Cascading effects of a disease outbreak in a remote protected area.

Disease outbreaks induced by humans increasingly threaten wildlife communities worldwide. Like predators, pathogens can be key top-down forces in ecosystems, initiating trophic cascades that may alter food webs. An outbreak of mange in a remote Andean protected area caused a dramatic population decline in a mammalian herbivore (the vicuña), creating conditions to test the cascading effects of disease on the ecological community. By comparing a suite of ecological measurements to pre-disease baseline records, we demonstrate that mange restructured tightly linked trophic interactions previously driven by a mammalian predator (the puma). Following the mange outbreak, scavenger (Andean condor) occurrence in the ecosystem declined sharply and plant biomass and cover increased dramatically in predation refuges where herbivory was historically concentrated. The evidence shows that a disease-induced trophic cascade, mediated by vicuña density, could supplant the predator-induced trophic cascade, mediated by vicuña behaviour, thereby transforming the Andean ecosystem.