Plant spatial aggregation modulates the interplay between plant competition and pollinator attraction with contrasting outcomes of plant fitness

Ecosystem functions such as seed production are the result of a complex interplay between competitive plant-plant interactions and mutualistic pollinator-plant interactions. In this interplay, spatial plant aggregation could work in two different directions: it could increase hetero- and conspecific competition, thus reducing seed production; but it could also attract pollinators increasing plant fitness. To shed light on how plant spatial arrangement modulates this balance, we conducted a field study in a Mediterranean annual grassland with three focal plant species with different phenology, <em>Chamaemelum fuscatum </em>(early phenology), <em>Leontodon maroccanus </em>(middle phenology) and <em>Pulicaria paludosa </em>(late phenology), and a diverse guild of pollinators (flies, bees, beetles, and butterflies). All three species showed spatial aggregation of conspecific individuals. Additionally, we found that the two mechanisms were working simultaneously: crowded neighborhoods reduced individual seed production via plant-plant competition, but they also made individual plants more attractive for some pollinator guilds, increasing visitation rates and plant fitness. The balance between these two forces varied depending on the focal species and the spatial scale considered. Therefore, our results indicate that mutualistic interactions do not always effectively compensate for competitive interactions in situations of spatial aggregation of flowering plants, at least in our study system. We highlight the importance of explicitly considering the spatial structure at different spatial scales of multitrophic interactions to better understand individual plant fitness and community dynamics.诸如种子生产在内的生态系统功能，源于植物间竞争互作与传粉者-植物互利互作之间的复杂交互过程。在这一交互过程中，植物空间聚集可能产生两种截然不同的效应：一方面会加剧异种与同种个体间的竞争，进而降低种子生产；另一方面则可能吸引传粉者，提升植物适合度。为阐明植物空间排布如何调控这一平衡，我们在一处地中海一年生草原开展了野外实验，选取3种物候期各异的目标植物：*Chamaemelum fuscatum*（褐花香甘菊，早物候型）、*Leontodon maroccanus*（摩洛哥毛莲菜，中物候型）与*Pulicaria paludosa*（沼泽蚤草，晚物候型），并涵盖了蝇类、蜂类、甲虫与蝶类等多样的传粉者类群。三种目标植物均表现出同种个体的空间聚集格局。此外，我们发现两种作用机制同时存在：拥挤的邻体通过植物间竞争降低了单株种子产量，但同时也让个体植物对部分传粉者类群更具吸引力，提升了访花频率与植物适合度。这两种作用力之间的平衡，会因目标植物物种与所考量的空间尺度不同而发生变化。因此，我们的研究结果表明，在显花植物空间聚集的情境下，互利互作并不总能有效弥补竞争互作带来的负面影响——至少在本研究系统中是如此。本研究强调，若要更好地理解植物单株适合度与群落动态，需明确考量不同空间尺度下多营养级交互的空间结构。