Interspecific variation in nectar and pollen reward among plant species commonly planted to support pollinators

Pollinators facilitate the reproduction of most wild plants. Declines in pollinator populations have led to interest in establishing floral enhancements to support them. However, knowledge about how food rewards vary across plant species planted to support pollinators remains scant. Using a greenhouse common garden, we quantified nectar volume, nectar sucrose, and pollen protein quantity for 11 species commonly planted to support pollinators in Mediterranean climates. We evaluated if variation was explained by life history and the potential for tradeoffs between nectar and pollen quality. There was high variation in nectar and moderate variation in pollen protein across species. The probability of producing nectar was higher for perennial plants. For flowers that contained nectar, there were no differences between annual and perennial plants in nectar traits. The species that produced the highest quality nectar (Scrophularia californica) produced pollen with the lowest protein amounts, and the species that produced the lowest quality nectar (Madia elegans) produced pollen with the highest protein amounts. There was no statistical association between pollen protein amounts and nectar traits across species. The nutrition pollinators obtain from foraging on most species studied here may come primarily in the form of carbohydrates or protein, but not both. However, one species (Epilobium canum) produced both high-quality nectar and pollen, so identifying others that produce similarly broad, high-quality rewards would be worthwhile. Our results also suggest that including perennial plants in pollinator gardens will facilitate pollinator acquisition of carbohydrates.