The effects of nitrogen on plant-pollinator mutualisms

Nitrogen is essential in all biological systems and can limit plant growth and dramatically affect plant quality, community structure and species interactions. Although the effects of nitrogen on productivity and typical consumer-resource interactions (e.g., plant-herbivore interactions) have been well documented, its effects on mutualistic consumers and their hosts have rarely been addressed. Using flowering plants in subalpine systems and their bee pollinators, I tested the degree to which nitrogen addition (control, low-nitrogen, high-nitrogen) affected producers, consumers, their interactions, and the mechanisms by which those responses occurred. I measured the effects of nitrogen on plant and floral traits and determined how nitrogen cascades up to affect fitness estimates in both plants and pollinators. In this 3 year study, I found that low levels of nitrogen addition enhanced plant traits important for pollinator attraction, including flower and nectar production. I also found that some bumblebee species spent more time per flower visit in low-nitrogen than high-nitrogen plots. These trends may be due indirectly to lower soil pH in high-nitrogen than low-nitrogen plots. This study indicates that nitrogen addition alters pollinator behavior via changes in floral and nectar production, and I predict that these changes in behavior will affect plant and pollinator fitness. Thus, nitrogen availability may influence not only plant biomass and quality but also pollination mutualisms and has implications for coupled plant-pollinator dynamics in patchy resource environments.