Mistletoes benefit from initiating cascading effects in a cactus–stem borers–arthropods system: A positive feedback

Parasitic plants may indirectly modify arthropod community structure by inducing resistance/susceptibility in host plants. In the Atacama Desert, the mistletoe Tristerix aphyllus initiates cascading effects on its cactus host, Echinopsis chiloensis, inducing susceptibility to stem‐borer beetles, whose brood chambers are colonized by a diverse arthropod community. We evaluated whether those cascading effects initiated by Tristerix generate a positive or negative feedback loop on this parasitic plant. A positive or negative feedback loop would occur if the mistletoe‐elicited indirect effects on arthropod community result in increased or decreased mistletoe fitness, respectively. Specifically, by way of field sampling, experimental manipulation of number of brood chambers, and structural equation modelling, we assessed the indirect impact of number of brood chambers on Tristerix fitness mediated by its influence on the abundance of arthropod trophic guilds (omnivores, nectarivores, herbivores and predators). We found that the number of brood chambers had a positive indirect effect on mistletoe fitness, likely mediated by the increased abundance of nectarivores. The number of brood chambers was positively associated with the abundance of predators, nectarivores and herbivores. However, nectarivores were the only trophic guild showing a positive association with both number of brood chambers and Tristerix fitness. This positive feedback loop, together with the known pattern of specific disperser‐mediated cyclic reinfection of cacti by Tristerix, would indicate that mistletoe infection may ultimately threaten cactus survival.