Ant richness within 14 Colombian dry forest fragments varies from at least 34–128 species. We have collected the little fire ant, Wasmannia auropunctata (Roger) in each of the fourteen forest fragments, by means of tuna baits, pitfall traps, sampling of logs, understory vegetation and forest litter sampling. A highly significant negative relation was found between the percentage of W. auropunctata samples per forest fragment and richness for the ant community. Additionally, a significant positive relationship was found between the abundance of W. auropunctata and the number of ant-plant associations in nine of the forest fragments. The displacement of other ants by W. auropunctata could be explained in terms of its competitive performance, natural history and ability to exploit disturbed sites. This opportunistic species displays high interspecific aggressiveness, recruiting workers massively to a variety of food resources. It also colonizes different substrates and forms unicolonial societies, which disperse broadly by detaching groups of workers along with multiple queens. The species can be easily monitored with tuna baits, which showed the same significant negative relation. We propose that the abundance of the little fire ant can be used as an indicator of low diversity ant communities in dry forest fragments of the Cauca river valley of Colombia.