Whether large carnivores indirectly influence vegetation via prey behavior remains controversial because available evidence is largely correlational, and recent field experiments have found weak associations between risk experienced by prey and vegetative responses to herbivory. We combined observational data and a field experiment to test whether an ambush predator—the puma (Puma concolor)—affected the antipredator behavior of its primary ungulate prey—the vicuña (Vicugna vicugna)—which in turn had cascading effects on vegetation. We predicted that strong protective effects of pumas on vegetation would be most apparent in habitats where cover and terrain (i.e., physical complexity) facilitated the ambushing strategy of pumas. In 3 different habitats, we evaluated the relationships among predation risk, vicuña behavior, and—by deploying vicuña exclosures— vegetation structure and productivity. We also examined habitat-specific rates of vegetative regrowth. Risky habitats presented high physical complexity, a disproportionately large number of vicuñas killed by pumas, and high relative abundance of pumas. In these habitats, vicuñas displayed strong antipredator behaviors and exclosures did not affect vegetation, suggesting that pumas indirectly protected plants. Conversely, a safe habitat presented low structural complexity, a disproportionately low number of vicuñas killed by pumas, and low relative abundance of pumas. Here, vicuñas relaxed antipredator behaviors and exclosures had strong effects on vegetation. In 1 habitat deemed as risky, however, cascading effects were offset likely because water and nutrients were readily available to plants and regrowth was rapid. Our results show that large ambush carnivores can positively affect vegetation via nonconsumptive effects on their prey, and that these effects are habitat mediated. However, primary productivity modulated the strength of such effects, with high primary productivity dampening the observed cascading effects.
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Vol. 97 • No. 3