Synergistic impact of invasive alien trees and the alien Argentine ant on local ant assemblages in the Cape Floristic Region

There is worldwide concern over the impact of invasive alien organisms on indigenous biota. Alien tree invasions have reached major proportions in the global biodiversity hotspot, the Cape Florsitic Region (CFR). Furthermore, the invasive Argentine ant (Linepithema humile) has also invaded the CFR. Both the invasive plants and the invasive ant are known to impact indigenous ant assemblages. This is of concern as certain indigenous ants are essential for dispersal of the seeds of almost 1300 species of indigenous plants. The impact of these invasives and the synergism between them, as they affect terrestrial epigaeic invertebrates, has been little studied, and is the subject of this study. Study sites varied from invaded to non-invaded by both plants and ant. Each site was sampled using four pitfall traps over ten months to sample ants. Forty ant species were sampled, and various analyses showed that invasive alien plants had a significant impact on the abundance and richness of the ant assemblage. The dense alien tree canopy cover changed the abiotic environment of the epigaeic ants' habitat. Furthermore, increasingly intense alien tree invasions correlated significantly with Argentine ant abundance. The Argentine ant displaced Pheidole capensis and certain Camponotus spp., and decreased the abundance of other indigenous ants, such as Lepisiota capensis and Plagiolepis spp. Displacement by the Argentine ant may be a result of indirect competition for food resources. The effects of invasive alien trees and the alien ant are synergistic, with a cascade effect from initial plant invasion to subsequent animal invasion.