Alien trees and the invasive Argentine ant Linepithema humile are known individually to impact indigenous ant assemblages, many species of which are important functional components of South African fynbos ecosystems. We studied the effect of the two alien taxa, tree and ant, on various species of indigenous ants. Alien trees significantly influenced abundance and species richness of local ant assemblages, as a result of the dense alien tree canopy changing environmental conditions of the ants' habitat. The Argentine ant displaced Pheidole capensis and certain Camponotus spp., and reduced abundance of other indigenous ants such as Lepisiota capensis and Plagiolepis spp. Displacement by the Argentine ant appears to arise from indirect competition for food resources. The effects of alien trees and the alien ant are synergistic, both positively and negatively, depending on the particular indigenous ant species in question. Thus the changes in the indigenous ant assemblage wrought by alien trees are further influenced by the presence of the alien ant. Removal of the alien trees disadvantages the Argentine ant and reverses the adverse synergism as a result of more sunlit natural conditions. We caution against the use of the word ‘synergistic’ to focus on particular species and interactions rather than being used as an umbrella term for general impact and threat to faunal assemblages.