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In fragile ecosystems, the introduction of exotic species could alter some ecological processes. The Austral parakeet (Enicognathus ferrugineous) shows close ecological and evolutionary relationships with the Andean Araucaria (Araucaria araucana), so any alteration in these interactions may have negative consequences for both partners and for ecosystem functioning and structure. We conducted extensive roadside surveys to estimate the abundance of parakeets in the northern Patagonian Andes over 4 years and recorded the food plants consumed by foraging flocks. The use of native habitats and humanized areas like villages and farms was influenced by the Araucaria seed crop. In masting years, the large seed crop allowed a massive use of this resource during the non-breeding season, and even during the breeding season. The exploitation of exotic plants was minor in the masting year, but became predominant in non-masting years, especially during the non-breeding season. This feeding switch towards exotic plants primarily arose because the low Araucaria seed crop in non-masting years is entirely consumed just after production by domestic and wild exotic mammals living in Araucaria forests year-round, thus forcing the displacement of parakeets towards anthropic habitats to exploit exotic plants. Given the degradation of the remaining Andean Araucaria forests due to the impact of exotic mammals on the ecological interaction between Araucaria and Austral parakeets, ambitious programs to exclude or reduce the density of these alien mammals, including livestock, are warranted., A Salvador de Madariaga sabbatical grant PRX12/00432 (to FH), Fundación Loro Parque project SEJI/2018/024 (to PRV), and Fundación Repsol funded this work.