The article that you linked does not provide any evidence that any of those animals require grass for their survival. Being able to digest grass and to survive in grass does not mean that those same animals wouldn't be just fine in a more diverse ecosystem. The only in-depth information that I've ever seen on the nutritional requirements of grazing mammals has actually concluded that adding non-grass vegetation to their diets would be of benefit. That makes sense, considering that natural meadows/prairies are (presumably) much more biodiverse places than the grasslands that we see today. The only (non-domesticated) animals that I've encountered that seem to only exist in the grass are blood-sucking insects and other parasites.