A fourth reason for having zoos is that they preserve species that would otherwise become extinct. As the destruction of habitat accelerates and as breeding programmes become increasingly successful, this rationale for zoos gains in popularity. There is some reason for questioning the commitment of zoos to preservation: it can be argued that they continue to remove more animals from the wild than they return. Still, zoo breeding programmes have had some notable successes: without them the Pere David Deer, the Mongolian Wild Horse and the European Bison would all now be extinct. Recently, however, some problems have begun to be noticed.