The mutualism between ants and trophobiont insects is widely known as an interaction where the trophobionts are usually defended from their natural enemies by the ants. Ants can also remove herbivores, causing changes in the arthropod community structure and altering the stability of ecological communities. However, few studies have been conducted in field conditions to test the effects of mutualism on the associated arthropod community. In this study, we tested the hypothesis that the mutualism between trophobiont insects and ants decreases the abundance and species richness of the associated arthropod community, supporting a more stable community. We also investigated whether the abundance of specific arthropod feeding groups is affected by the mutualism and whether plants with and without mutualism form a mosaic with different species composition, increasing overall species richness. Our assumptions were experimentally examined in a system comprising the host plant Psittacanthus robustus, three trophobionts and two tending ants. We showed that, locally, the abundance and species richness of the whole arthropod community did not decrease when mutualism was present, but the feeding group composed by predators was negatively affected by mutualism. Plants with trophobionts but without ants presented the highest stability. At the landscape level, plants with and without mutualism differed in arthropod species composition, suggesting that there was a mosaic formed by plants with and without mutualism, enhancing the overall species richness (or beta diversity). Overall, our results revealed that the mutualism can alter the structure and stability of the surrounding arthropod community.

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