An Ant-plant Mutualism Alters Local Arthropod Community Composition

Mutualisms are interspecific interactions with positive outcomes for both species. Much is known about the pairwise dynamics of these relationships; however, few experiments have addressed how mutualisms affect surrounding communities. Here we investigate how a facultative mutualism between ants and wild cotton (Gossypium thurberi) alters arthropod diversity in the Sonoran Desert of Southern Arizona, USA. In three wild cotton populations, we manipulated the presence of ants as well as the availability of extrafloral nectar. We monitored arthropod visitation to individual plants. At the site where ants were most beneficial to plant fitness, plants with ants had lower arthropod richness (both at the species and order levels) but higher total arthropod abundance. Thus, ants increased the dominance of a few taxa. These effects persisted even when ant-tended aphids were excluded from the analysis. In addition, across all sites, arthropod richness was highest when both ants and nectar were excluded – at least on some census dates. Thus, both ant presence and, surprisingly, extrafloral nectary availability can reduce the biodiversity of co-occurring arthropods. This study contributes to a growing body of literature demonstrating that mutualisms, like competition and predation, can shape community composition