Indirect biotic resistance: aggressive native ants reduce invasive plant seed set by enhancing exotic aphid infestation

DEVEGILI, A.M.; LESCANO, MARÍA N.; GIANOLI, ERNESTO; FARJI-BRENER, ALEJANDRO G.

Simposio; Primer Simposio Iberoamericano de Mirmecología; 2020

Third-party species may modulate species invasion by affecting biotic interactions between exotic and local species. Aphid-tending ants are excellent models for examining these indirect effects in the context of plant invasion. On the one hand, ants can deter aphid predators increasing aphid infestation and decreasing exotic plant fitness. On the other hand, ants can deter non-aphid herbivores increasing exotic plant fitness. In the Patagonian steppe (Argentina), we studied native aphid-tending ants (Dorymyrmex tener, Camponotus distinguendus, and D. richteri) on exotic thistles attacked by aphids and weevils. We examined the relationships between the presence and abundance of these insects and thistle seed set (N=215 plants). We compared ant species aggressiveness towards ladybugs and syrphid larvae (aphid predators) and towards weevils (non-aphid thistle herbivore) (N=26?31 trials). To determine the effects of ants on thistle seed set, we performed ant-exclusions on thistle stems (N=20 plants). Using path analysis, we analyzed whether ants affect thistle seed set through an effect on aphids and/or weevils. Seed set was negatively associated with the abundance of the most aggressive ant species (D. tener). Dorymyrmex tener successfully removed aphid predators from plants but did not affect weevils. Excluding D. tener from stems increased thistle seed set by 38%. Path analysis supported a negative link between D. tener and thistle seed set through aphids. Results suggest that aggressive aphid-tending ant species may enhance biotic resistance by increasing aphid infestation on exotic plants. This study highlights the importance of indirect biotic interactions in modulating the success of invasive species.