Accessibility and resource quality drive flower visitation patterns among native perennial species

Abstract

Pollinators navigate a complex and dynamic nutritional landscape while foraging for floral resources. Bees are a group of flower-visiting insects that rely on pollen as their sole protein source, and thus, bees have strong incentives to seek pollen with high protein content. Indeed, research has shown that bees may prefer to visit flowers with high-protein pollen, but the mechanisms by which bees are able to detect plants with this high-protein pollen are unknown. One hypothesis is that plants with high-protein pollen advertise this resource quality through volatile emissions. We established 17 native perennial plant species from three plant families (Fabaceae, Asteraceae, and Lamiaceae) in a large field experiment to explore the relationship between nutritional quality, inflorescence volatile emissions, and pollinator visitation. We sampled twenty garden plots composed of these native plant species for 2 years. Our results showed that floral morphology significantly affected pollinator visitation; floral morphology that restricted the accessibility of floral resources reduced the overall foraging female bee visitation rate. In contrast, the visitation rate of foraging female bumble bees was higher on plants with floral morphology that restricted access. Moreover, we showed that (1) plants with less accessible inflorescences had significantly higher pollen protein content and (2) lower volatile emissions, while (3) there was a significant interaction between accessibility and pollen protein for foraging female honey bee visitation; honey bees preferred accessible flowers with lower pollen protein. We found no evidence in support of the hypothesis that floral volatiles advertise pollen protein content. Overall, floral accessibility related significantly to both floral volatile emissions and pollen protein content, determining both the identity of floral visitors and affecting the frequency with which they visited.