SOURCES OF VARIATION IN POLLINATOR CONTRIBUTION WITHIN A BUILD - THE EFFECTS OF PLANT AND POLLINATOR FACTORS

Among plants visited by many pollinator species, the relative contribu tion of each pollinator to plant reproduction is determined by variati on in both pollinator and plant traits. Here we evaluate how pollinato r movement among plants, apparent pollen carryover, ovule number, reso urce limitation of seed set, and pollen output affect variation in con tribution of individual pollinator species to seed set in Lithophragma parviflorum (Saxifragaceae), a species visited by a broad spectrum of visitors, including beeflies, bees, and a moth species. A previous st udy demonstrated differences among visitor species in their single-vis it pollination efficacy but did not evaluate how differences in visita tion patterns and pollen carryover affect pollinator efficacy. Incorpo ration of differential visitation patterns and pollen carryover effect s - commonly cited as potentially important in evaluating pollinator g uilds - had minor effects (0-0.6% change) on the estimates of relative contribution based on visit frequency and single-visit efficacy alone . Beeflies visited significantly more flowers per inflorescence than t he bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating t hat apparent pollen carryover did not reduce per-visit efficacy of the se taxa. In contrast, Greya moth visits showed a decrease in seed set by 55.4% and the smaller bees by 45.4% from first to second flower. Th e larger carryover effects in smaller bees and Greya were diminished i n importance by their small overall contribution to seed set. Three va riable plant traits may affect seed set: ovule number, resource limita tion on seed maturation, and pollen output. Ovule number per flower de clined strongly with later position within inflorescences. Numbers wer e much higher in first-year greenhouse-grown plants than in field popu lations, and differences increased during 3 years of study. Mean polle n count by position varied 7-fold among flowers; it paralleled ovule n umber variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowerin g, with seed set in hand-pollinated flowers ranging from 66% in early flowers to 0% in the last two flowers of all plants. Variation in ovul e number and resource limitation of seed maturation jointly had a stro ng effect on the number of seeds per flower. Visitation to early flowe rs had the potential to cause more seed set than visitation to later f lowers. Overall, the most important sources of variation to seed produ ction contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potential ly differential phenology among visitor species. These effects are lik ely to vary among populations and years.