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

Authors: MANSON, JESSAMYN - Dartmouth College; Cook, Daniel; Gardner, Dale; IRWIN, REBECCA - Dartmouth College

Submitted to: Journal of Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2013
Publication Date: 11/1/2013
Publication URL: http://handle.nal.usda.gov/10113/62989
Citation: Manson, J.S., Cook, D., Gardner, D.R., Irwin, R.E. 2013. Dose-dependent effects of nectar alkaloids in a montane plant-pollinator community. Journal of Ecology. 1001:1604-1612.

Interpretive Summary: 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 significantly lower than those found in its leaves or flowers. By manipulating nectar alkaloid concentrations in the field and lab, 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 bumble bee 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 lab, the direct consumptive effects of nectar alkaloids on bumble bee pollinators were also concentration-dependent. Bumble bees exhibited reduced mobility and vigour but only at alkaloid concentrations more than 25-times higher than those found in natural nectar. We found that nectar alkaloids have dose-dependent effects on pollinator behavior and activity. While concentrations of nectar alkaloids rivaling 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.

Technical Abstract: 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 lab, 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 bumble bee 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 lab, the direct consumptive effects of nectar alkaloids on bumble bee pollinators were also concentration-dependent. Bumble bees exhibited reduced mobility and vigour but only at alkaloid concentrations more than 25-times higher than those found in natural nectar. We found that nectar alkaloids have dose-dependent effects on pollinator behavior and activity. While concentrations of nectar alkaloids rivaling 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.