Coevolved species should avoid competition through resource partitioning, but human-induced alteration of plant/animal communities may facilitate the onset of competitive interactions. In herbivores, access to high-quality forage in the warm months, that is, during nursing and weaning, influences growth and survival of offspring. In turn, resource exploitation by a reintroduced, superior competitor should affect offspring survival of the inferior one, by decreasing foraging efficiency and diet quality of mothers and young. We assessed the negative effects of reintroduced red deer Cervus elaphus on grassland, on foraging behavior of female Apennine chamois Rupicapra pyrenaica ornata (July–October 2012–2013) and on winter survival of chamois kids, across 3 study sites with different deer densities (great/intermediate/extremely low). The size of bare soil patches was positively associated with deer density and, in areas with deer, it increased throughout July–October. The volume of nutritious plants (i.e., legumes) in the diet of female chamois was lower and decreased faster between summer and autumn, in areas with deer than in that with an extremely low deer density. Feeding intensity (bite rate) of female chamois was significantly lower and their food searching (step rate) was greater in areas with deer. Chamois kids showed a significantly greater winter mortality, with a lower proportion of younger individuals, in areas with deer than in that with an extremely low deer density. In human-altered ecosystems, unpredictable consequences can follow interspecific interactions within restored animal communities. In turn, patterns of ecological relationships among ecosystem components may be modified, with an increase of the potential for competitive interactions.