Invasive species, not environmental changes, restrict the population and geographical range of the quokka ( Setonix brachyurus )

European arrival into Australia had large‐scale impacts on the local flora and fauna. Most notably, Europeans brought with them numerous non‐native species, including the European red fox ( Vulpes vulpes ), European rabbit ( Oryctolagus cuniculus ), and the goat ( Capra hircus ) among many others. The introduction of these species had significant consequences on native Australian mammals, causing some small‐ to medium‐sized herbivores to become geographically restricted to primarily islands. Here, we examined the dietary ecology of the quokka ( Setonix brachyurus ), a native marsupial herbivore with a restricted geographic range in Western Australia before and after European arrival. Fossils from south‐western Australia and modern specimens were examined via dental microwear texture analysis and stable isotope analysis to assess whether the diet of the quokka had changed dramatically over time. Collectively, we help clarify whether there were any ecological reasons as to why this marsupial became geographically restricted, aside from the presence of invasive predators on mainland Australia. The quokka maintains a browsing diet from the Pleistocene to the present on the mainland, but modern island populations eat drier and tougher foods when living on islands lacking invasive mammals. There is also an apparent shift in the feeding environment of the quokkas on mainland Australia, from more open forests/shrublands in the Pleistocene to denser and wetter forests. Multi‐proxy data collectively indicate that the restricted range of the quokka today is most likely a result of predation from non‐native taxa and/or other human influences—not because of a lack of suitable habitat.