Experimental manipulation of pollen protein demonstrates an indirect effect on plant fitness, mediated by pollinator behavior

Mutualism is fundamental to ecosystem function and increasingly at risk due to anthropogenic change. For example, soil quality can be affected by agricultural practices, which can affect floral reward quality (pollen protein content) with unknown consequences for plant-pollinator mutualisms. In a controlled field experiment, we manipulated the pollen protein of Helianthus annuus with chemical fertilizer to explore how soil quality affects plant reproduction as mediated by changes in pollen nutrient content. There were no direct effects of pollen protein on plant reproduction in the absence of visiting insects. Instead, pollinators spent more time foraging for pollen on fertilized plants and pollen foragers were more efficient at setting seed. Moreover, changes in visitor behavior had a significant effect on the number of seeds produced, independent of fertilizer treatment. These results suggest that soil quality can alter plant fitness mediated by changes in pollen quality and pollinator foraging behavior.