Abstract
Ant guards protect plants from herbivores, but can also hinder pollination by damaging reproductive structures and/or repelling pollinators. Natural selection should favour the evolution of plant traits that deter ants from visiting flowers during anthesis, without waiving their defensive services. The Distraction Hypothesis posits that rewarding ants with extrafloral nectar could reduce their visitation of flowers, reducing ant‐pollinator conflict while retaining protection of other structures.We characterised the proportion of flowers occupied by ants and the number of ants per flower in a Mexican ant‐plant, Turnera velutina. We clogged extrafloral nectaries on field plants and observed the effects on patrolling ants, pollinators and ants inside flowers, and quantified the effects on plant fitness. Based on the Distraction Hypothesis, we predicted that preventing extrafloral nectar secretion should result in fewer ants active at extrafloral nectaries, more ants inside flowers and a higher proportion of flowers occupied by ants, leading to ant‐pollinator conflict, with reduced pollinator visitation and reduced plant fitness.Overall ant activity inside flowers was low. Preventing extrafloral nectar secretion through clogging reduced the number of ants patrolling extrafloral nectaries, significantly increased the proportion of flowers occupied by ants from 6.1% to 9.7%, and reduced plant reproductive output through a 12% increase in the probability of fruit abortion. No change in the numbers of ants or pollinators inside flowers was observed. This is the first support for the Distraction Hypothesis obtained under field conditions, showing ecological and plant fitness benefits of the distracting function of extrafloral nectar during anthesis. Synthesis. Our study provides the first field experimental support for the Distraction Hypothesis, suggesting that extrafloral nectaries located close to flowers may bribe ants away from reproductive structures during the crucial pollination period, reducing the probability of ant occupation of flowers, reducing ant‐pollinator conflict and increasing plant reproductive success.
Highlights
Ant‐plants recruit ants by providing nesting sites and/food resources, and benefit from ant‐mediated reduction in damage by herbivores and pathogens (Bentley, 1977a; Chamberlain & Holland, 2009; Herrera & Pellmyr, 2009; Janzen, 1966; Rosumek et al, 2009; Trager et al, 2010)
Based on the Distraction Hypothesis, we predicted that preventing extrafloral nectar secretion should result in fewer ants active at extrafloral nectaries, more ants inside flowers and a higher proportion of flowers occupied by ants, leading to ant‐pollinator conflict, with reduced pollinator visitation and reduced plant fitness
Preventing extrafloral nectar secretion through clogging reduced the number of ants patrolling extrafloral nectaries, significantly increased the proportion of flowers occupied by ants from 6.1% to 9.7%, and reduced plant reproductive output through a 12% increase in the probability of fruit abortion
Summary
Ant‐plants recruit ants by providing nesting sites and/food resources, and benefit from ant‐mediated reduction in damage by herbivores and pathogens (Bentley, 1977a; Chamberlain & Holland, 2009; Herrera & Pellmyr, 2009; Janzen, 1966; Rosumek et al, 2009; Trager et al, 2010). Extrafloral nectaries on leaves associated with flowers of the Mexican ant‐plant Turnera velutina (Passifloraceae) secrete more nectar with higher sugar content than extrafloral nectaries on leaves bearing buds and fruits (Villamil, 2017) This increase in EFN secretion during anthesis is compatible with the Distraction Hypothesis, in that EFN secretion near flowers could lure and bribe ants that might otherwise enter flowers seeking floral nectar. If the Distraction Hypothesis is true, we predict that experimental elimination of extrafloral nectar secretion should: (a) reduce ant visitation to extrafloral nectaries, (b) increase the numbers of ants inside flowers, (c) increase the proportion of flowers occupied by ants, (d) leading to decreased levels of floral visitation by pollinators and (e) a reduction in plant fitness We evaluated the potential ecological and fitness consequences of the Distraction Hypothesis, addressing the following questions: (a) How often are flowers occupied by ants and how many ants are found in them? (b) Does preventing EFN secretion affect the number of ants patrolling extrafloral nectaries, the number of ants inside the flowers or the number of pollinators visiting flowers? (c) Does preventing EFN secretion increase the probability of a flower being occupied by ants? (d) Does the number of ants at extrafloral nectaries or inside flowers affect pollinator visitation? (e) Does preventing EFN secretion affect plant fitness? If the Distraction Hypothesis is true, we predict that experimental elimination of extrafloral nectar secretion should: (a) reduce ant visitation to extrafloral nectaries, (b) increase the numbers of ants inside flowers, (c) increase the proportion of flowers occupied by ants, (d) leading to decreased levels of floral visitation by pollinators and (e) a reduction in plant fitness
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