Defensive mutualists affect outcross pollen transfer and male fitness in their host plant

Ant guards can increase plant fitness by deterring herbivores, but they may also reduce it by interfering with pollination. While ant impacts on herbivory have been well-studied, much less is known about their impacts on pollinators and associated consequences for plant pollination, particularly pollen transfer dynamics and outcrossing/selfing rates. We used field experiments to quantify the effect of ant guards on pollinator community composition, frequency and duration of flower visits, and cascading effects on outcrossing pollen transfer and pollen exports in Turnera velutina (Passifloraceae). Although ant patrolling did not affect pollinator community composition or visitation frequency, it decreased flower visit duration and the time pollinators spent foraging inside flowers. Such behavioural changes resulted in reduced pollen deposition on stigmas, decreased pollen exports (a proxy for male fitness) and significantly doubled outcross pollen transfer. This study contributes to our understanding of how nonpollinator mutualists can shape plant reproductive processes. We discuss the downstream effects that variation in biotic defences, such as rewards for guarding ants, can have on plant pollen transfer patterns and fitness. In conclusion, guarding ants influence pollen transfer patterns in Turnera velutina, increasing outcrossing in a self-compatible species at the cost of male fitness. We show how non-pollinators, such as defensive ant mutualists, can shape plant reproductive traits and discuss the consequences these interactions may have for plant mating systems.