Beyond predator bodyguards: ant attendance at post-floral pericarpial nectaries increases fruit production, improves fruit morphology, and reduces nectar thief visitation in Declieuxia fruticosa (Rubiaceae)

Declieuxia fruticosa produces branches with inflorescences that simultaneously bear floral buds, distylous flowers, and fruits with post-floral pericarpial nectaries (PPNs). These branches attract ants, pollinators, herbivores, and nectar thieves. Ants feeding on PPNs may act as plant bodyguards against herbivores, but they can also influence floral visitors, potentially generating both costs and benefits for plant reproduction. However, the net effects of these multitrophic interactions on plant fitness remain unclear. We tested the hypothesis that ant presence at PPNs enhances fruit production and improves fruit morphological traits in D. fruticosa by repelling herbivores and nectar thieves, while not interfering with legitimate pollinators. Field experiments and observations were conducted in Uberlândia, MG, Brazil, manipulating ant presence and absence. We quantified fruit number, weight, width, and length, and evaluated stigma receptivity, pollen availability, and the assemblage of floral visitors and herbivores. Our results demonstrate that ant presence at PPNs significantly increases fruit set and all evaluated morphological traits. Fruits produced in the presence of ants were, on average, 25% larger and 2.4 times heavier than those from ant-excluded plants. Moreover, ant presence was associated with a 2.9-fold reduction in the frequency of nectar thieves and a 65% increase in visitation frequency by Apis mellifera, the main pollinator. These findings indicate that ant-mediated protection against herbivores and floral exploiters, without substantial interference in pollination, directly enhances plant fitness. Overall, our study highlights the role of ants in shaping plant reproductive success through direct and indirect multitrophic interactions.