Relevance of Allee effects in eusocial insects: experimental evidence in two ant species

The Allee effects gather phenomena in which per capita population growth rate increases with increasing population size or density: the more individuals, the better they perform, for example for survival or reproduction, and the better they get at the next time step. Conversely, under a given threshold, populations suffering from Allee effects can get increasingly smaller as their fitness decrease, driving them into a vortex of extinction. Allee effects are a direct consequence of individual aggregation and cooperation. Detecting and estimating the presence and strength of Allee effects has recently been shown to be of great relevance in conservation of endangered species and management of invasive species. Eusocial insects are strong candidates for many Allee effect mechanisms, due to the inherent role of cooperation in their social structure. Despite this, Allee effects have not been studied in eusocial insects. Here, we provide evidence of the presence of Allee effects in two ant species, Linepithema humile (Argentine ant) and Tapinoma nigerrimum. We set up laboratory colonies containing different initial number of queens (1, 3, 6) and workers (10, 30, 300) and monitored their brood production monthly and queen and worker survival during 18 weeks. We found that some measures (e.g. queen and worker survival) are positively related to different components of colony size (number of workers and queens). This is the first demonstration of the presence of Allee effects in ants, here for accomplished for two species.