Data from: Pollinator-mediated mechanisms for increased reproductive success in early flowering plants

Mating activities change within a season in many animal and plant populations. In plants, selection towards early flowering is commonly observed. Pollinator-mediated selection is hypothesized to be a pervasive evolutionary force acting directionally on flowering time. However, pollinator-mediated mechanisms have rarely been tested in realistic field conditions, especially in perennial plants visited by a diversity of generalist insect pollinators. We examined pollinator visitation in eight Echinacea angustifolia populations in western Minnesota, USA, to gauge the potential for pollinator-mediated selection. Echinacea is a common prairie perennial that persists in isolated remnant populations. Echinacea is self-incompatible and is pollinated by a diversity of generalist solitary bees. A previous study found that early flowering Echinacea plants have higher seed set and their reproduction is less pollen-limited than late flowering plants. Twelve times throughout a flowering season, we quantified pollinator visitation rates and pollinator community composition. In three sites, we also estimated the quality of pollinator visits by examining the composition of pollinators’ pollen loads brought to Echinacea plants. We found that three aspects of pollination dramatically decreased over the course of the flowering season. (1) Pollinators visited early flowering plants more frequently than late flowering plants. (2) The pollinator community was also less diverse late in the flowering season and became dominated by a single species of small bee, Augochlorella aurata. (3) Pollinators visiting Echinacea late in the season carried proportionally less conspecific pollen compared to pollinators visiting Echinacea early in the flowering season. Understanding within-season dynamics of pollination helps predict the prevalence of inbreeding, phenological assortative mating, and reproductive failure, especially in fragmented plant populations.诸多动植物种群的繁殖活动会随生长季发生动态变化。在植物类群中，针对早开花习性的定向选择常被观测到。传粉媒介介导的选择（pollinator-mediated selection）被假说为一种普遍存在、作用于开花时间的定向进化动力。然而，该选择机制极少在真实野外环境中得到验证，尤其是在由多种广谱昆虫传粉者访问的多年生植物类群中。 我们针对美国明尼苏达州西部的8个狭叶紫锥菊（Echinacea angustifolia）种群开展了传粉者访花调查，以评估传粉媒介介导选择的潜在作用强度。紫锥菊属（Echinacea）植物是北美草原常见的多年生类群，常以孤立的残存种群形式存续。该物种具有自交不亲和（self-incompatible）特性，依赖多种广谱独居蜂类完成传粉。既往研究表明，相较于晚开花植株，早开花的狭叶紫锥菊植株结籽率更高，且其繁殖过程受花粉限制程度更低。 在整个开花季内，我们共开展12次采样，量化了传粉者访花频率与传粉者群落组成。在其中3个样地中，我们还通过分析传粉者携带至紫锥菊植株的花粉载荷组成，评估了传粉访花的质量。 我们的研究发现，传粉过程的三个关键指标在整个开花季内呈现显著下降趋势： (1) 传粉者对早开花植株的访花频率显著高于晚开花植株； (2) 开花季后期的传粉者群落多样性更低，且逐渐被单一小型蜂类——金绿绿芦蜂（Augochlorella aurata）所主导； (3) 相较于开花季早期访问紫锥菊的传粉者，季后期访花的传粉者所携带的同物种花粉（conspecific pollen）比例显著更低。 解析传粉过程的季内动态变化，有助于预测近交、物候选型交配以及繁殖失败的发生频率，在植物种群片段化的背景下尤为如此。