Extrafloral nectar mediates food-for-protection mutualisms between plants and ants. Ant—plant mutualisms are keystone associations, occurring within a complex web of biotic interactions. As such, these interactions may affect plant fitness in a number of ways, both positive and negative. In Senna mexicana var. chapmanii (Isely) H. S. Irwin & Barneby (Fabaceae), the presence of ants has been shown to reduce herbivory and increase fruit and seed set. These effects, however, are not the result of only one interaction but the balance of many. We conducted a field study to determine the processes by which ants affect reproductive fitness in S. chapmanii. Thirty plants were established in a semi-natural area adjacent to native pine rockland habitat in southern Florida. Ants were excluded from half of the plants by painting a sticky resin (Tanglefoot™) around the base of each stem. Over the course of a single flowering season (Oct–May), we observed the effects of ants on the activity of herbivores, predators, pollinators, and pre-dispersal seed predators. We also observed the overall effects of ants on plant size and reproductive output. Plants with ants were quicker to establish, grew larger, and produced floral displays that attracted more pollinators. Contrary to our expectations, relative pollinator efficiency and rates of pre-dispersal seed predation were unaffected by ants. In S. chapmanii, ants did not appear to affect the outcome of other plant—insect associations, but they affected the scale at which they occurred. Ants facilitated plant growth and establishment in S. chapmanii, with subsequent effects on reproductive investment.