The Effect of Population Size on Stigma Pollen Load, Fruit Set, and Seed Set in Allium Stellatum Ker. (Liliaceae)

In Allium stellatum, a xenogamous self-compatible prairie species, we examined stigma pollen load, percentage of fruit set, and percentage of seed set in large and small populations over 2 yr. We tested for significant effects of population size on these characteristics, for relationships between stigma pollen loads and fruit and seed set, and for significant variation between years in reproductive characteristics. In 1994 stigma pollen loads were inversely related to population size and were significantly lower than in 1993. The lower stigma pollen loads in 1994 were associated with adverse weather conditions (high rainfall), which are known to affect pollinator activity and may have led to competition among plants for pollinators in large populations. However, population size was not correlated with percentage of fruit set or percentage of seed set in either year. The lack of fruit and seed set differences between large and small populations, particularly in 1994, likely resulted from the small amount of pollen needed to trigger fruit and seed production. Our results indicate that for some self-compatible species that similarly require small pollen loads, the effects of population size on reproduction are minimal, even if differences in pollen-movement exist between large and small populations.