Mixed responses of ant assemblages to Carpobrotus spp. and Rattus rattus eradications on a small Mediterranean island

Abstract

Abstract Eradication of invasive alien species (IAS) is often proposed to restore invaded ecosystems, with information on subsequent ecosystem recovery key to conservation policies. Although ants perform major ecological functions in the ecosystem, their response to IAS eradication has received relatively little monitoring. This study investigated ant response to Carpobrotus spp. (iceplant) and R. rattus (black rat) eradications on the small Mediterranean island of Bagaud (Var, France). Ant assemblages were monitored over a ten-year period, including two years before eradications, at six different sites, two invaded by Carpobrotus spp., two under high R. rattus pressure, and two control sites. We found inter-annual variations in ant assemblages but no before-after eradication trend at both control and rat eradication sites. However, there was a clear increase in ant species richness and abundance after the Carpobrotus eradication. A trend toward more homogeneous assemblages was observed, with an enhanced core of common Mediterranean ant species. Xerophilous and thermophilous species such as Pheidole pallidula, Messor bouvieri, and Plagiolepis pygmaea were more abundant, consistent with the return of native vegetation communities and possibly warmer and dryer microclimatic conditions. The trend was even stronger on the denser and thicker Carpobrotus eradication patch. Increased ant foraging activity is a good sign of restoration success, influencing different functions of the ecosystem, such as seed dispersal or nutrient cycling.