What's amazing to me is how little space is required to have a completely self-sustaining ecosystem. A 60km diameter circle just doesn't seem like a very big space to have enough plants to support "flourishing" numbers of multiple types of large herbivores, without migration, as well as all the different prey species required to keep things in balance.

Regardless of the arguments about radiation, it seems pretty clear that lack of humans is really the most important thing for animals to flourish.