Apex predators can have considerable impacts on meso-carnivore diets, through competition or facilitation. Facilitation occurs when smaller predators consume carrion created by larger predators, especially large-bodied prey species normally inaccessible to meso-carnivores. In contrast, apex predators can also negatively affect meso-carnivore consumption of important resources through competitive interactions. Thus, predicting meso-carnivore responses to trophic structure changes (i.e. apex predator extirpation or reintroduction) is often difficult. We investigated stable carbon and nitrogen isotope niche breadths of black-backed jackal (Canis mesomelas) in response to the reintroduction of an apex predator, the African lion (Panthera leo), to the Karoo National Park, South Africa. Jackal faecal isotopic niche widths were larger in post-lion than pre-lion samples, indicating a niche expansion to include pure C₃- and C₄-based food sources when lions were present. Most prey items of this nature in the study area are large-bodied ungulates. Our results agree with results of traditional scat analysis, which showed that prey species >92 kg were consumed more often after the lion reintroduction. Stable isotope data from carnivore faeces are effective for tracking responses of wildlife to changing ecological conditions, providing an alternative source of information about changes in community structure brought about by management interventions.