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

Ecosystem functions such as seed production arethe result of a complex interplay between competitive plant-plantinteractions and mutualistic pollinator-plant interactions. In thisinterplay, spatial plant aggregation could work in two different directions:it could increase hetero- and conspecific competition, thus reducing seedproduction; but it could also attract pollinators, increasing plant fitness.To shed light on how plant spatial arrangement modulates this balance, weconducted a field study in a Mediterranean annual grassland with three focal plant species with different phenology, Chamaemelum fuscatum (early phenology), Leontodon maroccanus (middlephenology) and Pulicaria paludosa (late phenology), and a diverse guild of pollinators (flies,bees, beetles and butterflies). All three species showed spatialaggregation of conspecific individuals. Additionally, we found that the twomechanisms were working simultaneously: crowded neighborhoods reducedindividual seed production via plant-plant competition, but they also madeindividual plants more attractive for some pollinator guilds, increasingvisitation rates and plant fitness. The balance between these two forcesvaried depending on the focal species and the spatial scale considered.Therefore, our results indicate that mutualistic interactions do not alwayseffectively compensate for competitive interactions in situations of spatialaggregation of flowering plants, at least in our study system. We highlightthe importance of explicitly considering the spatial structure at differentspatial scales of multitrophic interactions to better understand individualplant fitness and community dynamics.

Automated summary

The spatial aggregation of plants can both reduce seed production through competition and increase plant fitness through attracting pollinators. This study found that crowded neighborhoods decreased individual seed production but increased visitation rates and plant fitness by certain pollinator guilds. The balance between these forces varied depending on the focal species and spatial scale.