Extirpation is the localized extinction of a species. This can be due to over-hunting, human development, climate change, or competition from introduced species. It is especially common on islands where only a small and therefore vulnerable, population may exist. Sometimes species are reintroduced to an area from which they were extirpated, although this may cause controversy as during the reintroduction of wolves to Yellowstone National Park. In other instances, species return to an area on their own, as their range expands or their population recovers from some pressure, as seen in the return of wolves to the state of Michigan.