Resumen

Ecological theory has long supported the idea that species coexistence in a homogeneous habitat is promoted by spatial structure, but empirical evidence for this hypothesis has lagged behind theory. Here we describe a Neotropical ant–plant symbiosis that is ideally suited for testing spatial models of coexistence. Two genera of ants, Allomerus cf. demerarae and three species of Azteca are specialized to live on a single species of ant‐plant, Cordia nodosa, in a Western Amazonian tropical rain forest. Empirically, using census data from widely separated localities, we show that the relative colonization abilities of the two ant genera are a function of plant density. A parameterized model shows that this pattern alone is sufficiently robust to explain coexistence in the system. Census and experimental data suggest that Azteca queens are better long‐distance flyers, but that Allomerus colonies are more fecund. Thus, Azteca can dominate in areas where host‐plant densities are low (and parent colony–sapling distances are long), and Allomerus can dominate in areas where host‐plant densities are high. Existing spatial heterogeneity in host‐plant densities therefore can allow regional coexistence, and intersite dispersal can produce local mixing. In conclusion, a dispersal–fecundity trade‐off appears to allow the two genera to treat spatial heterogeneity in patch density as a niche axis. This study further suggests that a spatially structured approach is essential in understanding the persistence of some mutualisms in the presence of parasites