Effect of propagule pressure on the establishment and spread of the little fire ant Wasmannia auropunctata in a Gabonese oilfield

We studied the effect of propagule pressure on the establishment and subsequent spread of the invasive little fire ant Wasmannia auropunctata in a Gabonese oilfield in lowland rain forest. Oil well drilling, the major anthropogenic disturbance over the past 21 years in the area, was used as an indirect measure of propagule pressure. An analysis of 82 potential introductions at oil production platforms revealed that the probability of successful establishment significantly increased with the number of drilling events. Specifically, the shape of the dose-response establishment curve could be closely approximated by a Poisson process with a 34% chance of infestation per well drilled. Consistent with our knowledge of largely clonal reproduction by W. auropunctata, the shape of the establishment curve suggested that the ants were not substantially affected by Allee effects, probably greatly contributing to this species' success as an invader. By contrast, the extent to which W. auropunctata spread beyond the point of initial introduction, and thus the extent of its damage to diversity of other ant species, was independent of propagule pressure. These results suggest that while establishment success depends on propagule pressure, other ecological or genetic factors may limit the extent of further spread. Knowledge of the shape of the dose-response establishment curve should prove useful in modelling the future spread of W. auropunctata and perhaps the spread of other clonal organisms.