- Summary 1. Hutchinsonian niche theory posits that organisms have fundamental abiotic resource requirements from which they are limited by competition. Organisms also have fundamental biotic requirements, such as mutualists, for which they also might compete. 2. We test this idea with a widespread ant–plant mutualism. Ant-mediated seed dispersal (myrmecochory) in eastern North America involves a few ant species that can effectively disperse the seeds of many plant species. This imbalance suggests that ant-dispersed plants (myrmecochores) might compete for ant dispersers. We hypothesized that, because larger seeds are more attractive to ants, myrmecochores might segregate the timing of seed release by size to relieve competition. 3. Comparative literature analysis across plant species reveals that myrmecochore seed size increases with the fruiting season in a staggered pattern so that small- and large-seeded co-occurring species do not release seeds at the same time – a pattern not observed in plants using other dispersal modes. 4. We then presented foraging ants with small and large seeds in field trials throughout the fruiting season to test whether the observed temporal segregation in myrmecochore seed size is consistent with plant competition for ant dispersers. 5. Our results show that dispersal rates for smaller seeds increase across the growing season, but only in the absence of large seeds. Our combined literature and field data suggest that myrmecochores stagger fruiting by seed size so that small seeds are set earlier to avoid competition for dispersal mutualists with larger seeds. 6. Synthesis. Ecological interactions are often treated as either positive or negative, but our data blur this distinction by revealing that a positive interaction (mutualism) might be structured by a negative interaction (competition). Moreover, the recognition of biotic resources as critical niche requirements blurs the classic dichotomy between the fundamental (abiotic) versus realized (biotic limited) niche.