We report on the distribution, behaviour, interspecific competition and temporal persistence over four years of arboreal ants in a mangrove forest within which patches of habitat ranged from single isolated trees to groups of interlocking trees. Two ants, an undescribed species of Crematogaster and Anonychomyrma itinerans, were common but only one ever occurred within any occupied patch. Sampling of the same 108 trees in May 2008, December 2009 and May 2012 showed very little temporal change in occupancy by both species. In manipulative experiments where the branches of closely adjacent pairs of neighbouring Avicennia marina trees containing these different species were clamped together, the ants fought and one species succeeded in taking over both trees. Thirteen of 20 clamped pairs were taken over by Crematogaster and seven by A. itinerans. Occupancy of conjoined patches did not change during the following year, which suggests the ant species present is not determined by any inherent heterogeneity among patches. Since ants can affect herbivory, community composition and tree health, we suggest the success of attempts to restore or conserve mangroves may be improved by long-term comparative studies of the effects of ants, including possible differences among species, in natural and artificially planted mangrove forests.