Ants enhance reproductive output in a tropical plant species independent of herbivore suppression

In mutualistic ant–plant interactions, ants are widely recognized for reducing herbivory, benefiting plants through decreased leaf damage, lower florivory, and enhanced fruit production. Although broadly supported, the magnitude of these benefits can vary with local biotic and abiotic conditions, and few studies have simultaneously quantified ant activity, herbivore abundance, and multiple components of plant reproduction, leaving the mechanisms underlying ant-mediated benefits unresolved. To address this gap, we conducted a field experiment in the Brazilian Cerrado, experimentally manipulating ant presence on the extrafloral nectar–bearing plant Senna rugosa (Fabaceae). We monitored ant activity, herbivore abundance, and reproductive traits—including bud, flower, and seed production, as well as seed weight—over a full growing season to assess whether ants reduced herbivore abundance and whether this translated into measurable gains in plant reproductive output. Using structural equation modelling, we quantified the relative importance of direct effects of ants on plant reproduction and indirect effects mediated through changes in herbivore abundance. Ant exclusion effectively reduced ant abundance and, unexpectedly, also decreased herbivore abundance, while simultaneously lowering bud and flower production. However, seed number and seed weight did not differ between treatments. Structural equation modelling revealed strong direct positive associations between ant presence and multiple reproductive traits, including bud and flower production, seed number, and seed weight, but no evidence for indirect effects through herbivore suppression. These results suggest that ants enhance plant reproduction primarily through mechanisms other than herbivore control. Overall, our findings highlight the complexity of ant–plant 0mutualisms and provide new mechanistic insights into how ants influence reproductive success in extrafloral nectar systems.