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 exami ned stigma pollen load, percentage of fruit set, and percentage of seed set in large and small populations over 2 yr. We tested for significant effect s of population size on these characteristics, for relationships between st igma pollen loads and fruit and seed set, and for significant variation bet ween years in reproductive characteristics. In 1994 stigma pollen loads wer e inversely related to population size and were significantly lower than in 1993. The lower stigma pollen loads in 1994 were associated with adverse w eather conditions thigh rainfall), which are known to affect pollinator act ivity and may have led to competition among plants for pollinators in large populations. However, population size was not correlated with percentage o f fruit set or percentage of seed set in either pear. 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 fru it and seed production. Our results indicate that for some self-compatible species that similarly require small pollen loads, the effects of populatio n size on reproduction are minimal, even if differences in pollen-movement exist between large and small populations.