Pollinator visitation on Na-enriched plants in a subalpine meadow

Many plants have evolved nutrient rewards to attract pollinators to
flowers, but most research has focused on the sugar content of floral
nectar resources. Concentrations of sodium in floral nectar (a
micronutrient in low concentrations in nectar) can vary substantially both
among and within co-occurring species. Sodium concentrations in floral
nectar might play an important and underappreciated role in
plant-pollinator interactions, especially because many animals, including
pollinators, are sodium-limited in nature. Yet, the consequences of
variation in sodium concentrations in floral nectar have gone largely
unexplored. Here, we investigate whether enriching floral nectar with
sodium influences the composition, diversity, and abundance of pollinator
interactions. We experimentally enriched sodium concentrations in four
plant species in a subalpine meadow in Colorado, USA. We found that
flowers with sodium-enriched nectar received more visits from a greater
diversity of pollinator visitors throughout the season. Each pollinator
species foraged more frequently on flowers enriched with sodium and showed
evidence of other changes to pollinator foraging behavior, including
greater dietary evenness. These findings are consistent with the ‘salty
nectar hypothesis,’ providing evidence for the importance of sodium
limitation in pollinators and suggesting that even small nectar
constituents can shape plant-pollinator interactions.