Little is known about the biology of invasive ants in their native geographical zones, even though such knowledge can potentially provide useful insights into the factors that promote the ants' success when introduced into new areas. Here we focus on the demographic and social features of populations of the little fire ant Wasmannia auropunctata in disturbed and undisturbed habitats in French Guiana, one of its native geographical ranges, and compare these results with the situation in one of its introduced ranges, the island of New Caledonia. In disturbed areas (e.g., plantations, forest edges) of French Guiana, the populations can reach very high densities that totally saturate the ecosystem, as observed in its introduced range, with a resulting negative impact on the ant fauna in both situations. Both populations also have a unicolonial social organization with, nevertheless, one major difference: one single colony has invaded all of New Caledonia, while the Guianian populations form several distinct entities strictly limited to disturbed areas and highly aggressive toward each other. Conversely, colonies are rare in pristine forests where the species is found only occasionally and then mostly in flooded areas along river banks. In the latter case, colonies belonging to two major behavioural entities were found to intermingle along a 7 km-long area and were also aggressive towards each population in disturbed sites. This thus demonstrates the important role disturbances generated by human activity play in the apparition of potentially invasive species and also shows that biological invasions can sometimes occur first in a species' native range before the propagules are transported to a new region.