Mixed responses of ant communities to the eradication of black rats and iceplants on a small Mediterranean island

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 iceplant (Carpobrotus spp.) and black rat (Rattus rattus) eradications on the small Mediterranean island of Bagaud (Var, France). Ant communities were monitored over a ten-year period, including two years before eradications, at six different sites: two invaded by iceplants, two under high rat pressure, and two native vegetation sites without intervention. We found inter-annual variations in ant communities but no before-after eradication trend at both native vegetation and rat eradication sites. However, there was a clear increase in ant species richness and abundance score after the iceplant eradication. A core of common Mediterranean species, including Pheidole pallidula, Messor bouvieri, and Plagiolepis pygmaea, increased their foraging activity after the removal. As xerophilous and thermophilous species they would benefit from the return of native vegetation with possibly warmer and dryer microclimatic conditions, but also from habitat and resource diversification. The trend was even stronger on the denser and thicker iceplant eradication patch. Our results emphasize the relevance of implementing ant monitoring to evaluate the effectiveness of such restoration and conservation strategies.