive more outcross pollen and produce more seeds in the presence of a co-flowering species, Coreopsis leavenworthii, indicating that facilitation occurs in this species, through increased visit quality (more repeated foraging trips to patches) in the presence of the co-flowering species. In addition, a density-dependent matrix model for population growth in P. caroliniana (Chapter 4) demonstrates that populations may experience an Allee effect, growing more slowly at low densities due to reduced seed production, but increased seed production in the presence of the co-flowering species did not alter this Allee effect. Thus, facilitation may be more likely to occur among plants at low density through disproportionate increases in visit quality rather than through increases in numbers of visits to patches of plants. These effects may strongly affect individual plant fitness, but the population-level effects of facilitation may be stronger in plant species that are short-lived.