Plant size affects mutualistic and antagonistic interactions and reproductive success across 21 Brassicaceae species

Plant size has been hypothesized to be a major driver of biotic interactions. However, it is little understood how plant size affects plant mutualists vs. antagonists and the plant's resulting reproductive success. We established a common garden experiment covering an interspecific plant size gradient (from 10 to 130 cm height) across 21 annual Brassicaceae species, thereby standardizing features of habitat and surrounding landscape. We assessed flower-visiting pollinators and florivores (pollen beetle adults and larvae) and the resulting effects of all these flower-visiting insects on plant reproductive success. Besides flower characteristics (size, abundance, color), plant size had a generally positive effect on abundance and species richness of pollinators as well as on abundance of pollen beetle adults and larvae. Pollen beetles reduced seed number as well as thousand-seed weight, whereas pollinators increased seed number only. Overall, increasing plant size led to less thousand-seed weight but had no effect on seed number, indicating counterbalancing effects of herbivory and pollination. In conclusion, seed number of large plant species should benefit from locations with many pollinators and few herbivores and small plant species' seed number from locations with few pollinators and many herbivores.