Pellmyr Olle, Thompson John N
Departments of Botany and Zoology, Washington State University, 99164, Pullman, WA, USA.
Oecologia. 1996 Sep;107(4):595-604. doi: 10.1007/BF00333953.
Among plants visited by many pollinator species, the relative contribution of each pollinator to plant reproduction is determined by variation in both pollinator and plant traits. Here we evaluate how pollinator movement among plants, apparent pollen carryover, ovule number, resource limitation of seed set, and pollen output affect variation in contribution 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 study demonstrated differences among visitor species in their single-visit pollination efficacy but did not evaluate how differences in visitation patterns and pollen carryover affect pollinator efficacy. Incorporation of differential visitation patterns and pollen carryover effects -commonly cited as potentially important in evaluating pollinator guilds - 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 the bees and the moth. Seed set remained virtually constant during the first three visited flowers for beeflies and larger bees, indicating that apparent pollen carryover did not reduce per-visit efficacy of these 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. The larger carryover effects in smaller bees and Greya were diminished in importance by their small overall contribution to seed set. Three variable plant traits may affect seed set: ovule number, resource limitation on seed maturation, and pollen output. Ovule number per flower declined strongly with later position within inflorescences. Numbers were much higher in first-year greenhouse-grown plants than in field populations, and differences increased during 3 years of study. Mean pollen count by position varied 7-fold among flowers; it paralleled ovule number variation, resulting in a relatively stable pollen:ovule ratio. Resource limitation of seed set increased strongly with later flowering, 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 ovule number and resource limitation of seed maturation jointly had a strong effect on the number of seeds per flower. Visitation to early flowers had the potential to cause more seed set than visitation to later flowers. Overall, the most important sources of variation to seed production contribution were differences among pollinators in abundance and absolute efficacy (ovules fertilized on a single visit) and potentially differential phenology among visitor species. These effects are likely to vary among populations and years.
在被多种传粉者访问的植物中，每种传粉者对植物繁殖的相对贡献取决于传粉者和植物性状的变异。在此，我们评估传粉者在植物间的移动、明显的花粉携带、胚珠数量、结实的资源限制以及花粉输出如何影响小花假虎刺（虎耳草科）中单个传粉者物种对结实的贡献变异，小花假虎刺是一种被多种访客访问的物种，包括牛虻、蜜蜂和一种蛾类物种。先前的一项研究表明访客物种在单次访问的授粉效率上存在差异，但未评估访问模式和花粉携带的差异如何影响传粉者效率。纳入差异访问模式和花粉携带效应（在评估传粉者群落时通常被认为具有潜在重要性）对仅基于访问频率和单次访问效率的相对贡献估计产生的影响较小（变化0 - 0.6%）。牛虻每花序访问的花朵数量显著多于蜜蜂和蛾类。对于牛虻和较大的蜜蜂来说，在前三次访问的花朵中结实率几乎保持不变，这表明明显的花粉携带并没有降低这些类群的每次访问效率。相比之下，从第一朵花到第二朵花，灰蛾的访问使结实率下降了55.4%，较小的蜜蜂使结实率下降了45.4%。较小的蜜蜂和灰蛾中较大的携带效应因其对结实的总体贡献较小而重要性降低。三个可变的植物性状可能影响结实率：胚珠数量、种子成熟的资源限制和花粉输出。每朵花的胚珠数量随着花序内位置靠后而大幅下降。第一年温室种植的植物中的胚珠数量远高于田间种群，并且在三年的研究期间差异增大。按位置计算的平均花粉计数在花朵间相差7倍；它与胚珠数量变化平行，导致花粉与胚珠的比例相对稳定。结实的资源限制随着开花时间靠后而大幅增加，人工授粉花朵的结实率从早期花朵的66%到所有植物最后两朵花的0%不等。胚珠数量的变化和种子成熟的资源限制共同对每朵花的种子数量产生强烈影响。访问早期花朵比访问后期花朵有可能导致更多的结实。总体而言，对种子生产贡献的最重要变异来源是传粉者在数量和绝对效率（单次访问受精的胚珠）上的差异以及访客物种之间潜在的物候差异。这些影响可能因种群和年份而异。
