Nectar quality changes the ecological costs of chemically defended pollen

Plants often compete in a marketplace that involves the exchange of floral rewards for pollination service [1]. This marketplace is frequently viewed as revolving around a single currency, typically nectar. While this focus has established pollinators such as bees as classic models in foraging ecology, in reality many plants provide both pollen and nectar, which vary in composition within and across species [2]. How this complexity impacts interactions between plants, pollinators, and co-flowering competitors is unknown. We explored how variation in two axes of reward chemistry - nectar sugar and pollen alkaloid content - impacted competition for bumblebee visits. The effect of variation in one reward depended on the presence and quality of the other - bees discriminated against flowers with more defended pollen when all flowers offered the same quality nectar. However, bees preferred flowers with highly defended pollen when they offered higher quality nectar, suggesting that attractive nectar can overcome the ecological costs of defended pollen. Recognizing the interdependence of these floral currencies may help identify traits that drive indirect interactions between plants and clarify broader evolutionary patterns of floral reward phenotypes.