Numerical Dominance of the Argentine Ant vs Native Ants and Consequences on Soil Resource Searching in Mediterranean Cork-Oak Forests (Hymenoptera: Formicidae)

Abstract

This study is focused on the dominance exerted by the invasive Argentine ant over native ants in a coastal Mediterranean area. The impact of this invasive ant on native ant assemblages and its consequences on total ant biomass and on the intensity of habitat exploration were evaluated. Foraging ants were observed and their trajectories recorded during 5-minute periods in two study zones, one invaded and the other non-invaded. Ant species detected, ant worker abundance, ant biomass and the intensity of soil surface searching done by ants were compared between the two zones. The Argentine ant invasion provoked a drastic reduction of the ant species richness. Apparently only one native ant species is able to coexist with the Argentine ant, the cryptic Plagiolepis pygmaea. Ant worker abundance was also modified after the invasion: the number of Argentine ant workers detected, which represented 92% of the invaded zone, was two times higher than the number of native ant workers detected in the non-invaded zone. The total ant biomass was inversely affected, becoming four times lower in the invaded zone highly dominated by Linepithema humile. The higher number of Argentine ant workers and their fast tempo of activity implied an alteration of the intensity of soil surface searching: scanning by the Argentine ants in the invaded zone was higher than that done by the native ants in the non-invaded zone, and the estimated time for a complete soil surface scan was 64 minutes in the invaded zone and 108 minutes in the non-invaded zone. Consequently, resources will be discovered faster by ants in the invaded zone than in the non-invaded zone. The increase of the mean temperature and the decrease of the relative humidity from May to August reduced the ant activity in the two study zones but this reduction was greater in the invaded zone