Ant-aphid mutualisms in the face of ant invasions: do novel partners affect diversity within mutualist and parasitoid guilds?

Background/Question/Methods

Invasive ants often form protection mutualisms with plants or insects that offer sugar-rich food rewards. The infiltration of mutualisms by non-native ants can disrupt such interactions, and negatively affect mutualist partners and give rise to broader ecological effects. Here we examine how the Argentine ant affects diversity in a multi-species ant-aphid mutualism on arroyo willow (Salix lasiolepis). To test this hypothesis, we selected six pairs of willow-dominated riparian sites with invaded and uninvaded areas in close proximity. We addressed the following questions. (1) Does diversity within the aphid mutualist guild change following invasion by the Argentine ant? (2) Do parasitoid wasp assemblages differ in diversity in areas with the Argentine ant compared to areas with native ants? To determine aphid and parasitoid identity, we used PCR to amplify the COI barcode region of aphids tended by ants. DNA obtained from parasitized aphids amplified as the COI for each wasp species, allowing us to determine level of parasitism and diversity.

Results/Conclusions

Within our six pairs of sites, ant diversity was greater in areas not yet invaded by the Argentine ant (1 +/- 0 SE vs. 2.8 +/- 0.3 SE species). We sampled 660 aphids from 220 aggregations living on willows from five genera within the family Aphididae. Aphid richness was greater on willows at uninvaded sites (2.3 +/- 0.3 SE vs. 1 +/- 0 SE species) compared to areas invaded by the Argentine ant. Furthermore, aphid diversity increased with greater ant diversity. Overall, we observed similar levels of parasitism between areas with and without the Argentine ant (6 percent). For the most prevalent native aphid (Chaitophorus nigrae), the Argentine ant was not consistently better at preventing parasitism, and the Argentine ant allocated similar numbers of foragers compared to native ants when protecting aphid aggregations. Changes in parasitoid communities appear to result from decreased host richness rather than increased aphid protection. These results illustrate the potential for non-native mutualists to change the composition of mutualist guilds with effects also ramifying to higher trophic levels.