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Regular dispersion patterns in the spatial distribution of ant nests are linked to both defense of foraging areas and competition for food. They are frequently observed amongst colonies of conspecifics when the availability of colony foundation sites is homogeneous. In order to evaluate the degree of intraspecific competition in the dominant Neotropical ectatommine ant Ectatomma ruidum and the importance of interactions with another dominant sympatric ectatommine ant, E. tuberculatum, we examined the spatial relationships among colonies of these two species in an area with a naturally very high nest density (up to 11500 and 1500 nests/ha, respectively). Data were analyzed by calculation of the Clark and Evans nearest neighbor index and by statistical comparison of the observed mean distance to the nearest neighbor and a theoretical value obtained by Monte-Carlo simulation. The latter procedure allowed analysis of spatial relationships despite the non-haphazard location of E. tuberculatum nests, which were always found at the base of a tree. Whatever the nest density, the mean distance between nearest neighbor nests ofE. ruidum was always higher than that expected for random distribution. This regular dispersion pattern strongly supports the hypothesis that E. ruidum colonies are monodomous. In the three quadrats where they co-occurred, E. ruidum and E. tuberculatum were independently distributed, suggesting a low level of interaction between them. Even at high colony densities, the dominant character of each of these species does not appear to be incompatible with their co-existence, due to the low degree of niche overlap between them. Inglés

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