Dose-dependent effects of nectar alkaloids in a montane plant-pollinator community

Although secondary metabolites are prevalent in floral nectar, the ecological consequences for pollinators and pollination remain relatively unexplored. While often deterrent to pollinators at high concentrations, secondary metabolite concentrations in nectar tend to be much lower than secondary metabolite concentrations in leaves and flowers; yet, they may still affect the maintenance of pollination mutualisms.Delphinium barbeyi, a common montane herb, contains norditerpene alkaloids in its nectar but at concentrations that are substantially lower than those found in its leaves or flowers. By manipulating nectar alkaloid concentrations in the field and laboratory, we assessed the degree to which varying concentrations of alkaloids in nectar influenced pollinator behaviour and activity and plant reproduction. In the field, nectar alkaloids significantly reduced both the number of flower visits and the time spent per flower by free-flying bumblebee pollinators, but we only observed effects at alkaloid concentrations 50 times that of natural nectar. When we supplemented D.barbeyi nectar with alkaloids at concentrations almost 15 times that of natural nectar, we found no evidence for direct or pollinator-mediated indirect effects on female plant reproduction. In the laboratory, the direct consumptive effects of nectar alkaloids on bumblebee pollinators were also concentration dependent. Bumblebees exhibited reduced mobility and vigour but only at alkaloid concentrations more than 25 times higher than those found in natural nectar.Synthesis. We found that nectar alkaloids have dose-dependent effects on pollinator behaviour and activity. While concentrations of nectar alkaloids rivalling those found in leaves would negatively affect pollinator behaviour and pollination services, the natural concentrations of nectar alkaloids in D.barbeyi have no negative direct or pollinator-mediated indirect effects on plant reproduction. These results provide experimental insight into the dose-dependent ecological consequences of nectar secondary metabolites for pollinators and pollination, suggesting that low nectar alkaloid concentrations incurred no ecological costs for D.barbeyi.