Temporal shifts in interactions between alien trees and the alien Argentine ant on native ants

Synergistic effects between various stressors on native biodiversity are poorly understood, especially adverse synergisms between different invasive organisms. While it is known that alien trees and the invasive Argentine ant Linepithema humile individually impact indigenous ant assemblages, little is known about how the impacts of these two types of aliens have a joint effect, and even less about the temporal dynamics of this interaction. We found that the Argentine ant benefited from invasion by alien trees through creation of less extreme environmental conditions at the hottest and coolest times of the year. We also found that the impacts of the two types of aliens were synergistic on the native ant assemblage. However, this synergistic impact also varied in intensity throughout the year, especially so in the more open natural sites. The alien tree canopy, when dense, created more constant environmental conditions throughout the year, leading to more constant Argentine ant levels and consequently more even impacts on the native ants. The various native ants varied considerably in their response to the impacts of both types of aliens, with many formicine ants being particularly sensitive. Our results show that the synergistic impacts, both negative and positive, of the two alien types are particular to each native ant species. Furthermore, the intensity of the adverse synergism is highly variable across the year.