Chemical Defenses Against Floral Enemies: Costs and Benefits of Toxic Nectar to Nectar Robbing, Pollination, and Plant Fitness

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ADLER - 0211480In natural systems, organisms face simultaneous direct and indirect interactions with mutualists andantagonists, yet our understanding of the relative importance of these competing pressures inshaping the evolution of phenotypic traits is still rudimentary. For example, flowering plants mayface the dilemma of attracting pollinators while discouraging floral enemies, such as nectarrobbers. Because robbers are competing with pollinators for the same resource (e.g., nectar), it maybe particularly difficult for plants to evolve defensive mechanisms that are effective againstantagonists without also deterring mutualists. The goal of the proposed research is to determinehow a ubiquitous but poorly understood floral trait, toxic nectar, influences interactions with bothnectar robbers and pollinators, and how these simultaneous interactions shape selection on thisphenotypic trait.Toxic nectar, or nectar that contains secondary compounds, is a widespread trait that may conferresistance to floral robbers, but at a potential cost to pollinator attraction. Currently, the costs andbenefits of toxic nectar are unknown. In addition to resistance traits, plants may cope withantagonists via tolerance. The concepts of resistance and tolerance have traditionally been used inreference to herbivores; however, nectar robbers, like herbivores, have profound effects on plantfitness and plants are likely to use resistance and tolerance as defensive mechanisms againstrobbers. The proposed research will extend resistance and tolerance concepts to a wider range ofplant-animal interactions through an examination of the role of nectar traits, such as nectar toxicityand production, as mechanisms of resistance and tolerance to nectar robbing.The aim of this work is to understand the ecological role of toxic nectar within the context ofinteractions between mutualists (pollinators) and antagonists (nectar robbers), and to determine theextent to which resistance and tolerance mediate the impact of nectar robbers on plant genotypes.The research proposed will experimentally manipulate nectar toxicity in Gelsemium sempervirens,a perennial vine with documented alkaloids in its nectar, to determine the impact of toxic nectar onthe community of insect pollinators and robbers as well as on male and female plant reproductionand offspring success. In addition, this study will manipulate nectar robbing and pollination ingenotypes of G. sempervirens with known variability in nectar toxicity to determine the direct andindirect effects of nectar robbing on plant fitness, genetic variation in resistance and tolerance tonectar robbing, and costs of toxic nectar in the absence of nectar robbing. The research combinesmanipulative field experiments with observations to address four main questions: 1) Is theregenetic variation in nectar toxicity? 2) Does toxic nectar benefit plants by increasing resistance tonectar robbing without reducing pollinator attraction? 3) Does nectar robbing reduce plant fitnessdirectly or indirectly, and do plant genotypes vary in their resistance or tolerance of nectar robbing?and 4) Is there a cost of toxic nectar in the absence of robbing?The work proposed here is novel in that it integrates the costs and benefits of resistance associatedwith a ubiquitous but poorly understood floral trait, toxic nectar, in the context of joint forces ofselection exerted by both mutualistic and antagonistic floral visitors. This work extends an existingconceptual framework and provides empirical insight into the diversity of roles a single floral traitcan serve, from attraction to defense. Consideration of the broader context in which organismsevolve is fundamental to our understanding of how multiple-species interactions shape genetic andbiological diversity at ecological, microevolutionary, and macroevolutionary scales.