Experimental resource supplementation shifts ant‐mediated defence on silver cholla

Abstract Extrafloral nectaries (EFN) often mediate facultative mutualisms between plants and ants. Ants obtain a nutrient‐rich food source from plants in exchange for defending the plant from herbivores. Ant species can differ in effectiveness as plant defenders and compete for access to mutualism partner resources. The partitioning of mutualist‐provided resources between ant species can have important implications for both ant coexistence and plant fitness. How plant investment into defensive traits translates into partner identity, and thus defensive quality, is a fundamental question in facultative mutualisms because plants cannot restrict visitation to only the most effective mutualist. We conducted an artificial nectar supplementation experiment in the Mojave Desert using a population of the EFN‐bearing cactus species silver cholla ( Cylindropuntia echinocarpa ), replicated over two spring growing seasons. We measured the response of the ant community to supplemented and natural EFN resources and evaluated the effectiveness of ant defence against herbivores and for plant fitness. Resource availability—natural and experimental—impacted the abundance and identity of ants associated with silver cholla. Experimentally increasing sugar resources disrupted the existing resource partitioning between ant species. Cactus coreid ( Chelinidea vittiger ) abundance on unsupplemented silver cholla increased over the study period but not on resource‐supplemented plants. The most important mutualist was Crematogaster depilis , at least during the early spring, because this species was associated with lower herbivore abundance and increased seed set. This trend suggests that increases in EFN resources provided by plants directly translate to increased services from mutualists. Collectively, these results suggest that the energetic investment into EFN is beneficial to silver cholla, but the return on investment is mediated by competition hierarchies between mutualists. Synthesis . The partitioning of EFN resources between ant species influences c and subsequent fitness effects on silver cholla.